Solarisation controls soilborne fungal pathogens in nursery pottingmixesJ.D. DuffA and A. BarnaartB

APlant Pathology Section, Department of Primary Industry and Fisheries, GPO Box 990, Darwin, NorthernTerritory 0801Blnstitute of TAFE, Department of Horticulture, Northern Territory University, PO Box 40146, Casuarina, NorthernTerritory 0811

Abstract

Solarisation of a 30 cm high mound (2.3 x 1.1 m) of pot­ting mediuml(local fine peat, sand, composted pine bark= 1:1:1), using either single- or double-layer, clear, poly­ethylene mulch raised temperatures by as much as 14.6°Cand 17,2°C, respectively at 5 cm when compared to anuncovered mound. Solarisation of a 25 em mound, with adouble layer of clear polyethylene mulch, killed pathogens(Pythium myriotylum, Phytophthora nicotianae var. tiicoti­anae and Sclerotium rolfsit), placed at 10 cm within 7,3 and7 days and at 25 cm within 7, 7 and 10 days, respectively.

Introduction

To ensure consistently high quality, disease-freeplants, nursery potting mixes must be treated to con­trol soilborne pests and diseases. Such soil treat­ments usually involve the use of pasteurisation byaerated-steam or fumigation by methyl bromide ordazomet (Goss 1979). Because there are large capi­tal costs incurred when using steam, and possiblerisks to the operator and environment when usingfumigants, soil solarisation has emerged as a viablealternative for managing soilborne pests anddiseases.

Solarisation has been extensively studied since1976 when Katan et et. (1976) investigated itsbenefits on field soils in Israel. Since then severalreports have shown that solarisation has been par­tially successful for controlling soilborne diseasesin nurseries (Old 1981; Kassaby 1985; Barbercheckand von Broembsen 1986; Kaewruang et al. 1989;Katan, personal communication).

In nurseries in Darwin, three important root dis­eases of Spathiphyllum, Petunia, Bougainvillea,Caladium, Araucaria and Eucalyptus species arecaused by Pythium myriotylum Drechsl., Phytoph­thora ntcottenee B. de Haan var. nicotianae andSclerotium rolfsii Sacco These diseases have beencontrolled elsewhere using soil solarisation (Katanet al. 1976; Katan 1980; Stapleton and DeVay 1986;Old 1981; Kassaby 1985; Barbercheck and vonBroembsen 1986; Pullman et al. 1981).

Solarisatlon is best carried out during periods ofhigh temperatures and intense solar radiation (Katan1981). These two factors are readily available dur­ing the spring/summer and early autumn in Darwin.Our preliminary results using the methods of Ray­mundo and Alcazar (1986) and Ben-Ysphet et at.(1987) indicated that required temperatures can beachieved by using clear polyethylene mulch. Thus,

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two experiments were set up to study the efficacyof solarisation for controlling the above mentionedsoilborne pathogens in a nursery potting mix.

Methods

Two unreplicated experiments were conducted atthe Berrimah Agricultural Research Centre, Darwin,during December 1989 and March 1990. Potting mix(peat, sand and composted pine bark = 1:1:1) wasmoistened to 25-30% water holding capacity andthen mounded either to 30 cm or 25 cm high on nur­sery trolleys (2.3 x 1.1 m). Each mound was insul­ated from the metal tray of the trolley by a 40 mmlayer of polystyrene foam.

The mounds of potting mix were covered witheither a single or double layer of clear polyethylenemulch (CPM, Visqueen, ICI, Australia, 50 J.tm thick­ness) or left uncovered. The double layer of plasticwas separated by a 15 cm air space between eachlayer. A pathogen control experiment using 25 cmhigh mounds of potting mix, was either covered witha double layer of CPM or left uncovered. In bothexperiments the uncovered mounds were wateredon a daily basis.

Temperature profiles Temperature readings forthe temperature profile experiment were taken overa period of 11 days (6-17 December, 1989) and wererecorded with stainless steel 12.5 cm long HS-U typethermister probes. These were placed at 5 cm inter­vals from 5 cm .to 30 cm depth in the moundscovered with CPM and from 5 cm to 20 cm depth inthe uncovered mound. The pathogen control experi­ment ran for 13 days (6-19 March, 1990). One probewas placed at 5 em, and two at 25 em, in both themound covered with CPM and the uncoveredmound. The thermister probes were attached toGrant Squirrel Data Loggers which recorded tem­peratures every 30 min. Ambient temperatures wereobtained from the Bureau of Meteorology Station atDarwin Airport.

Preparation of inoculum Pathogens used in thisstudy were P. myrioty/um and P. nicotianae var, nlco­tienee isolated from the roots of a severely stuntedBougainvillea sp. and S. rottsll which was isolatedfrom a basal rot on a Caladium sp.

Australasian Plant Pathology Vol. 21 (1) 1992

Healthy washed root pieces (10-20 mm inlength x 1-2 mm diameter) of Bougainvillea sp., wereplaced in two 250 mL flasks to a depth of 1-2 cm(with no additional water) and autoclaved at 120°Cfor 30 min (Old 1981).Each flask was inoculated withfive discs bearing mycelia from 10-day-old culturesgrown on corn-meal agar (CMA) of either P. myrio­ty/um or P. nicotianae var. nicotianae. The flaskswere incubated at 27.5°C for 2 weeks. To test thatthe root pieces were successfully colonised, 50 rootpieces from each flask were plated onto CMA.Sclerotes of S. rolfsii were harvested from 4-week­old cultures grown on potato-dextrose agar (PDA).Fifty sclerotes were plated on PDA to test theirviability.

Effect of solarisation on pathogen sur­vival Either ten root pieces colonised by P. myrio­ty/um or P. nicotianae var. nicotianae or ten sclerotesof S. rolfsiiwere placed in fine polyester/cotton bags(5 cm square) and closed with a paper clip. Theywere kept moist and cool for 12-20 h prior to bury­ing in the potting mix (Old 1981).

Three bags, each containing a different pathogenwere buried at each of 32 sites at both 10 cm and25 cm depths. The sites at each depth werearranged in four rows with eight sites per row. After

3, 7, 10 and 14 days, one site was chosen randomlyfrom one of the eight sites in each of the four rows.The three bags were recovered from each of the foursites at each time of sampling. Removals were madeearly in the morning before the potting mix startedto heat up. The root pieces were surface sterilisedin 70% ethanol for 1 min and then rinsed in steriledistilled water and blotted dry. Five root pieces fromeach bag were randomly chosen and placed ontoCMA plates. All ten sclerotes were plated directlyonto PDA. All plates were examined daily for up to1 week for the presence of the pathogens. Samplingcontinued until the viability was lost for all threepathogens at both depths in the solarised moundof potting mix.

Results

Temperature profiles Temperature details for theDecember experiments are given in Table 1. In allthree treatments there was a gradual reduction intemperature with increasing depth. The differentialbetween the single and double layer CPM for themean maximum temperature ranged from 2.6°C at5 cm down to 0.8°C at 30 cm. The differentialbetween these treatments was usually slightly larger

Table 1 Maximum, mean maximum, mean minimum and minimum temperatures recorded at various depthsin nursery potting mix uncovered or covered with clear plastic mulch at Darwin

Date Treatment Depth (cm) Max Mean max Mean min Min

6-17 Dec, 1989 Air temperature 34.8 33.8 25.8 22.2Control 5 42.2 37.3 28.6 26.6Single CPM 5 57.5 51.9 33.8 31.6Double CPM 5 62.2 54.5 37.8 33.0Control 10 38.2 34.7 39.8 27.6Single CPM 10 52.1 47.6 35.2 32.7Double CPM 10 56.1 49.9 37.8 33.8Control 15 37.3 34.0 30.8 28.4Single CPM 15 47.4 44.1 36.6 34.8Double CPM 15 51.0 45.9 37.4 34.1Control 20 36.2 33.4 31.3 28.6Single CPM 20 44.9 41.7 37.0 35.9Double CPM 20 47.8 43.2 38.6 34.5Control 25Single CPM 25 44.2 41.2 37.4 36.3Double CPM 25 46.7 42.2 38.8 34.8Control 30Single CPM 30 44.2 41.3 37.4 36.3Double CPM 30 46.7 42.1 39.3 34.5

6-19 March, 1990 Air temperature 33.6 31.5 25.2 21.8Control 5 42.2 36.8 27.7 24.4Double CPM 5 61.1A 58.9A 38.6A 37.0A

Control 25 37.0 32.9 30.0 27.6Double CPM 25 49.2A 47.2A 41.2A 38.8A

ATemperature readings are average values recorded for 13 to 19 March due to equipment failure.

Australasian Plant Pathology Vol. 21 (1) 1992 21

with the mean minimum temperatures, 3.0°C at5 cm and 1.9°C at 30 cm (Table 1). Both CPM treat­ments had higher temperatures than the control. Thedifferential between the double layer of CPM andthe control ranged from 17.2°C at 5 cm to 9.8°C at20 cm and 8.2°C at 5 cm to 7.3°C at 20 cm for meanmaximum and mean minimum temperatures, res­pectively. The differential between the single layerof CPM and the control ranged from 14.6QC at 5 cmto 8.3QC at 20 em and 5.2°C at 5 cm and 5.7°C at20 cm for mean maximum and mean minimum tem­peratures, respectively (Table 1).

Temperatures under the double layer of CPM inMarch were not recorded during the first week ofthis experiment due to a recorder malfunction. Theweather during this week was overcast and rainy,while the second week was clear and sunny, result­ing in high mean temperatures (Table 1). Thetemperatures in the control treatment in both experi­ments were similar, but the March temperatures inthe double layer of CPM tended to be higher thanthose recorded in the December experiment at both5 cm and 25 em depth.

In the March experiment the period above 45°Cin the covered mound at 5 cm and 25 cm rangedfrom 8-14 h (an average of 11 h/day) and 6-13.5 h(an average of 8.5 h/day), respectively. The temper­ature never reached 45QC in the uncovered mound.

Inoculum All 50 root pieces from the flaskscolonised by P. nicotianae var. nicotianae or P.myriotylum gave pure cultures when grown ontoCMA and all colonies from the 50 sclerotia were con­firmed as S. roltsi:

Effect of solarisation on pathogen survival Allthree pathogens were killed within 10 days at both10 and 25 cm depth with the double layer of CPM

(Table 2). After 7 days no viable inoculum of P.myriotylum and P. nicotianae var. nicotianae wasrecovered at either depth whereas viable sclerotiaof S. rolfsii were recovered from the 25 cm depth.An additional 3 days of solarisation with the doublelayer CPM was required to kill all the sclerotes atthe 25 cm depth. The inoculum in the control treat­ment remained highly viable after 10 days, althoughthe survival of P. nicotianae var. nicotianae wasreduced (Table 2).

Discussion

The temperatures obtained with a single layer CPMwere equal to or higher than those previously repor­ted at Frankston and Irymple in Victoria (Porter andMerriman 1985), and other sites in Australia (Old1981; Kassaby 1985; Hardy and Sivasithamparam1985; Kaewruang et a/. 1989). Temperatu resobtained with a double layer CPM were higher thanthose previously recorded in Australia, and similarto those obtained in Israel (Ben-Yephet et a/. 1987).The maximum temperatures obtained during thetemperature profile were, however, 4-5°C higherthan those recorded by Ben-Yephet et a/. (1987) at15 cm and 30 cm depth. The reason for these differ­ences is unknown but may be a result of an addi­tional 9 cm air space between the two layers ofplastic. Soil texture, soil colour, or a combination ofall these differences may have also influenced theseresults.

This study demonstrates that the three soilbornepathogens can be eliminated to 25 cm depth withinthe nursery potting mix following 7 to 10 days solar­isation (Table 2). Previous reports have shown thatPythium and Phytophthora species are particularlysusceptible to high soil tempeatures (Old 1981; Bar­bercheck and von Broembsen 1986; Pullman et al.

Table 2 Effect of solarisation on the survival of three soil pathogens at two depths in a nursery potting mixuncovered or covered with clear plastic mulch

Percentage survival of pathogensPathogens Soil depth Treatment Number of days after solarisation commenced

(ern) 3 7 10 14

Pythium 10 Control 100 90 100 90myriotylum 10 Solarised 5 0 0 a

25 Control 100 100 90 7525 Solarised 75 0 0 0

Phytophthora 10 Control 40 50 60 60nicotisnse 10 Solarised 0 0 0 0var. 25 Control 60 20 35 65nicotianae 25 Solarised 3 0 0 0Sclerotium 10 Control 100 100 90 100rottstt 10 Solarised 100 0 0 0

25 Control 100 100 100 10025 Solarised 100 10 0 0

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1981). In our study both were controlled in a shorterperiod of time than S. rolfsii. The control of S. rolfsiidemonstrated in this study would be due to achiev­ing and maintaining a temperature of 45°C andhigher for periods ranging from 6-13.5 h. Mihail andAlcorn (1984) found that sclerotes of S. rolfsii sur­vived 12 h at 45°C, and 4-6 h at 50°C.

The time required in this study for solarisation tocontrol the above three pathogens is a vast improve­ment on the 4 weeks or longer recommended byKatan (1981), the 14 days to eradicate P. cinnamomiand Pythium spp. in potting mix at 20 cm depth asshown by Kassaby (1985), the 21 days necessaryto control Phytophthora cryptogea Pethybr. and Laff.in plastic bags as demonstrated by Kaewruang etal. (1989), and the 28 days for partial control of S.rolfsii and Pythium irregulare Buisman at 26 cm soildepth as reported by Porter and Merriman (1983).

The reduced recovery of P. nicotianae var. nico­tianae in the control mound at both soil depths indi­cates that factors other than soil temperature wereaffecting its viability. Antagonistic organisms (Tricho­derma and Bacillus species) and volatile gases inthe soil (ethylene, carbon disulphide and carbondioxide) have been implicated in controlling Phytoph­thora spp., Fusarium oxysporum and Rhizoctoniasolani in solarised soil by other workers (Katan 1981;Old 1981; Barbercheck and von Broembsen 1986;Kaewruang et al. 1989). These factors may havecontributed to the loss in viability in our experimentbut determining what factors were involved wasbeyond the scope of this study.

Solarisation therefore appears to be a promisingalternative for the treatment of nursery media inDarwin and other areas of northern Australia thatexperience similar climatic conditions. Further work,however, needs to be carried out to determine ifsolarisation can control the same soil pathogens andothers during the cooler, winter (dry season) months.

Acknowledgements

The authors thank Mrs M. Connelly and Ms L. Ulyattfor their technical assistance.

References

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Manuscriptreceived 13December 1990, accepted8 August1991.

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