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The New Zealand Institute for Plant & Food Research Limited
Fungicide effects on Sclerotinia rot of lettuce
Peter Wright and Robert Beresford
The New Zealand Institute for Plant & Food Research Limited
Sclerotinia rot – White mold – ‘Drop’
• Caused by the fungi Sclerotinia minor (Sm) and S. sclerotiorum (Ss)
• Serious pathogens of several other horticultural crops
• Soilborne pathogens - can survive in soil as sclerotia for >3 years
• Infect plants by hyphal germination of sclerotia that are in contact with senescing lower leaves on the soil surface (Ss & Sm)
• Infect plants by airborne ascospores, which infect senescent or dead lettuce leaves (Ss) (see apothecia in photo ->)
The New Zealand Institute for Plant & Food Research Limited
Sclerotinia rot – White mold – ‘Drop’
• All commercial cultivars of lettuce are susceptible to both Sclerotinia species
• Fungicides are the primary method of disease control (others include bio-control agents, crop rotation and soil amendments)
• The only chemical currently registered in NZ for control of Sclerotinia rot of lettuce is the benzimidazole fungicide carbendazim
• Benzimidazoles are generally at high risk for development of fungicide resistance
The New Zealand Institute for Plant & Food Research Limited
Fungicide resistance
• Sclerotinia isolates may be developing resistance to some
fungicides – including carbendazim
• Resistance arises from repeated use of an at-risk fungicide – the
fungus changes genetically allowing it to survive in the presence of
the fungicide
• Resistance development can be slowed by limiting use of the at-
risk fungicide groups through a resistance management strategy
• When resistance develops, local data are required to scope the
problem and to develop a strategy tailored to the local situation.
The New Zealand Institute for Plant & Food Research Limited
Fungicides to control Sclerotinia in NZ
Product Chemical Group CropCarbendazim Benzimidazole Lettuce, bean, tomatoTopsin M-4A Benzimidazole Bean, tomatoSumisclex 500SC Dicarboximide Bean, cucurbit, tomatoFluazanim Dicarboximide Potato, tomatoIprodione Dicarboximide Ornamentals, kiwifruitTaratek 5F Nitrile & benzimidazole Bean, tomatoChlorocarb Nitrile & benzimidazole TomatoPristine Carboxamide & strobilurin KiwifruitFlint Strobilurin Kiwifruit
The New Zealand Institute for Plant & Food Research Limited
Fungicides to control Sclerotinia in lettuce overseas
Chemical Fungicide GroupPossible
resistance issues?
Boscalid Carboxamide (7) yesIprodione Dicarboximide (2) YesVinclozolin Dicarboximide (2) YesProcymidone Dicarboximide (2) Yes
Cyprodinil/FludioxonilAnilinopyrimidine (9)/Phenylpyrrole (12)
-
Dichloran Aromatic hydrocarbon (14) -Tebuconazole Triazole (3) Yes
Carbendazim Benzimidazole (1) YesGroup numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program.
The New Zealand Institute for Plant & Food Research Limited
Fungus FungusSensitive Resistant
Fungicide resistance methodology
Disease Disease notcontrolled controlled
Low EC50 High EC50
EC50= Effective concentration of fungicide that gives 50% inhibition
(Fungus tested at different fungicide concentrations)
Fungicide applications
Decreasing sensitivity Increasing
resistance
The New Zealand Institute for Plant & Food Research Limited
EC50= Effective concentration of fungicide that gives 50% inhibition of the fungus
The New Zealand Institute for Plant & Food Research Limited
Growth of Mycelium in Fungicide-amended media
The New Zealand Institute for Plant & Food Research Limited
Inhibition of mycelial growth of S. sclerotiorum on PDA containing fungicide
0
2
4
6
8
10
12
14
16
0.1 0.5 1 5 10
Fungicide concentration (ppm)
Radi
al g
row
th (m
m) o
f m
ycel
ium
eac
h 24
hou
rs
Carbendazim
Dichloran
Trifloxystrobin
Tebuconazole
Iprodione
Procymidone
The New Zealand Institute for Plant & Food Research Limited
Nation-wide survey
• Nation-wide survey to determine S. sclerotiorum and S.
minor sensitivity to several fungicides.
• S. minor and S. sclerotiorum isolates used in this study will
originate from mycelium from a single sclerotium of each
fungal species taken from naturally infected lettuce leaves.
• Laboratory assay – growth of Sclerotinia isolates on
fungicide-amended media.
• If/when resistance has occurred we can look at developing
disease control strategies tailored to the local situation.
The New Zealand Institute for Plant & Food Research Limited
To collect Sclerotinia isolates (sclerotia) for me
for my fungicide resistance tests