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 ProductChemical GroupCrop CarbendazimBenzimidazoleLettuce, bean, tomato Topsin M-4ABenzimidazoleBean, tomato Sumisclex 500SCDicarboximideBean, cucurbit, tomato FluazanimDicarboximidePotato, tomato IprodioneDicarboximideOrnamentals, kiwifruit Taratek 5FNitrile & benzimidazoleBean, tomato ChlorocarbNitrile & benzimidazoleTomato PristineCarboxamide & strobilurinKiwifruit FlintStrobilurinKiwifruit

The New Zealand Institute for Plant & Food Research Limited Fungicides to control Sclerotinia in lettuce overseas ChemicalFungicide Group Possible resistance issues? BoscalidCarboxamide (7) yes IprodioneDicarboximide (2) Yes VinclozolinDicarboximide (2) Yes ProcymidoneDicarboximide (2) Yes Cyprodinil/Fludioxonil Anilinopyrimidine (9)/ Phenylpyrrole (12) - DichloranAromatic hydrocarbon (14) - TebuconazoleTriazole (3) Yes CarbendazimBenzimidazole (1) Yes Group 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 SensitiveResistant Fungicide resistance methodology DiseaseDisease notcontrolled Low EC 50 High EC 50 EC 50 = Effective concentration of fungicide that gives 50% inhibition (Fungus tested at different fungicide concentrations) Fungicide applications Decreasing sensitivityIncreasing resistance

The New Zealand Institute for Plant & Food Research Limited EC 50 = 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

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