Fungicide Resistance

Fungicide use in apple production can be intensive and the range of fungicides available for control is often limited to a few fungicide groups, increasing the risk that fungal strains may arise or be selected which are resistant or less sensitive to the fungicides, leading to reduced control or control failure.

Fungal diseases which readily develop strains resistant to fungicides include:

Botrytis cinerea
Apple scab (Venturia inaequalis)
Penicillium expansum
Gloeosporium sp.
Colletotrichum spp.

Fungicides are at risk from fungi developing less sensitive or resistant strains because their mode of action works on a single site in the target fungi.

Fungicides at risk from fungal resistance

Fungicide group	Fungicides	Resistance type
anilinopyrimidine	cyprodonil pyrimethanil	reduced sensitivity
benzimidazole	thiophanate-methyl	resistant/sensitive
DMI (triazole)	e.g. myclobutanil penconazole	reduced sensitivity
guanidine	dodine	resistant/sensitive
dicarboximide	iprodione	resistant/sensitive
strobilurines (QoI)	kresoxim-methyl pyraclostrobin	resistant / sensitive
phenylamide	metalaxyl-M	resistant/sensitive
hydroxypyrimidine	bupirimate	reduced sensitivity

Where fungi develop reduced sensitivity to fungicides they may still be partially effective.
Where fungi such as Botrytis become resistant to a fungicide such as thiophanate-methyl the fungus isolate is totally resistant and the fungicide completely ineffective.

The current incidence of fungicide resistance in apple diseases is summarised in the following table.

Summary of main apple diseases and fungicide resistance

Fungal disease	Resistance identified	Fungicides where identified and resistance type (incidence)
Apple scab	Yes	dodine resistant/sensitive (L) carbendazim } resistant/sensitive (WS) thiophanate-methyl } pyrimethanil reduced sensitivity (L) kresoxim-methyl (QoI) resistant / sensitive (not UK) DMI (triazole) reduced sensitivity (L)
Apple powdery mildew	Yes?	DMI (triazole) reduced sensitivity . (localised)
Blossom wilt (Monilinia laxa f.sp. mali)	None
Nectria canker	None
Crown rot/collar rot (Phytophthora cactorum)	None
Silver leaf	None
Sooty blotch Fly speck	None None
Gloeosporium rot	Yes	carbendazim } resistant/sensitive (WS) thiophanate-methyl }
Penicillium rot	Yes	carbendazim } resistant/sensitive (WS) thiophanate-methyl }
Botrytis rot	Yes	carbendazim } resistant/sensitive (WS) thiophanate-methyl } iprodione resistant/sensitive (rare)
Phytophthora rot	Yes?	metalaxyl reduced sensitive (rare)
Brown rot	None
Botryosphaeria	None
Diaporthe	None
Fusarium	None?

Key WS = widespread, L = low incidence

Apple Best Practice Guide