fresh march issue eight 2020 quarterly...a post-harvest disorder and fruit that has passed optimal...

FOR THE DECIDUOUS FRUIT GROWER

FRESHQUARTERLY

SunburnWhy it happens

& how to prevent it

ISSUE EIGHTMarchMarch 20202020

2 FQ ISSUE 08 MARCH 2020

FRESH QUARTERLYIssue 8, MARCH 2020

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Executive editor Elise-Marie Steenkamp

Editor Anna Mouton

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QF

This issue of Fresh Quarterly is a milestone marking our first two years of publication. What have we learnt so far and how do we plan to go forward?

Our original objective was to repackage past and present research results to

create an accessible and absorbing product for our growers. We envisaged

a magazine that engaged readers by a combination of compelling content

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and the benefits gained from research — the focus had to be on practical

recommendations and implementation.

All of this still holds true. We haven’t deviated from our original objective

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The success of Fresh Quarterly depends on its relevance to our members.

Please get in touch to share your suggestions — we welcome your feedback.

Hugh Campbell,

letter from the Publisher

QF pUblISHER’S Letter

3 ISSUE 08 MARCH 2020 FQ

QF CONTENTS

COVER STORY: CROP PRODUCTION

The world keeps warming and growers are feeling the heat. In this issue we

discuss sunburn what is the impact and what can you do about it?

4 Sunburn What are the diff erent kinds? When are fruit

at risk? How do you prevent it?

10 Sunburn researchsummarised

An overview of the highlights from projects funded by Hortgro Pome and

Hortgro Stone.

CROP PROTECTION 16 A tale of two fl ies

The Mediterranean fruit fl y is an old enemy of growers. Now a new threat has appeared:

the Oriental fruit fl y. How do we fi ght it?

20 Going the distanceThe surprising habits of Oriental fruit fl ies.

Spoiler alert: males and females have diff erent priorities.

19 Research inventory

POST-HARVEST

22 Internal browning in Cripps Pink

A post-harvest expert explains the diff erence between diff use and radial

browning and why it matters.

24 Keep your apples in the pink

Research results confi rm the importance of step-down cooling in preventing

internal browning.

26 Best post-harvest practices

The protocol for Cripps Pink as developed by the Store-it Group and Hortgro Science.


REGULARS2 Letter from the Publisher

29 COLUMN - A citrus perspective on Oriental fruit fl y.

30 COLUMN - Back to earth.12 Net essentialsPome and stone fruit production under nets is surging but are nets appropriate for all orchards? Six people across the industry

provide answers.


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Sunburn can cause signifi cant economic losses in apples and other fruit. Why it

happens & how to prevent it. By Anna Mouton

prevent it. By Anna Mouton

SunburnST

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QF CROP PRODUCTION


is the damage caused by too much sun exposure — anyone

who spends time outdoors knows this. People

tend to � nd a tan attractive in humans

but not so much in apples and other fruit.

Sunburn is a major reason why fruit is

rejected as unsuitable for export and ends up

in informal markets or as juice.

“Sunburn is most noticeable in pome

fruit,” says Prof. Stephanie Midgley of the

Department of Horticultural Science at

Stellenbosch University. The worst-affected

apple cultivars are Granny Smith, Golden

Delicious and Fuji.

The impact of sunburn on sensitive

cultivars is significant, according to

Midgley. “In my experience an average

orchard that is well-managed and not under

drought stress will often have 10%–15%

Sunburn

Department of Horticultural Science at

Stellenbosch University. The worst-affected

apple cultivars are Granny Smith, Golden

Delicious and Fuji.

The impact of sunburn on sensitive

cultivars is significant, according to

Midgley. “In my experience an average

orchard that is well-managed and not under

drought stress will often have 10%–15%

Sunburnsunburn. It can go up to 30%, sometimes

even more than that.”

Stone fruit are also susceptible to

sunburn. Prof. Karen Theron holds the

Chair in Applied Preharvest Horticulture at

Stellenbosch University. “There are seasons

when there are sunburn problems in certain

plum cultivars,” she confirms.

Sunburnt fruit frequently suffer from

post-harvest disorders because of the role

of heat stress in both sunburn and many

post-harvest problems.

� e sunburn spectrumSunburn is a collective term for three

preharvest conditions that di� er in ways that

are relevant to control. � e descriptions below

refer to apples but similar conditions can

occur in other fruit and even some vegetables.

Sunburn necrosis is the most dramatic

manifestation of sunburn. Affected fruit

have dark brown or black areas of dead

tissue on the surface. The damage may

extend for several millimetres into the flesh

and is a perfect substrate for the growth

of spoilage organisms. Sunburn necrosis

renders fruit unfit for anything other than

perhaps juicing.

Research has shown that sunburn necrosis

occurs when the fruit surface temperature

attains 52 °C for as little as ten minutes.

Heat — not light — causes the tissue

damage and death. Sunburn necrosis can be

experimentally induced in complete darkness

by raising fruit surface temperature.

Sunburn browning is the equivalent of

a tan. The fruit responds to excess light

and heat by developing yellow, brown,

bronze or dark discolouration of sun-

exposed areas of the peel. The underlying

flesh is not damaged. Affected fruit can

have higher sugar levels and lower acidity

but consumers dislike sunburn browning.

Anything more than mild discolouration

will lead to rejection by the market.

Both light and heat are necessary for

sunburn browning — it will not develop

in darkness. Apple cultivars vary in their

sensitivity to browning. The dark colours of

certain red apple and stone fruit cultivars

may also mask browning.

The third type of sunburn is called

photooxidative sunburn. It affects fruit

that are suddenly exposed to sunlight after

having grown in shaded conditions. This

can happen due to management practices

such as summer pruning or even when

newly picked fruit are left lying in full

sun. Light exposure — not heat — is the

primary driver of photooxidative sunburn.

Fruit affected by photooxidative sunburn

develop a white — or photobleached —


Photooxidative sunburn has resulted in bleaching of the

colour on the exposed surface of these apples.

area. Tissues in the centre of the bleached

area may eventually die off and the fruit is

left with a necrotic area similar to that seen

in sunburn necrosis.

Sometimes fruit only develop signs

of sun damage while in storage. Minor

sunburn browning at harvest can progress to

extensive darkening in Granny Smith apples

during storage. � is is called sunburn scald

or sunscald. Sunscald — like all scalds — is a

post-harvest disorder. Sunscald develops on

the exposed side of the fruit whereas other

scalds tend to develop on the shaded side.

Factors that contribute to sunburnTemperature and sunlight are the main

culprits — as described above — but there

are several indirect factors that also in� uence

the development of sunburn. One of these —

cultivar — has already been mentioned.

Individual fruit within a cultivar respond

differently to sun exposure. Some fruit

— like some people — just never seem to

burn. Scientists are still trying to work out

the reasons for this individual variation.

Sunburn browning and necrosis are more

common in fruit that are at least 45

millimetres in diameter. Photooxidative

sunburn can afflict fruit of any size.

Fruit are at risk of sunburn above a

threshold fruit surface temperature.

The threshold is higher in fruit that are

acclimated to the sun. All fruit are capable

of acclimation — given sufficient time.

Apples that set and grow in sunlight can

tolerate far more light and heat than

those that have lived all their life in the

cool shade. Apples that have experienced

a long period of overcast weather may be

vulnerable to sunburn if the days suddenly

turn bright and hot.

Fruit surface temperature influences

sunburn and therefore so does anything

that heats or cools the fruit. Low relative

humidity may promote sunburn by

increasing fruit surface temperature. Air

movement can retard sunburn by decreasing

fruit surface temperature. Even modest air

circulation can cool fruit by 4–5 °C.

Strategies to reduce sunburn“� e main thing that farmers want to do is

canopy management,” says Midgley. Fruit

that were exposed to sunlight from a small

size have had time to acclimate and build up

their defences against strong light and high

temperatures. Fruit that have been shaded are

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not acclimated and they will burn if suddenly

exposed during summer pruning. For this

reason summer pruning is best done as soon

as possible after � nal fruit set or when doing

� nal hand-thinning.

The trend toward high-density

plantings of smaller trees on dwarfing

rootstocks increases the risk of sunburn

due to increased light exposure. Dwarfing

rootstocks may also predispose trees to

drought stress if irrigation is not well-

managed because their root systems are

smaller. Optimal irrigation is important in

mitigating the risk of sunburn but there is

no benefit in over-irrigating the trees.

Correct row orientation will help reduce

sunburn. “In South Africa the standard

is north-south for that reason,” explains

Midgley. “It’s actually a little off north-

south. There are still orchards out there

Photooxidative sunburn can afflict fruit of any size.

QF CROP PRODUCTION


that are east-west and

when you walk through

them there’s no sunburn

on the south. But there’s

sometimes serious sunburn on

the north.”

Researchers have tested a number of

interventions for sunburn. � eron recalls

experiments with kaolin as a sunblock. “Yes,

it reduces sunburn, but if you don’t wash it o�

one hundred percent, it looks as though there’s

spray residues on the fruit, which is most

de� nitely not acceptable to the market.”

Midgley adds that see-through

sunscreen products were also trialled but

never caught on.

Evaporative cooling is effective at

reducing sunburn but it’s not a viable

solution for South Africa. “We don’t have

enough water to use this,” says Theron.

“And in plums you have the problem

that the fruit often burst if you apply

evaporative cooling close to harvest.”

Shade net has proved the most practical

option for combatting sunburn. “When

we did the trials almost ten years ago, it

was absolutely clear that there’s almost

no sunburn under shade net,” confirms

Midgley. “The intensity is reduced and

the numbers are really low. So that’s what

people are using.”

Shade net is likely to become even more

Sunburn necrosis is characterised by a well-circumscribed, dark

brown or black area of dead tissue on the fruit surface.

widespread with the trend toward high-

density plantings. “What I saw in Australia

was that every single one of those orchards

where they’re doing two-dimensional

plantings has shade net,” states Midgley.

“There would have to be research done

here where we have a very two-dimensional

planting to see how that changes the risk

for sunburn. To be absolutely safe one

would advise farmers to put up a net if they

want a system like that.”

� e inexorable rise in global temperatures

are leading us to a hotter and drier future

where sunburn and related disorders can

only become more troublesome. Canopy

management and shade nets will play an

important role in helping growers to safeguard

their production in years to come.

Reference: Racsko J. and Schrader L.E.

2012. Sunburn of apple fruit: historical

background, recent advances and future

perspectives. Critical Reviews in Plant

Sciences 31(6):455‒504.


interventions for sunburn. � eron recalls


� e inexorable rise in global temperatures

that are east-west and

when you walk through

them there’s no sunburn

on the south. But there’s





Sunburn necrosis is characterised by a well-circumscribed, dark

brown or black area of dead tissue on the fruit surface.

widespread with the trend toward high-

density plantings. “What I saw in Australia

was that every single one of those orchards

where they’re doing two-dimensional

plantings has shade net,” states Midgley.

“There would have to be research done

here where we have a very two-dimensional

planting to see how that changes the risk

for sunburn. To be absolutely safe one

would advise farmers to put up a net if they


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Sunburn browning and necrosis tend to occur on fruit that are at least 45 millimetres in diameter.


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Sunburn browning causes yellow to brown discolouration on the fruit surface.

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QF CRop pRodUCtIon

Completed projects2003. Midgley SJE, Holcroft DH, Jacobs G and

Steyn WJ. Improving red colour development

in blushed apples. [The project assessed

evaporative cooling.]

2006. Midgley SJE, Gindaba J, Theron KI, Holcroft

DH, Jacobs G, Steyn WJ and various MSc

students. The influence of climate stress and

source/sink manipulations on gas exchange, size

and colour development of bi-colour apples.

[Included studies on nets and particle films.]

2010. Steyn WJ, North M, Cook NC, Midgley

SJE and various MSc students. Improvement of

apple colour.

2011. Brink D, Pretorius JJB and Steyn WJ.

Developing a sunburn control strategy for Granny

Smith, Fuji and Braeburn using Raynox Plus.

2014. Brink D, Kotze WP and Steyn WJ. The

effect of different coloured shade nets on the

productivity of Granny Smith and Fuji apple trees

in the EGVV area.

2015. Steyn WJ, Midgley SJE, Lötze E, Schmeisser

M, Theron KI and students. Inherent and acquired

resistance to fruit sunburn and poor colour in

various apple/pear cultivars.

2017. Steyn WJ, Dzikiti S and Schmeisser M. Tree-

water relations and sunburn in pome fruit.

2018. Midgley SJE and Steyn WJ. Acclimation

of apple peel to light and temperature and the

effect thereof on red colour development and

tolerance to sunburn.

2019. Steyn WJ, Jooste M, Schmeisser M and

Lötze E. Heat damage in plums.

A list of research projects and publications related to sunburn and funded

by Hortgro Pome and Hortgro Stone.

RESEARCH InVEntoRY

Current projectsLabuschagne IF, Louw ED, Booyse M, Truter W,

Hörstmann and Van Zyl W. Adaptability indexing

of new pome (apple) and stone fruit (plum)

cultivars in diverse South African growing areas.

Midgley SJE, Dzikity S, Volschenk T and

Lötze E. Investigating the potential of fixed and

draped netting technology for increasing water

productivity and water savings in full-bearing

apple orchards under micro-irrigation.

Midgley SJE, Shülze RE and Davis NS. Scientific

and practical guide to climate change and pome/

stone fruit production in South Africa.

Theron KI. Effect of nets on growth, yield and

fruit quality as well as economic feasibility in plums.

Publications Journal publicationsFouché JR, Roberts SC, Midgley SJE and Steyn WJ.

2010. Peel color and blemishes in Granny Smith

apples in relation to canopy light environment.

HortScience 45(6):899–905.

Gindaba J and Wand SJE. 2005. Comparative

effects of evaporative cooling, kaolin particle film

and shade net on sunburn and fruit quality in

apples. HortScience 40:592–596.

Gindaba J and Wand SJE. 2007. Do

fruit sunburn control measures affect leaf

photosynthetic rate and stomatal conductance

in ‘Royal Gala’ apple? Environmental and

Experimental Botany 59: 160–165.

Hengari S, Theron KI, Midgley SJE and Steyn WJ.

2014. The effect of high UV-B dosage on apple

fruit photosystems at different fruit maturity

stages. Scientia Horticulturae 170:103–114.

Hengari S, Theron KI, Midgley SJE and Steyn

WJ. 2014. Response of apple (Malus domestica

Borkh.) fruit peel photosystems to heat stress

coupled with moderate photosynthetic active

radiation at different fruit developmental stages.

Scientia Horticulturae 178:154–162.

Hengari S, Theron KI, Midgley SJE and Steyn

WJ. 2016. Differential dependence of apple

(Malus domestica Borkh.) cultivars on the

xanthophyll cycle for photoprotection. South

African Journal of Plant and Soil 33(1):69–76.

Makeredza B, Marais H, Schmeisser M, Lötze

E and Steyn WJ. 2015. Ripening associated red

color development masks sunburn browning in

apple peel. HortScience 50(6):814–818.

Makeredza B, Schmeisser M, Lötze E and Steyn

WJ. 2013. Water stress increases sunburn in

Cripps Pink apple. HortScience 48(4):444–447.

Mupambi G, Schmeisser M, Dzikiti S, Reynolds

S and Steyn WJ. 2018. Ineffectiveness of

foliar S-ABA application as an apple sunburn

suppressant explained through effects on peel

biochemistry and leaf ecophysiology. Scientia

Horticulturae 232:256–263.

Wand SJE, Theron KI, Ackerman J and Marais

SJS. 2006. Harvest and post-harvest apple

fruit quality following applications of kaolin

particle film in South African orchards. Scientia

Horticulturae 107:271–276.

Conference proceedingsBarasu PD, Steyn WJ, Schmeisser M and

Midgley SJE. (In press). Sensitivity of apple

(Malus domestica Borkh.) fruit peel to high

irradiance and temperature as influenced by

a range of rootstocks in South Africa. Acta

Hortic. Proceedings of the 30th International

Horticultural Congress, Istanbul, Turkey, 12–16

August 2018.

Le Grange M, Wand SJE and Theron KI. 2004.

Effect of kaolin applications on apple fruit quality

and gas exchange of apple leaves. Acta Hortic.

636:545–550.

Gindaba J and Wand SJE. 2008. Comparison

of climate ameliorating measures to control

sunburn on ‘Fuji’ apples. XXVIIth International

Horticultural Congress, August 2006, Seoul,

Korea. Acta Hortic. 772:59–64.

Makeredza B, Jooste M, Lötze E, Schmeisser M

and Steyn WJ. 2018. Canopy factors influencing

sunburn and fruit quality of Japanese plum

(Prunus salicina Lindl.). Acta Hortic. 1228:121–128.

Mupambi G, Reynolds JS and Steyn WJ.

2014. Foliar S-ABA application does not reduce

sunburn in ‘Granny Smith’ apple. Acta Hortic.

1042:303–309.

Mupambi G, Schmeisser M, Lötze E, Malan

C, Dzikiti S and Steyn WJ. 2017. Effect of

supplementary irrigation at high ambient

temperatures on sunburn, plant physiology,

soil and canopy environment of ‘Granny Smith’

apple. Acta Hortic. 1150:239–244.

Smit A, Steyn WJ and Wand SJE. 2008. Effects

of shade netting on gas exchange of blushed

apple cultivars. Acta Hortic. 772:73–80.

Wand SJE, Steyn WJ, Holcroft DH, Mdluli

MJ, Van den Dool K and Jacobs G. 2005. Use of

evaporative cooling to improve ‘Rosemarie’ and

‘Forelle’ pear fruit blush colour and quality. Acta

Hortic. 671:103–111.

Wand SJE, Van den Dool K, Smit A and Steyn

WJ. 2008. Heat injury thresholds in apples

measured using chlorophyll fluorescence

are influenced by orchard heat reduction

technologies. Acta Hortic 772:273–277.

Highlights of projects funded by Hortgro Pome and

Hortgro Stone

Sunburn research summarisedHEL

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QF CROP PRODUCTION

development of sunburn is thought to be associated with both heat

exposure and UV-B irradiation. Laboratory

trials showed that di� erent cultivars respond

di� erently to UV-B but all are more sensitive

to UV-B if they have been growing in the

shade. � e researchers con� rmed that

photodamage in acclimated fruit starts at

temperatures of around 45 °C.

Sensitivity to sunburn was tested for Rosy

Glow apples grown on a range of dwar� ng

to vigorous rootstocks. � e researchers

concluded that the rootstock does not a� ect

the innate sensitivity of the apple peel

to high light and temperature. Sunburn

prevalence on a tree level is likely to be

driven by the exposure of more fruit to

potentially damaging levels of light and heat.

Water stress was found to increase

fruit surface temperature and sunburn

in Cripps Pink and Granny Smith apples

as well as in African Delight and Laetitia

plums. Researchers tested various levels

of irrigation against normal irrigation for

different time periods in a series of trials.

Sunburn was both more prevalent and more

severe in fruit from trees suffering from

water stress. Whereas irrigation at half

normal levels increased sunburn compared

to the control, irrigation at twice normal

levels did not reduce sunburn compared to

the control.

Research on the effect of pulsing

irrigation in apples was inconclusive.

Pulsing irrigation improved the plant water

status and reduced sunburn in Granny

Smith and Golden Delicious compared

to a control that did not receive pulsing

irrigation. However, the control experienced

water deficits. Therefore the difference in

the occurrence of sunburn could reflect

increased sunburn in the control rather

than a true decrease in the treatment.

Researchers measured the colour of

Granny Smith apples at different positions

in the canopy from fruit set until harvest

to see how colour and sunburn relate to

light levels and fruit surface temperature.

They found that Grannies require good light

until around eighty days after full bloom

to develop high chlorophyll concentrations

and dark green colour. Too much light in

the latter half of the season reduced green

colour and increased the risk of sunburn

and red blush. These results indicate that

shade netting — especially draped nets —

should significantly improve the colour of

green apple cultivars.

The red colour of apple cultivars such as

Topred and Cripps Pink may mask sunburn

browning. Studies confirmed that red

and blushed cultivars are not protected

from sunburn by anthocyanin — the red

pigment just hides the sunburn. Analysis

of the tissue response of different cultivars

revealed that the severity of visible signs

of sunburn is not directly correlated to the

stress levels of the fruit.

Sunburnt fruit tend to have higher

sugar levels but higher sugar levels do

not predispose fruit to sunburn. Fruit do

become more sensitive to sunburn closer

to harvest for a large variety of reasons but

high sugar levels are not one of the causes.

Two years of trials on Braeburn,

Fuji, Royal Gala and Cripps Pink apples

demonstrated that 20% black shade net

was not detrimental to photosynthesis and

carbon assimilation rates. The implication

is that nets will not have a negative effect

on seasonal carbohydrate production and

growth. Nets with a high shade percentage

may have a negative effect on the extent

and intensity of red colour.

Plums are susceptible to sunburn and

internal heat damage. Shade net and

optimal irrigation reduced sunburn.

The

Hortgro Pome and Hortgro Stone have funded and continue to fund extensive research into the development and control of sunburn. The goal is to better understand the causes and mechanisms of

sunburn so that control measures can be developed. This article summarises some of the highlights.

Consult the research inventory for more on the many scientists involved and their research outputs.


What you need to know to get started.

By Anna Mouton

Pome and stone fruit production under nets is surging but are nets appropriate for all orchards? Fresh Quarterly spoke to six people across the industry about their practical experience of using nets.

Net e

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Do you need nets?“You need to determine the commodity and the

purpose. You can put nets up for hail and sleet,

or for sunburn, or for wind damage,” says Charl

Stander, agricultural consultant at Franschhoek

Marketing. “� e pioneering work on nets was

done in the northern parts of South Africa,

primarily for hail in peaches and nectarines.”

Most people agree that nets are essential in

areas where hail and sleet occur. “If you don’t

erect hail nets, you may as well give up,” asserts

Daan Brink, technical adviser at Fruitmax Agri.

Hail nets tend to be fixed nets — these

are nets that are supported by permanent

structures. The nets are usually pitched over

the rows to encourage the hail to fall to the

ground instead of accumulating on the nets.

Brink contrasts nets for hail with nets for

sunburn. “You want hail nets to transmit

as much light as possible because they’re

usually covering high-value crops for

most of the season and you don’t want to

influence the red colour. For sunburn on

green varieties you want a dark net so you

can prevent as much sunburn as possible.”

Sunburn can reduce the yield of export-

quality fruit to as little as 40% in susceptible

cultivars such as Granny Smith. “Put a net

up and you can shift your pack-out to 85%,”

says Brink, “and your fruit quality improves

dramatically. � e fruit is greener.”

Nets for sunburn can be either fixed or

draped. Draped nets are — as the name


Retractable nets are supported by permanent structures but can be

opened or closed.

suggests — draped over trees instead of

being supported by posts and wires.

Sunburn can also affect plums but the

primary driver for using nets in plums is

often wind damage. “I’ve erected about

fifteen hectares of nets to prevent wind

damage,” remarks horticultural adviser

Petru du Plessis of his own orchards in

Stellenbosch. Nets have increased his yield

of export-quality fruit by 10%–12%.

What are the challenges?Cost is an obvious drawback. But there

is general agreement that nets pay for

themselves when they are used appropriately.

Horticultural adviser Anton Müller shares

data from trials he conducted with draped

nets on Granny Smith apples.

“If your yield was around sixty tonnes,

you could make an additional R100 000.00

per hectare with your nets, which is roughly

the cost of the draped nets You pay for the

net in one year and it lasts twelve to fifteen

years. The question isn’t how much it costs,

but how much money I’ll get back.”

Orchards under nets require careful

management as the trees grow more

vigorously. This is a potential advantage

when establishing orchards under fixed

nets. “The growth you get from your trees

is substantially better,” says Brink. “The day

that the trees come into production, you

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have a higher production because the trees

are larger.“

The downside of too much growth in

older trees is reduced yield. “Farmers who

are considering nets must implement

very good summer pruning,” advises Du

Plessis. “I think anyone who uses nets and

doesn’t have optimal summer pruning will

eventually run into fertility problems.”

The effect of nets on colour development

QF CRop pRodUCtIon


Fixed nets are supported by a permanent structure

of posts and wires.

can be positive — think greener Grannies —

or negative. Brink warns that you sacrifice

red colour in bicolour apples, although less

so with white or translucent nets. “Your

production gains have to be enough to

justify that loss of colour,” he says. Some

growers also report that apples are slightly

smaller under nets. Brink believes this could

be due to over-irrigation, insufficient fruit

thinning or increased tree vigour.

Colour development in plums can likewise

be less intense under nets. However fruit

tend to be larger.

To fix or not to fixThe majority view is that fixed nets are the

preferred option for protection against hail

and wind damage. Both fixed and draped nets

are suitable for controlling sunburn.

Fixed nets are available as so-called

retractable nets that can be pulled back to

expose trees to full light. Retractable nets are

more expensive than non-retractable nets.

Retractable nets are the recommended

option for trees that require pollination by

bees. “Bees and nets are not compatible,”

says Stander. Nets also impact the quality

of flower buds and this can lead to reduced

fertility and yields over several seasons.

Stander advocates retracting the nets

immediately after harvest and leaving the

trees exposed until pollination is complete.

“If you have fixed nets that can’t open, they

need to be fairly high,” cautions Stander. This

allows room for bees to move over the rows.

Growers who experience snow should

also invest in retractable nets. “We made

the mistake of not having retractable nets

where there’s snow,” recalls Willie Kotze,

technical adviser at Dutoit Agri. “You have

the risk of the whole structure collapsing.

That’s not much fun.”

Draped nets are supported by the tree itself

and don’t require an expensive structure

Pest control under nets“If there is a pest and you cover it with a

net, it will cause problems,” warns Brink.

“You need more monitoring. But you don’t

necessarily need to spray more.” Fixed nets

improve spray conditions and can facilitate

pest control. It is possible to spray effectively

through draped nets by using higher air

volume settings.

Fruit flies and codling and false codling

moths appear to be less numerous under nets.

Growers need to be aware that chemicals

break down more slowly under nets. “The

market is fanatical about residues,” cautions

Stander, “so you must adapt to ensure that

you stop spraying a few days earlier.”

Stander also raises concerns about

potential residues in draped nets. “If the

spray accumulates in the nets and it touches

the fruit it could leave a residue.” Similar

to keep them in place. So could they be a

better option than fixed nets for sunburn?

It depends, says Müller. “If you’re in the

Grabouw area there’s no real reason for a fixed

structure. You only need to control sunburn

at certain times of the year. But for hail you

need nets for a much longer period.”

Draped nets are especially valuable for older

trees. Müller recounts that his initial research

into draped nets was prompted by old Granny

Smith trees. “Their harvest was lost because of

sunburn and everyone was saying you should

take them out. But the market is clamouring

for Grannies. With a net we can keep those

orchards for many more years.”

Does net colour matter?Professor Wiehann Steyn, research

and development manager at Hortgro

Science, recommends that growers stick to

monochromatic nets — black, grey, white

and translucent. The difference between these

shades is the amount of light they let through.

Steyn stresses that growers need to be clear

about the problem that the net is intended to

solve. Is it sunburn on Grannies? Consider a

black net. Worried that a black net will reduce

the productivity of your Goldens? A grey net

may be a better option. And for damage that

isn’t caused by light — for example hail —

look at white or translucent nets.

“The one thing we’ll have to keep in mind

in future is the environmental impact,”

says Steyn, “the visual impact.” The Italian

province of South Tyrol allows only black

nets for aesthetic reasons. “Black is a little

extreme for our blush cultivars. Grey is a

potential compromise between the visual

disturbance of white nets and too much

shade cast by black nets.”

“Bees and nets are not

compatible.”

QF crop production

15 iSSuE 08 MArcH 2020 FQ

problems have occurred in the past with

chemicals transferring from crates to fruit.

What lies ahead for nets?Everyone agrees that the number of orchards

under nets will proliferate further. But, as

Kotze points out, there are some constraints.

“This is a growing industry and there’s a

shortage of contractors to erect nets. And

there aren’t enough dwarfing rootstocks.”

Kotze explains that the success of

apple trees under nets relies on dwarfing

rootstocks. “We tested nets in the past but

we could never control the growth of the

trees and we lost productivity. Now that

we’re on the right rootstocks we can put the

trees under nets.”

He describes the next challenge as

sourcing trees in the numbers that he

needs to establish new orchards under nets.

“We’re reliant on the right rootstocks and

this is likely to limit the expansion.”

Climate change is set to boost net use

even more. Everyone agrees that nets

lower the water requirements of trees —

but by how much? Work done in Australia

found reductions of up to 25% but a study

funded by Hortgro and the Water Research

BOTH

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Commission indicate savings of only 12%.

“This was in the mild Witzenberg valley,”

reports Steyn. “Savings tend to be greater

under more extreme conditions.”

Stander is positive about the expansion

of nets. “Nets will play a much more

prominent role, given the drought in the

Klein-Karoo and the growers that need to

generate income. If water is limiting your

new plantings then you have to increase the

yield of what you have. You have to increase

your pack-outs and how do you do that? By

reducing wind damage and sunburn.”

“Nets will play a much more prominent role, given the drought in the Klein-Karoo and the growers that need to generate income. If water is limiting your new plantings then you have to increase the yield of what you have. You have to increase your pack-outs and how do you do that? By reducing wind damage and sunburn.”

Draped nets are supported

by the tree itself.

� e Mediterranean fruit � y has long been

established across South Africa and now

a new kid on the block has appeared: the

Oriental fruit � y. While both Mediterranean

and Oriental fruit � ies cause their fair

share of damage, the latter is a di� erent

type of trouble. Known for its aggression,

competitiveness and rapid reproduction, the

Oriental fruit � y might pose the bigger risk to

South Africa’s deciduous fruit industry.

Di� erent journeys but similar life histories� e Oriental fruit � y (Bactrocera dorsalis)

and the Mediterranean fruit � y (Ceratitis

capitata) underwent di� erent journeys to

South Africa. Originally from Asia, the

Oriental fruit � y was � rst discovered on the

African continent in Kenya in 2003. From

there, the pest spread south. In 2013, it

invaded South Africa. Today, Oriental fruit

� y is o� cially established in the northern

and northeastern parts of the country, but

not in the Western Cape or Langkloof.

Mediterranean fruit fly, on the other

hand, is established over a wider area.

It is believed to originate from tropical

Africa and invaded Europe more than 150

years ago. From there it spread across the

globe, including to South Africa. Currently,

Mediterranean fruit flies are found in most

parts of South Africa.

Both Oriental and Mediterranean fruit

flies have a typical fruit-fly life cycle: the

female and male mate; the female lays eggs

under the skin of fruit; the larvae hatch

and feed on the fruit; the final larval stage

jumps into the soil where it pupates; the

adult emerges from the soil. This process

takes about a month under optimal

conditions of 25°C and slows down in

winter. Fruit flies do not become dormant

during cold months, but overwinter as

adults or larvae. Because fruit flies are

polyphagous and attack a wide variety of

fruit, there is usually an available host, no

matter the season.

� e pit bull versus the chihuahua� e Oriental fruit � y is a tropical insect that

prefers tropical fruits like mangoes, but it

can also infest deciduous fruit, including

nectarines, plums, apples and pears.

“We wanted to know whether Oriental

and Mediterranean fruit flies could use

the same hosts,” says Dr Welma Pieterse,

an entomologist with the Department

of Agriculture, Land Reform and Rural

Development. “The conclusion is that both

fly species can complete their life cycles on

various deciduous hosts. In fact, Oriental

fruit flies can do it more efficiently.”

How Oriental fruit fl ies diff er from Mediterranean

fruit fl ies. By Grethe Bestbier

Two FliesA TA L E O F


The Oriental fruit fl y Bactocera dorsalis resembles a wasp but is

much smaller.

Two Flies QF CROP PROTECTION

18 FQ ISSUE 08 MARCH 202018 FQ ISSUE 08 MARCH 2020

Oriental fruit flies easily outcompete

other flies. Pieterse says that while

observing both species for her research, the

Oriental fruit flies’ hostility was striking.

“They are incredibly aggressive. They

pushed and shoved each other,” she says.

“The Mediterranean flies mostly kept out

of the way while the Oriental flies patrolled

the fruit.”

It is not only their aggressive nature that

makes the Oriental fruit fly the pit bull and

the Mediterranean the chihuahua in this

fight. The former is also a better invader in

terms of reproductive rate.

“An invasive insect wants to fit in as

many generations in a year as possible,

so being able to reproduce quickly is

advantageous,” explains Prof. Pia Addison

of the Department of Conservation Ecology

and Entomology at Stellenbosch University.

Pieterse’s research found that various

factors enable Oriental fruit � ies to reproduce

faster than Mediterranean fruit � ies. Oriental

fruit � ies live longer as adults, so can lay eggs

for longer and thus generate more o� spring.

� ey also attempt more oviposition and lay

more eggs than Mediterranean fruit � ies.

Female Oriental fruit � ies lay around � fty

eggs over ninety days — about twice as many

as Mediterranean fruit � ies.

Oriental fruit flies have other strategies

as well. When the females probe fruit with

their ovipositors, they deposit pheromones

that deter other flies from infesting that

host. Lastly, Oriental fruit fly larvae hatch

earlier than Mediterranean fruit fly larvae,

which gives them a competitive edge. The

earlier larvae hatch, the better their chances

in the fruit.

Oriental fruit � y’s apple fetishWhile nectarines proved to be the best host

for both � y species in laboratory studies,

apples were surprisingly popular with

Oriental fruit � ies. “We weren’t expecting

Oriental fruit � ies to do so well on apple,

because it is far removed from a tropical

fruit,” says Addison. “But, it did really well.

� e reproductive rate was high on apple

compared to Mediterranean fruit � y.

Pieterse observed both species on apples for

ninety days in the laboratory. By the end of

the experiment, the Oriental fruit � ies were

still very active and laying eggs. Apples seemed

to extend the Oriental fruit � y’s lifespan,

especially compared to the Mediterranean fruit

� y. Relative to other fruit, however, apple is

not the number one host.

“Apple is definitely not their primary

host, but they will most likely be able to use

it as a survival host. Because it is a thick and

sturdy fruit, pupae can possibly survive in it

over the winter. Then, when more suitable

temperatures and crops appear, it can live

on in the preferred hosts. In that way, the

population is sustained,” says Pieterse.

Addison believes that the Oriental fruit

fly’s potential to establish in deciduous

fruit production areas in the Western

Cape is very high. “Climatic models, the

Oriental fruit fly’s ability to compete with

Mediterranean fruit fly, and its suitability

for deciduous hosts, indicate that it could

do well.” However, she warns that these

are predictions based on a controlled

laboratory study. “We don’t know how it

will respond to a Mediterranean climate.

But, as we speak, those insects are busy

adapting, and we know that insects

adapt quickly.”

“Early detection and elimination

of Oriental fruit flies are of utmost

importance,” emphasises Pieterse.

“Monitoring is crucial, because on

deciduous fruit they can sustain their

population. To avoid establishment in the

Western Cape, we must be extra careful

here. Monitoring systems, traps, and people

who check it regularly, are important to

know what is going on.”

The Mediterranean fruit fl y Ceratitis capitata is established throughout

South Africa.

“We don’t know how it will respond to a Mediterranean

climate. But, as we speak, those insects are busy

adapting, and we know that insects adapt quickly.”


QF CROP PROTECTION

Completed projects2008. Addison P and Marakhan A. Pest risk

analysis of fruit fl y introductions and assessment

of current fruit fl y management practices.

2011. Grout T. Cold susceptibility and

disinfestation of Bactrocera invadens (Diptera:

Tephritidae) in oranges.

2014. Terblanche JS and Hill MP. Bactrocera

dorsalis environmental niche modelling.

2015. Pringle KL. Bactrocera dorsalis: pest risk

analysis for the deciduous fruit industry in the

Southwestern Cape.

2017. Weldon C. Dispersal capacity of Bactrocera

dorsalis.

2019. Addison P and Pieterse W. Forecasting

Bactrocera dorsalis invasion potential using trait-

based modelling approaches.

Current projectsWare T. Cold sterilization treatments for

Bactrocera dorsalis in deciduous fruit.

PublicationsAnguelov R, Dufourd C and Dumont Y. 2017.

Simulations and parameter estimation of a trap-

insect model using a fi nite element approach.

Mathematics and Computers in Simulation,

133:47‒75.

De Villiers M, Manrakhan A, Addison P and

Hattingh V. 2013. The distribution, relative

abundance, and seasonal phenology of

Ceratitis capitata, Ceratitis rosa and Ceratitis

cosyra (Diptera: Tephritidae) in South Africa.

Environmental Entomology, 42(5):831‒40.

Dufourd C, Weldon C, Anguelov R and Dumont

Y. 2013. Parameter identifi cation in population

models for insects using trap data. Biomath,

2: 1312061.

Grout TG, Daneel JH, Mohamed SA, Ekesi

S, Nderitu PW, Stephen PR and Hattingh V.

2011. Cold susceptibility and disinfestation of

Bactrocera invadens (Diptera: Tephritidae)

in oranges. Journal of Economic Entomology,

104(4):1180‒8.

Hill MP and Terblanche JS. 2014. Niche

overlap of congeneric invaders supports a

single-species hypothesis and provides insight

into future invasion risk: implications for global

management of the Bactrocera dorsalis complex.

PLoS ONE 9(2): e90121.

Karsten M, Barton M, Addison P, Addison

MF and Terblanche JS. 2018. A synthesis

for managing invasions and pest risks

simultaneously for tephritid fruit fl ies in South

Africa. Entomologia Experimentalis et Applicata.

166 (5):344‒356.

Makumbe LDM, Manrakhan A and Weldon

CW. 2017. Optimisation of fl uorescent pigment

marking for Bactrocera dorsalis (Diptera:

Tephritidae). African Entomology, 25:220‒234.

Manrakhan A and Addison P. 2014. Assessment

of fruit fl y (Diptera: Tephritidae) management

practices in deciduous fruit growing areas in South

Africa. Pest Management Science, 70(4):651‒60.

Pieterse W, Benítez HA and Addison P.

2017. The use of geometric morphometric

analysis to illustrate the shape change induced

by diff erent fruit hosts on the wing shape

of Bactrocera dorsalis and Ceratitis capitata

(Diptera: Tephritidae). Zoologischer Anzeiger,

269:110‒116.

Pieterse W, Manrakhan A, Terblanche JS and

Addison P. 2019. Comparative demography

of Bactrocera dorsalis (Hendel) and Ceratitis

capitata (Wiedemann) (Diptera: Tephritidae)

on deciduous fruit. Bulletin of Entomological

Research, 27:1‒10.

A list of research projects and publications related to Oriental fruit fl y and funded by Hortgro Pome and Hortgro Stone.

RESEARCH INVENTORY


Factors infl uencing dispersal in Oriental fruit fl ies. By Grethe Bestbier

G O I N GThe Distance

“At the moment the Western Cape is an Oriental fruit � y free area and we

want to maintain it like that,” says Prof. Chris

Weldon from the Department of Zoology and

Entomology at the University of Pretoria. “If

you want to keep Oriental fruit � ies out of the

Western Cape, then you need to know what

is driving them, why and where they want to

move, and what determines how far they � y.”

Weldon explains that knowledge of

dispersal can be used to establish bu er

zones. � ese are areas from which fruit

should be excluded to avoid fruit � y invasion.

� en again, if the pest does establish in the

Western Cape, dispersal ability will determine

how large an area needs to be quarantined

and treated to eradicate it.

From 2015 to 2017, Weldon led a research

project on the dispersal of Oriental fruit

flies, studying how far they can fly in

relation to physiological and environmental

variables. Fly maturity and sex, and the

availability of host fruit were studied

as relevant physiological factors, while

environmental factors included temperature

and rainfall. Flies marked with fluorescent

pigments were released and recaptured

weekly for four weeks, providing valuable

information on flight distances and drivers.

Sex and age� e results showed that younger males cover

greater distances than older males. Young

males tend to disperse to � nd unrelated

females for mating, so as to spread their genes.

� is behaviour is also seen in other species of

the same genus as the Oriental fruit � y.

“The young males have this tendency to

want to move away from the place where

they emerged,” says Weldon. “There seems

to be a pattern where the males disperse

when they are young, whereas older males

settle down.”

However, this is not the case for females.

The distances that females fly is determined

by the availability of host plants. Females

are stronger fliers when released amongst

non-host plants.

“Because females lay their eggs in the

developing fruit on host trees, we suspect

that they are less likely to move away from

host plants. But if they are in a place with

no host fruit, the females will need to move

away to find somewhere to lay their eggs,”

explains Weldon.

Interestingly, the researchers also found

that females would not move away from

host plants even when these were not in

fruit. There is still some uncertainty about

what causes this behaviour, says Weldon.

“Perhaps the females were using some

information from the tree other than host

fruit to decide whether to stay put, or

perhaps it relates to the environment that

those particular trees provide.”

Temperature and rainfallAccording to Weldon, South Africa has

favourable environmental conditions for

Oriental fruit � ies to disperse, especially

in terms of temperature. His team found

that the frequency and duration of � ight

increases in the optimal range of 24°C to

32°C. When temperatures drop below 20°C,

� ies become relatively inactive, while very

hot conditions — 36°C and above — increase

duration of resting.

“The temperature [in South Africa] is

only below the threshold of 20°C in the

evening or in the early morning. After that,

during the most of the day, the temperature

conditions for flies are great, unless it gets

too hot,” says Weldon.

The impact of rainfall is still uncertain.

Although there were no clear effects on

dispersal, increased rainfall did result

in more female recaptures. One possible

explanation is that rainfall increases

humidity and females prefer humid

environments, becoming more active and

more likely to be caught. In contrast, as

rainfall increased, fewer males were caught.

Weldon says that they don’t yet fully

understand these behaviours, but are

looking into them. The first step was

studying dispersal patterns, while further

research will examine the underlying causes.

“Most control tactics require a really good

understanding of the biology of the insect.

That is where going into the detail comes

into play,” stresses Weldon. “If you want to

keep the flies out of the Western Cape, then

you need to know what determines how far

they fly.”

South Africa has favourable environmental conditions for Oriental fruit flies to disperse


QF CROP PROTECTION

The Oriental fruit fl y Bactrocera dorsalis.


browning is apost-harvestdisorderinwhichthefleshof

applesturnsbrownwhileremainingfirm.

Thetwomaintypesarediffusebrowning

andradialbrowning.Thesecanoccurin

combination—thisconditioniscreatively

calledcombinationbrowning.

Internalbrowningisamongthemost

importantcausesoflossesduringthe

post-harvestperiod,accordingtoDrElke

Crouch,apost-harvestphysiologistwith

theDepartmentofHorticulturalSciencesat

StellenboschUniversity.But,shesays,it’s

veryimportanttodistinguishwhichtypeof

browningaffectsyourfruit.

Diffuse browning: all about maturityDiffusebrowningisaprobleminfruitthat

washarvestedatpost-optimalmaturity

—characterisedbystarchbreakdownof

40%–50%orgreater.Preharvestfactorsthat

speedupripeningincreasetheriskofdiffuse

browningbecausefruitaremorelikelyto

beharvestedwhenbeyondtheiroptimal

maturity.“Ifyouseeyourorchardismoving

fast,thechancesofthatorchardstoringwell

arenotgood,”cautionsCrouch.

Preharvestfactorsthatareassociatedwith

agreaterriskofdiffusebrowninginclude

youngertreesandorchardsonsandysoils.

Croploadalsoplaysaroleinthatlargerfruit

ripenfasterandstorelesswell.

Crouchexplainsthetensionbetween

optimalmaturityandcolourdevelopment.

Bettercolourtranslatesintohighervalue—

butbetterstorabilityenablesgrowerstosell

atamorefavourableprice.“Peoplewantto

storelong-termandtheywantredfruit.Not

allfruitwillbeabletomakethatgrade.”

What have we learnt over the past ten years? By Anna Mouton

Inte

rnal

bro

wni

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in C

ripp

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k internal


QF poSt-HARvESt

Diffusefbrowningfaffectsfthefcortexf whilefthefvascularftissuefisfunaffected.

Radialfbr

ownin

gfaffectsft

hefvas

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Combinationfbrowningfaffectsfb

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Ian

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ExpE

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o

Diffuse browning

can develop as early

as three months into

the storage period.

Fruit that is left longer

on a tree so that it can

colour — typically those on

the inside of the canopy — has

a greater risk of browning and Crouch

doesn’t recommend storing these fruit for

more than three months.

Radial browning: a seasonal problemRadial browning seems most common

when fruit are exposed to cool weather

during the cell enlargement phase. This

may lead to denser fruit which could slow

the escape of carbon dioxide — high tissue

carbon dioxide levels are damaging to

cells. Maturity may also impact

gas exchange but is not a

determinant of radial

browning to the same

extent as it is for

diffuse browning.

Tree age and soil

type do not affect

the occurrence of

radial browning.

Radial browning was

thought to develop early

in storage and packhouses

use hyperspectral sorting

to eliminate brown fruit prior to

shipping. “But you risk that fruit goes brown

on the water,” says Crouch. Her team’s data

showed that radial browning can become

worse during shipping and shelf life. This is

especially true for high-risk seasons and after

prolonged storage.

Best practices for storageThe stage at which you harvest is critical

and fruit that has passed optimal maturity

is likely to develop internal browning. “The

bulk of your fruit will have to be sold before

six months,” states Crouch. “If you want to

risk storing fruit, which fruit are you

going to choose for that?” She

advises growers to measure

starch breakdown to

inform their decision.

Step-down cooling

has been effective

in reducing the

occurrence of diffuse

browning. New

research confirms

this and shows that

step-down cooling is also

essential for the control of

radial browning. Crouch thinks that

newer cultivars may be more sensitive to

chilling injuries. “All the new protocols have

very long step-down cooling,” she observes,

“not like the instant we used to have.”

Internal browning in Cripps Pink is well-

understood, according to Crouch. “We know

that if you harvest them over-mature and

store them at really low temperatures you

will get diffuse browning,” she summarises.

“We know that in some seasons we get

radial browning and in some seasons we

don’t and generally the cooler seasons your

risk might be higher.”

Browning in other cultivars is not always

as simple. “In other cultivars, like Fuji,

we’re still trying to figure all the browning

patterns out,” says Crouch. “One needs

research to make a storage recipe for each

specific fruit and each type of browning.”

24 FQ ISSUE 08 MARCH 202024 FQ ISSUE 08 MARCH 2020

“Internal browning touches

people’s pockets,” says Dr Elke Crouch, a post-

harvest physiologist with the Department

of Horticultural Sciences at Stellenbosch

University. “Particularly in Cripps Pink, because

it’s a high value crop.” Crouch is part of a group

investigating the impact of harvest maturity

and storage conditions on internal browning in

Cripps Pink apples. � e team is led by Heleen

Bergman from ExperiCo.

The researchers sampled apples from five

farms each in Ceres and Grabouw. They

collected fruit during three harvest seasons

pinkKe e p yo u r a p p l e s i n t h e

Strategies to mitigate the risk of internal browning

By Anna Mouton


QF POST-HARVEST

Five diff erent storage protocols were tested. Treatment 2 delivered the fewest defects and most consistent results.

from 2017 to 2019. The apples were

evaluated for maturity using a range of

measurements including firmness, colour,

total acidity, total soluble solids, and starch

breakdown. The aim was to harvest at

optimum maturity which is characterised by

starch breakdown of 30%–40%.

Apples were harvested in April and kept

in controlled atmosphere storage. Five

treatments were used — see table below.

Fruit were then subjected to a simulated

shipping period under regular atmosphere

for six weeks followed by a ten-day shelf life.

“The first season we stored for about

nine months and we didn’t get any internal

browning,” says Crouch. “This season we

realised, if you have a high-risk browning

year, no matter what you do, don’t push

it.” Stored fruit showed a high prevalence

of internal browning and superficial

scald leading Crouch to caution against

attempting to store Cripps Pink apples for

as long as nine months.

Don’t store up troubleApples can su� er from di� use browning, radial

browning, or a combination of these. Di� use

browning has historically been more common

in South Africa — but radial browning caused

signi� cant losses last season.

“Diffuse browning is managed by having

the right harvest maturity,” says Crouch.

“If you harvest anything after 40%–50%

starch breakdown, that’s high risk and you

shouldn’t store longer than four months.”

This can be difficult in practice especially in

trees with a large canopy.

“One should harvest the inside and

outside fruit separately, because we know

their starch breakdown levels are completely

different,” says Crouch. “Unfortunately the

fruit that hangs longest is also the ripest.”

Crouch recommends measuring starch

breakdown in apples from the inside of the

canopy to avert long-term storage of fruit

with starch breakdown of greater than 50%.

Radial browning is highly seasonal

because it is strongly influenced by pre-

harvest factors. Radial browning is more

common in years with lower temperatures

during the period in spring when fruit

undergo early cell division and expansion.

Harvest maturity is not a predictor of

radial browning. Crouch notes that their most

recent data indicated that fruit harvested

early were more susceptible to radial

browning than fruit harvested later. More

research is needed to clarify the relationship.

The risk of both radial and diffuse

browning is reduced by using step-down

cooling. “The message here is, don’t store

at a single temperature of 1°C. When we

did the long-term step-down cooling, the

firmness was better, the quality of the fruit

was better — step-down cooling is a definite

must,” emphasises Crouch.

Step down the browningResults so far suggest that the best protocol

for step-down cooling starts at 3°C for 30

days, followed by 2°C for 30 days, followed

by 1°C for the remainder of storage. Crouch

points out that any step-down cooling

treatment is better than taking fruit directly

to 1°C.

All the apples undergoing step-down

cooling as well as one group stored

directly at 1°C were treated with 1-MCP

— 1-methylcyclopropene (marketed as

SmartFresh) — within seven days of

storage. The application of 1-MCP to

maintain fruit quality is a standard industry

practice. A control group was stored directly

at 1°C and not treated with 1-MCP.

Both groups stored at 1°C had high levels

of internal browning. Treatment with 1-MCP

reduced di� use browning — as expected.

Di� use browning is related to increased

maturity and 1-MCP retards maturity.

This research was conducted by a team

from the Department of Horticultural

Sciences at the University of Stellenbosch

in collaboration with ExperiCo. The work is

funded by Hortgro.

TreatmentInitial 1-MCP application

Storage temperature

0‒30 days 31‒60 days 61‒90 days SeptemberJanuary end of trial

1 + 4°C 3°C 2°C 1°C 1°C

2 + 3°C 2°C 1°C 1°C 1°C

3 + 0‒14 days 2°C 15‒30 days 1°C 31+ days 0.5°C 1°C 1°C

4 + 1°C 1°C 1°C 1°C 1°C

5 - 1°C 1°C 1°C 1°C 1°C


post-harvest practicesC r i p p s P i n k a p p l e s

Compiled by Store‐it Group for Hortgro Science

Bestfor

What is the correct harvest maturity for long-term storage?◆ Starch breakdown <40%. Optimum is 20%‒

30%. Can be released at 15% depending on

other parameters.

◆ Firmness >7.8 kg when measured with an

11.1 mm tip.

◆ Total soluble solids should be >12.5%.

◆ Titratable acidity 0.75%‒0.55%.

◆ Maturity is the biggest risk factor for

development of diff use browning after

storage. Post-optimum harvested fruit

should never be stored long-term.

Dealing with variability◆ Rank orchard maturity four weeks before

harvest and assess pre-delivery samples to

determine ripening rate and storability.

◆ Market fruit from orchards with faster

ripening rates fi rst and do not store in CA.

◆ Determine inside- and outside-canopy fruit

maturity. The outside, well-coloured fruit

will be harvested fi rst due to block colour

standards of 40‒60%. The inside pale fruit

will be left longer to colour up.

◆ Inside-canopy fruit may be riper even at

the fi rst pick and should not be stored in

CA for longer than 12 weeks.

◆ Consider leaf-stripping after fi rst pick

when sunburn risk has diminished. Rather

consider refl ective mulching should

sunburn be a risk.

◆ The second pick may be put into CA for

long-term storage depending on the

starch breakdown.

◆ Avoid a long picking window.

◆ Do not CA-store fruit with starch

breakdown >50% for longer than 12 weeks.

It may be acceptable for RA and RA +

1-MCP and short-term CA storage.

Orchard history e� ects on long-term storage◆ Do not store fruit from young trees for

longer than 12 weeks.

◆ Light crop-load fruit may be more

susceptible to earlier ripening

and disorders.

◆ Under- or over-irrigation may result in

faster ripening rates and poor storability.

◆ Fruit from orchards on sandy soils tend to

ripen faster.

◆ Mineral nutrition is important in ripening

rate and storage quality.

◆ Consider the history of progressive defects

and ripening rates of each orchard.

◆ Topworked trees may have a higher internal

browning risk due to increased stress.

◆ Fruit that received any type of stress during

the season should be sold within 6 weeks.

Post-harvest and storage handling◆ Harvested bins should not remain in the sun.

Accumulate in a shaded area and transport

to the packhouse as soon as possible.

◆ At the packhouse fruit should be loaded

into the cold room as quickly as possible.

Fruit should not be accumulated outside

the packhouse in the shade or overnight to

facilitate initial cooling.

◆ Be aware that cold-room temperature can

fl uctuate excessively while adding warm

fruit. Separate warm and cooled fruit with

curtains in the cold room to help avoid this.

◆ Make sure that the cold room has

suffi cient cooling capacity for the volume

of warm fruit.

◆ Load two or three rooms simultaneously to

get even cooling rates.

◆ Attain pulp temperature of 4 °C after 48 hours.

Fruit should further be cooled stepwise.

◆ CO2 levels should be monitored when

warm fruit are closed in a cold room.

◆ CO2 levels should never reach levels >1%.

Use additional lime or new generation

scrubbers to reduce CO2 to 0.5%.

◆ Fruit will benefi t from ethylene inhibitors

1-MCP to reduce ripening rate,

27 ISSUE 08 MARCH 2020 FQ 27 ISSUE 08 MARCH 2020 FQ

QF POST-HARVEST

greasiness, superfi cial scald and internal

browning. Treat within 7 days of harvest

especially if stored for up to 3 months in RA

or >3 months in CA.

◆ Diff use and radial browning is reduced when

using step-down cooling rather than single-

storage-temperature regimes of 1°C for long-

term >7 months storage in CA.

Industry guidelines for CA O2 and CO2 gas regimes ARC/HORTGRO◆ Nitrogen fl ushing: attain gas regime of 3%

O2 + 1.5% CO2 within 48 hours of sealing

the room.

◆ Optimum O2 level is 1.5%. Minimum 1%.

Maximum 2%.

◆ Optimum CO2 level is 0.5%. Minimum 0%.

Maximum 1%.

DCA guidelinesDCA- RQ / ACR SYSTEM (VAN AMERONGEN) | (PACKHOUSE EXPERIENCE / VA)◆ Room fi lling 5-7 days. Keep CO2 <1%.

◆ Room full 3 days. Fruit on temperature.

Keep CO2 <1%.

◆ Pull down O2 to max 6% 2 days. Keep

CO2 <1%.

◆ O2 reduction to 1.2% 7 days. Keep CO2 <1%.

◆ O2 1.2% 14 days. CO2 <1%. Total 28 days.

◆ ACR mode from 28 days onwards. O2

>0.6%. CO2 <1%.

◆ Keep O2 between 0.5%‒0.7% depending on

the season.

◆ Always keep CO₂ as close to 0.5% as possible.

◆ It is a good idea to monitor the fruit from

DCA for ethanol levels.

◆ Please contact your service provider for

their specifi c guidelines.

DCA-CF SYSTEM (ISOLCEL / GAS AT SITE) (ARC GUIDELINE)◆ Load the room and cool to core

temperature within 48 hours of harvest.

◆ Allow 5 days for loading the room.

◆ Then seal and pull down to 3% O2 + 1.5%

CO2 over 2 days.

◆ For the next 2 days pull down to 1.5% O2 +

1% CO2.

◆ Pull down to lower O2 limit + <1% CO2

within the next 2 days.

◆ Therefore, the total time from harvest to

lower O2 limit is 11 days.

◆ After spiking, adjust to 0.2% O2 above

spiking and keep CO2 <1% during storage.

Monitor for spiking during storage period.

◆ Please contact your service provider for

their specifi c guidelines.

Internal browning considerations◆ Fruit maturity starch breakdown is

linked to development of diff use browning

after storage and ripening.

1. Starch breakdown of 15%‒40% and

storage up to three months: lower risk of

diff use browning.

2. Starch breakdown of 15%‒40% and >3

months’ storage: risk of diff use browning

depends on other factors.

3. Starch breakdown of 40%-50% and

storage up to three months: medium risk of

diff use browning.

4. Starch breakdown of 40%-50% and

>3 months’ storage: higher risk of

diff use browning.

5. Starch breakdown more than 50%:

high risk of diff use browning within three

months storage.

◆ Low storage temperature is linked to

the development of diff use and radial

browning. Storage temperatures

of minus 0.5 °C up to 1 °C have

previously been linked to diff use

browning after only 3 months

of storage.

◆ The step-down cooling treatment

protocols described below

performed better during the past

season in terms of diff use and radial

browning for long-term CA storage

>7 months compared to single-storage-

temperature regimes at 1°C. Cooling

regime number 2 seemed to perform the

best in terms of fruit quality. But note that

any stepwise cooling is better than none

at all.

1. 1-MCP within 7 days + 30 days at 4 °C, 30

days at 3 °C, 30 days at 2 °C, remaining days

at 1 °C.

2. 1-MCP within 7 days + 30 days at 3 °C, 30

days at 2 °C, remaining days at 1 °C.

3. 1-MCP within 7 days + storage at

commercial CA facility: 2 °C for 14 days +

1 °C for 14 days + remainder at 0.5 °C until

room opened. Transferred to US-CA-storage

from September 2018 and stored onwards

at 1.0 °C with 1.5% O2 and 0.5% CO2.

◆ Pre-harvest factors that aff ect maturity

see above have been linked to diff use

browning.

◆ Cripps Pink is CO2 sensitive and readily

shows CO2 browning in cases where CO2

increased above 1%.

◆ Non-perforated bags could lead to

the build-up of CO2, especially with

fl uctuating temperatures during shipping

and distribution.

◆ Consider having shipping container vents

15% open due to CO2 sensitivity and risk of

temperature fl uctuations during the voyage.


QF POST HARVEST

Completed projects2010. Lötze E. Modelling internal browning of

Pink Lady/Cripps Pink apples.

2011. Crouch IJ and Crouch EM. Factors

infl uencing fl esh browning in Cripps Pink apples.

2012. Bergman H, Crouch IJ, Crouch EM and

Majoni TJ. The eff ect of rate of cooling and

storage temperature on the storage potential of

Cripps Pink apples cold stored under controlled

atmosphere conditions.

2015. Crouch EM, Bergman H, Crouch IJ and

Butler L. The relation between Cripps Pink

internal browning and preharvest temperatures,

mineral nutrition, tree age, soil type etc. in two

production areas after long term CA storage

and the evaluation of the non-destructive NIR

techniques for sorting internal, brown fruit.

2016. Tayler H. Physiological profi ling on Rosy

Glow apples harvested at diff erent maturities,

with special reference to internal browning

development potential.

2018. Crouch EM. Harvest and storage condition

plus duration infl uencing internal browning and

fruit quality of Rosy Glow.

Current projectsTayler H, Viljoen D and Crouch EM. To investigate

diff erent step-down cooling regimes, utilised in

conjunction with SmartFresh, to reduce the risk

of internal browning of Cripps Pink apples and

provide year-round supply of good quality fruit.

PublicationsCrouch EM, Bergman H, Majoni TJ, Jooste M

and Crouch IJ. 2012. Factors infl uencing fl esh

browning in ‘Cripp’s Pink’ apples. 2012 CIGR

Section VI International Technical Symposium on

“Innovating the Food Value Chain” Postharvest

Technology and Agri-Food Processing.

Stellenbosch, South Africa.

RESEARCH INVENTORYA list of research projects and publications related to internal browning in Cripps Pink and Rosy Glow apples funded by

Hortgro Pome.

Crouch EM, Jooste M,

Bergman H, Crouch IJ and

Majoni TJ. 2013. Post-harvest

factors infl uencing fl esh browning in

South African `Cripps Pink` apples. Post-

Harvest Innovation Programme: Symposium

2013. Spier Conference centre, Stellenbosch,

South Africa.

Crouch EM, Jooste M, Majoni TJ, Crouch IJ

and Bergman H. 2014. Harvest maturity and

storage duration infl uencing fl esh browning

in South African ‘Cripps’ Pink’ apples. Fifth

Postharvest Unlimited ISHS International

Conference, Aphrodite Hills, Cyprus. Acta Hortic.

2015(1079):121‒127.

Doe JW, Schoeman L and Crouch EM. 2018.

Eff ect of harvest maturity, storage condition and

duration, on internal fl esh browning and quality

of ‘Rosy Glow’ apples grown in South Africa. Acta

Hortic. 2018(1201):41‒48.

Majoni TJ, Jooste M and Crouch EM. 2013.

The eff ect of 1-MCP and storage duration

on the storage potential and fl esh browning

development on ‘Cripps’ Pink’ apples stored

under controlled atmosphere conditions. Acta.

Hortic. 2013(1007):49‒56.

Crouch EM, Jooste M,

Bergman H, Crouch IJ and

Majoni TJ. 2013. Post-harvest

factors infl uencing fl esh browning in

South African `Cripps Pink` apples. Post-

Harvest Innovation Programme: Symposium

2013. Spier Conference centre, Stellenbosch,

29

QF COLUMN

Grethe Bestbier

ISSUE 08 MARCH 2020 FQ

A citrus perspective onOriental Fruit Fly

except for placing more traps around,” she

says. “When we talk about incursions per se,

it is more than one fly in an area and this

triggers ‘eradication actions’.”

However, Manrakhan warns, if Oriental fruit

� y incursions are not eradicated in the Western

Cape and if conditions are suitable, the Western

Cape could have the same experience as the

North: � ies all around, constantly being found

in traps, indicating an established pest.

How fond of citrus are Oriental fruit

flies? Compared to, for example, mangoes

grown in the North or deciduous fruit grown

in the Western Cape, citrus is not their

preferred host. It presents certain barriers to

infestation, such as the peel’s toxicity to eggs

and developing larvae.

A study done in 2016–2017 looked at the

infestation of sweet oranges in the northern

Limpopo and Mpumalanga Provinces. Rates

of Oriental fruit � y infestation were low. � e

researchers also found that damaged or vulnerable

fruit, such as those already infested or dropped

from trees, were more susceptible to infestation.

Two top-notch techniques for control“� e pest is actually fairly easy to control,

because of two techniques that are available,

with one using a speci� c male lure,” says

Manrakhan. “With these two methods, we target

males and females to bring populations down.”

Where the pest is present, growers are

advised to apply bait sprays that target

predominantly females, in combination with

male annihilation technique. The latter uses

blocks of wood impregnated with a lure and a

pesticide that attracts and kills males. These

two techniques, together with good orchard

sanitation, can reduce fly catches to as low as

two flies per trap per week.

“We’ve tested these techniques in

some citrus plantations for two years, and

we consistently found that you can keep

to the threshold that has been established

by DALRRD [Department of Agriculture,

Land Reform and Rural Development],” says

Manrakhan. “Below three flies per trap per

week is acceptable, and it is possible to do that

with the combination of these techniques plus

orchard sanitation.”

Citrus grows abundantly in the Western

Cape and international markets are

increasingly intolerant of Oriental fruit

fly. Hence the pressure to keep these pests

out. According to Manrakhan, alertness has

never been so important. “Now is the right

time to take action. There are high chances

for incursions in the Western Cape, so we

need to increase vigilance. Traps need to be

continuously checked. If a fly is detected, it

should be reported immediately and actions

should be rolled out fast. We need to make

sure our fruit is pest-free.”

What can the Western Cape learn?Up to now the Western Cape has dodged the

Oriental fruit � y bullet, but other parts of South

Africa have not been so fortunate. According to

Dr Aruna Manrakhan, research entomologist

and Fruit Fly Programme Coordinator at Citrus

Research International, Oriental fruit � ies

are becoming increasingly established in the

northern and northeastern parts of the country.

What does a single � y tell us?� e Oriental fruit � y � rst invaded South Africa in

2013. Since then it has established in Limpopo,

Mpumalanga, North-West and Gauteng Provinces

as well as parts of KwaZulu-Natal.

Manrakhan has been studying the pest for

several years. The Oriental fruit fly population

is growing in the north and a higher

population poses a threat to other areas of the

country. “There is a lot of pressure on other

areas, ” says Manrakhan.

It is important to distinguish between

incursions versus establishment of flies.

Manrakhan explains that a single detection

of a male only indicates that it is necessary to

determine whether a population is present.

“One fly is considered ‘no control action’,

Fungi that cause wood-rot and cankers can

infect young trees leading to die-back and

death. Anecdotal accounts suggest that die-

back in new orchards is increasing. Could this

be related to chipping old orchards for mulch?

No one knows. We also don’t know if harmful

fungi can be spread by other forms of mulch

such as pine chips or straw.

A project led by Prof. Lizel Mostert from the

Department of Plant Pathology at Stellenbosch

University seeks to answer these questions.

The study will identify the fungi associated

with wood rots and cankers and develop better

testing methods for them. These new testing

methods will be used to study the occurrence

and spread of harmful fungi in the orchard.

“We also need to determine the e� ect of using

straw or pine chips on the orchard � oor and soil

ecology,” says Addison. “Will we import pathogens?

Chipping and mulching with apple wood is by far

the highest risk but there are others.”

Counting on collembolaCollembola — also called springtails — are

minuscule insect-like creatures that mostly live

on detritus. Every grower should care about

collembola because these tiny organisms help

to keep soils healthy and productive and are

important food for spiders and predatory mites.

� ere are more than a hundred known species

of collembola in South Africa and research

has shown their potential as indicators of

biodiversity and soil health.

A new project will gather data on collembola

in orchards and vineyards subject to di� erent

interventions. � ese include four types of cover

crop as well as no cover crop. Plantings with

and without shade net will also be compared.

Dr Charlene Janion-Scheepers, an expert on

collembola at the University of Cape Town, is

leading the study.

“It feeds into, what are we doing on our orchard

� oor? How important is it?” says Addison. He

believes that a healthy litter layer � lled with

collembola contributes carbon to the soil and

provides shelter and food for bene� cial organisms

such as predatory mites. � e complex interactions

between these mini-beasts and cover crops can

ultimately bring about a more stable orchard

ecosystem with fewer pest outbreaks.

Mites make rightMites live everywhere from your pillow to your

eyelashes to your orchard — there are nearly

50 000 species described globally. A project led

by Prof. John Terblanche of the Department

of Conservation Ecology and Entomology at

Stellenbosch University will examine how pest and

predatory mites � t into the orchard ecosystem.

� e researchers will use a combination of

approaches. Mites will be tested in the laboratory

to see which prey they prefer. Mites and potential

prey species — such as collembola — will also

be sampled from orchards and their body tissues

examined using a technique called stable isotope

analysis. � is will provide information about

diet across seasons and from di� erent orchards.

Understanding who eats who on the orchard � oor

is critical to maintaining a healthy orchard.

“Is the grass litter important or is it broad-

leaved plants? Are the predatory mites eating

collembola?” wonders Addison. Predatory mites

usually do a good job of keeping the pest species

under control. � e question is how to boost the

predators without amplifying the plant-eaters.

Biological control is set to become ever more

important if growers want to access high-value

markets. These new crop protection projects

are another step in helping the industry reduce

its dependency on chemicals while enhancing

sustainability and resilience.

� ere’s an old saying that nature abhors a

vacuum and this certainly applies to orchards.

Establish a block of fruit trees and other

life follows: plants of one kind or another

colonise bare soil. Insects — some of them

pests — take up residence. Bacteria and fungi

grow everywhere. What was intended as a

monoculture soon becomes an ecosystem.

How should growers view this proliferation

of species in their orchards? � e traditional

response was to take up arms — mostly

chemical — to protect tree health. But, argues

Matthew Addison, programme manager for crop

protection at Hortgro Science, it’s better to have

a diverse orchard with minor sporadic problems

than no diversity.

“With no diversity you have chronic problems

that cost you a lot of money,” says Addison. He

explains how three new research projects aim to

better understand how orchard ecology interacts

with soil condition and tree health.

Fungal communities and cankerChipping an old orchard to mulch a new one might

seem like a great idea. But is it a disease risk? “I

think it is,” says Addison. “I’m worried that you

bring in wood-rot fungi that’s been sprayed for

forty years with fungicide and it then establishes.”

Back to earth

Collembola are tiny but vital for soil health.

ANDY M

URR

AY | A

CHAOS OF DEL

IGHT

New crop protection projects focus on orchard ecology

QF COLUMN

Anna Mouton

important food for spiders and predatory mites.

� ere are more than a hundred known species

of collembola in South Africa and research

has shown their potential as indicators of

Collembola mostly go unnoticed

because of their sm

all size.


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fresh march issue eight 2020 quarterly...a post-harvest disorder and fruit that has passed optimal...

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