As apple trees emerge from dormancy one of the first priorities of the season is a fungicide application for apple scab control. The apple scab fungus overwinters in infected leaves on the orchard floor. Coinciding with the buds showing green is usually the first discharge of apple scab spores from the leaves on the ground. Spore numbers develop slowly from green tip to the tight cluster stage of blossom development. From tight cluster to bloom the number of developing spores increases and peaks right around bloom. However, changes in weather can impact the patterns of apple scab spore dispersal. To determine the predicted ascospore maturity and infection events in your area, refer to the NEWA (Network for Environment and Weather Applications) Apple Disease Model for apple scab to determine optimal spray timings for fungicides (Figure 1). Infection events are based on the minimum duration of wetness required at different temperatures for apple scab infections to occur.

Effective fungicide applications should be made based on predicted infection events, presence of fungal inoculum, cultivar susceptibility, and the features of the fungicide(s) to be applied. Apple scab control during the early season (green tip to ½ inch green) is a critical time for preventative spray!

Some conventional fungicides for apple scab control during this period include:

• Captan
• Mancozeb (be aware of the 77-day PHI)

Some organic fungicides for apple scab control during this period include:

• Copper – should be applied before the quarter-inch green stage; one or two pre-bloom copper applications can be made.
• Sulfur – use as a protectant; needs to be applied before scab infections take place. Do not use within 10 days of applying oil or captan or when temperatures exceed 85°F.

The Midwest Fruit Pest Management Guide 2021-2022 provides additional listings of fungicides for apple scab control. Before making fungicide applications, be sure to consult the label and follow the instructions.

This article was posted in Apples, Disease and tagged Apple Scab, Apples, disease, Leslie Holland.