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