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Scab fungicide resistance management

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Fungicide Resistance Management - Scab

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 manage 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. 
  • 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.
  • 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.

In organic production low metallic copper equivalent (MCE) copper fungicides, sulfur and lime sulfur are the most commonly used chemical control options. These fungicides may be used through primary season, but often result in some fruit and foliar damage.