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.