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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

Further Reading

FRAG-UK Fungicide resistance in apple and pear pathogens – 1999.