citrus root diseases · – causes brown rot of fruit ... • sweet orange • cleopatra mandarin...
TRANSCRIPT
Citrus Root Diseases
Citrus Pathology - Fall 2019
Evan JohnsonPlant PathologistUF-IFAS-CREC
Role of Roots
• Anchor tree– Structural roots
• Uptake of water and nutrients– Fibrous or feeder roots
Structural
Fibrous
Anchoring tree
• Wind
Courtesy of Mongi Zekri
Courtesy of A. Singerman
Fibrous root system
• Gathers water and nutrients
• Short-lived and mines the soil
Pathogens can compromise root health
• Fibrous roots– Limits water uptake capacity
• Structural roots– Poorly anchored– Reduces fibrous roots
Root damage directly or indirectly affects
• Fruit size
• Yield
• Post harvest quality
• Tree survival
Key pathogens, pests, and their interactions affecting citrus roots
• Phytophthora diseases– Phytophthora spp.
• Diaprepes root weevil (DRW)– Diaprepes abbreviates– Diaprepes/Phytopthora complex
• Huanglongbing (HLB)– Candidatus Liberibacter spp.
Phytophthora causes disease in all parts of the tree
• Root Rot
• Foot Rot
• Brown rot of fruit
• Yield reduction estimate = 3-6% per yr
Oomycetes and Fungi are Not Closely Related
Stramenopila/Oomycota
(“Oomycetes”)
Fungi
Differences Between Oomycota & EumycotaFeature Oomycete True Fungi
Neighboring taxonomic groups Diatoms and golden brown algae Animals
Hyphal architecture Aseptate tubular hyphae Either single cell or septated hyphaeOne or more nuclei per compartment
Vegetative hyphae ploidy Diploid except in cells leading to gametes (haploid)
Typically haploid or dikaryotic; often with stable or semi-stable diploid stage post-mating
Typical genome size (Mb) 50-250 10-40
Major components of cell walls Cellulose and glucose polymers Chitin and/or chitosan and glucans
Asexual spore types Undesiccated, unicellular sporangia (multinucleated cells)
Desiccated single or multicellular conidia (one nucleous/cell)
Motile asexual spores Nearly universal biflagellated Rare, only chytrids, and monoflagellated
Sexual spores Oospores on the end of specialized hyphae with one zygotic nucleous
Varied types, often occur in large numbers within complex structures
Modified from Howard S. Judelson & Flavio A. BlancoNature Reviews Microbiology 3:47-58
Causal species of Phytophthora diseases
• Phytophthora nicotianae(parasitica) – common cause of foot rot and root
rot
• Phytophthora palmivora– causes brown rot of fruit – root rot in poorly drained soils
• Phytophthora citrophthora– Causes gummosis in Mediterranean
climates
P. palmivoraSporangia elongated
P. nicotianaeSporangia round
Life cycle of Phytophthora
Phytophthora infection cycle
• During favorable (warm, wet) conditions, zoospores release from sporangia– Swim or splash to root, bark, or
fruit
• Spores encyst, germinate, and infect tissues within 24 hr
Phytophthora root rot
• Damages fibrous (feeder roots)
Healthy Damaged
(Fibrous) Root Rot
• Preferentially infects young growing root tips
• Root cortex soft and water soaked
• Cortex and epidermis slough leaving thread-like tips
• Water and nutrient uptake impaired
• Stored carbohydrates depleted
Young tree canopy symptom of root rot is ‘Yellow vein chlorosis’
• Yellow veins in fall/winter foliage
• Poor growth and dieback of shoots
• Can resemble HLB
Root rot decline is difficult to diagnose in mature trees
• Reduced fruit size and/or number
• Loss of leaves• Twig dieback• Decline in yield is slow but
annual• Confounded with HLB
Foot rot
• Infects bark of roots and trunk
• Kills cambium layer
• Blocks carbohydrate movement to the roots
• Initial canopy symptoms similar to root rot ‘yellow vein chlorosis
Foot rot symptoms
• Bark cracks– Remains firm
• Water soluble gum exuded• Lesions spread around the trunk
or crown below the soil line• Lesions may heal
Terminal symptoms of foot rot
• Leaf drop
• Branch dieback
• Fruit drop
• Death of tree
Girdling of the trunk results in tree collapse
Brown rot of early season fruit
• Light brown leathery decay• White fungal growth on surface
under humid conditions• Infected fruit have sharp,
pungent odor• Infection spreads post-harvest
Hamlin
Grapefruit
Brown Rot affects early season varieties
• Rains coincident with color break• Most fruit loss in older, larger
trees• Fruit cannot be harvested until
infected fruit fall– Post harvest spread
• Hamlin most affected• Navel and grapefruit (shows up in
the packinghouse box)
Periodic rain and prolonged leaf wetness = Brown rot
Date
Rainf
all (i
nche
s)
0
1
2
3
4
June July Aug Sept Nov Dec JanOct
12Brown rot recorded in 1997
8/4/97
10/3/
97
11/12
/97
11/25
/97
12/12
/97
12/29
/976 13 20 27 4 11 18 25 1 8 15 22 29 5 12 19 26 3 10 17 24 7 14 21 28 5 12 19 26 2 9 16 23 30
Date
Leaf
wet
ness
(hou
rs)
0
4
8
12
16
20
24
June July Aug Sept Nov Dec JanOct6 13 20 27 4 11 18 25 1 8 15 22 29 5 12 19 26 3 10 17 24 7 14 21 28 5 12 19 26 2 9 16 23 30
Brown rot control with cultural and fungicide management
• Avoid premature drop of fruit to reduce inoculum– Not possible with HLB
• Raise tree skirts– Increase air movement and drying– Reduce contact with inoculum
• Apply preventative phosphite or copper sprays
• Phosphite sprays in July– October if rainy season is prolonged
Complete Disease Cycle
• Roots most susceptible during very wet to very dry cycles
• Wetting and drying increase root exudation that attracts zoospores
• HLB-affected roots also exude more sucrose that promotes infection
Root and bark age and activity: periods of susceptibility
• Young roots > old• Root flushes more susceptible• Foot rot risk highest in spring and
fall– Carbohydrates moving in trunk
• Fall roots store carbohydrates for next spring flowering
• Phytophthora infection depends on carbohydrate concentration in roots
Soil temperature, rain and root flushes
• Phytophthora is dormant in winter– Soil temp <59oF
• Spring root flush – After shoot flush/fruit set with soil
temp >68oF and rains start• Fall root flush after last summer
shoot flush• Soil populations increase on root
flushes
Root Flushes and Seasonal Phytophthora Activity
Month
Soil
prop
agul
es (p
rop/
cm3 )
0
10
20
30
40
50
60
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Spring Shoot Flush Shoot Flush Shoot Flush
Flowering/Fruit Set
Root Flush Fall Root FlushRoot Flush
Flooding damage to roots predisposes them to infection
• Occurs in soils with restricted drainage
• Over irrigation• High rainfall in summer• Tropical storms and hurricanes
Prolonged O2 deprivation kills roots
• Root regeneration depends on proper aeration
• Disfavors Phytophthora attack of new roots
• Monitor water table to maintain drainage– Allow root regeneration– Reduce infection
Field monitoring of Phytophthora spp.
• Soil populations used to predict damage and need for management
• Soil sampling is most important step
• Commercial labs available• P. palmivora can be distinguished
from P. nicotianae on selective medium
Sample collection• Sample sites at random in the
problem area – Worst trees may have too few roots to
support large population• Collect root/soil cores halfway
between trunk and dripline• 20-40 samples/10 acres• Composite and store in resealable
bag– cool not cold
• Ship to lab within 24-48 hr
Interpretation of population counts
• Vary seasonally and annually– Resample each year
• Thresholds to predict damage are imprecise• Populations of < 10 propagules/cm3 are considered non-
damaging• 10-20 propagules/cm3 depends on soil type, irrigation method,
rootstock, etc.
Phytophthora prevention
• Avoid highly susceptible rootstocks– Plant graft union 6-9 inches above soil
• Adequate drainage and proper irrigation are essential
• Keep area around trunk clear of weeds and avoid wounding
• Remove trunk wraps early in spring and treat for fire ants
• Fire ants feed on and damage bark
Tolerance/Resistance of Swingle citrumelo (SC) P. nicotianae compared to Cleopatra mandarin (CM)
(mg/
cm3 s
oil)
UndisturbedDisturbed
01234
Prop
agul
es (c
fu/c
m3 s
oil)
Roo
t mas
s de
nsity
0102030
Rainfall (cm)
CM
SC
Apr Jun Oct DecAug
Month
Apr Jun Oct DecAug
Month
Disturbed Undisturbed
0
20
40
600
20
40
60SC
CM
01234
Susceptibility of scions to Foot Rot
• Grapefruit - Very susceptible• Lemons - Very susceptible• Limes - Susceptible• Mandarins, tangerines, other hybrids - Very susceptible• Sweet oranges - Very susceptible Because all scions are susceptible they are grown on Phytopththora ‘tolerant’ or ‘resistant’ rootstocks
Rootstock susceptibility - Foot rot• Very Susceptible• Tolerant to susceptible• Tolerant to susceptible• Tolerant• Tolerant• Moderately resistant• Resistant• Resistant
• Sweet orange• Cleopatra mandarin• Rough lemon• Sour orange• Volkamer lemon• Carrizo citrange (hybrid)• Swingle citrumelo (hybrid)• Trifoliate orange
Rootstock susceptibility to root rot depends on Phytophthora spp.
P. nicotianae P. palmivora
Sweet orange Susceptible ?
Cleopatra Susceptible Tolerant
Rough lemon Susceptible ?
Sour orange Susceptible Tolerant
Volkamer lemon Tolerant Tolerant
Carrizo citrange Tolerant Susceptible
Swingle Moderately Resistant Susceptible
Trifoliate orange Resistant Susceptible
Cleopatra mandarin rootstock trees infected in the nursery showing damage that started belowground
Cleopatra mandarin rootstock trees showing damage that started on the sweet orange due to burial of the budunion at planting
Fungicide use in young trees
• Warranted only if cultural practices fail
• Treatment decision based on:- Rootstock susceptibility- Likelihood of nursery
infection- History of site, e.g. Diaprepes
Management of root health in mature groves
• Most commonly used rootstocks are susceptible
• Proper irrigation scheduling and soil drainage are essential
• In high water table areas -installation of tile drains and ditch maintenance
5 minute Break
Phytophthora/Diaprepes ComplexHLB of citrus roots
After the break