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Resistance management: apple fungicides

Fungicide resistance can make apple scab management much more difficult.  Apple scab has developed resistance to most fungicides in at least a few parts of the U.S., and the risk of resistance is high for single-site fungicides. Once resistance to a fungicide class is established in an orchard, that class is no longer a control option. It is important to keep options, and manage fungicide risk.

The following risk management rules are recommended.

  • Use a sanitation program to reduce inoculum.
  • Use a multi-site fungicide in every spray - FRAC groups M3 and M4 - captan, mancozeb or metiram. 
  • Change site-specific fungicides - FRAC groups 3, 7, 9 and 11. 
  • Use at least three active ingredients from three different FRAC groups over primary scab season. 
  • If possible, do not use any one class of single-site fungicide more than twice in a season. For many fungicides, labels limit applications to no more than 4 per season. 
  • Whenever possible fungicides should be applied preventatively, before infection periods. Labels may suggest post-infection uses, but these should be used only as a last resort.
  • Apply the maximum label rate of single-site fungicides.
  • Pre-mix fungicides containing two single-site ingredients  – Merivon, Luna Sensation and Luna Tranquility – should still be mixed with a multi-site fungicide.
  • Each ingredient in a pre-mix fungicide counts as an application. For example, Luna Sensation with both fluopyram (FRAC group 7) and trifloxystrobin (FRAC group 11) would count as an application of a Group 7 and an application of a Group 11.

General resistance management guidelines are here.