Wisconsin apple growers are facing the start of the 2025 season with a challenging setup: high levels of Venturia inaequalis (apple scab) infection were reported in many orchards last year. This may mean a higher-than-normal amount of overwintering inoculum is likely present in leaf litter, increasing the risk of early and aggressive infection this spring.

When scab pressure is high going into the season, early, well-timed fungicide applications are critical to prevent the disease from gaining a foothold. Primary infections from overwintering ascospores are the main driver of seasonal epidemics. Once primary lesions appear, secondary spread becomes much harder to control.

• Dormant to silver tip copper-based product applications can offer some early scab suppression and also help reduce fire blight inoculum.
• Start at green tip with a protectant such as copper and/or mancozeb.
• Do not delay sprays—initiate your first application before the first predicted infection event based on NEWA forecasts.
• Tighten spray intervals (e.g., 5–7 days) during the primary scab season, especially during wet weather or when NEWA indicates high infection risk.

The NEWA Apple Scab model (https://newa.cornell.edu/apple-scab/) provides real-time data on ascospore maturity, release, and infection risk using weather data from your closest station. This tool is especially valuable following a high disease pressure year, as it helps time fungicide applications precisely to infection events.

• Begin monitoring the model at green tip and continue through petal fall.
• Use model output to target fungicide sprays before and after predicted infection periods (based on your reapplication intervals and product residuals).

To manage fungicide resistance and improve efficacy:

• Use FRAC Group 3 (DMI), 7 (SDHI), or 9 (AP) products in rotation with multi-site protectants like mancozeb or captan.
• NOTE: Growers are strongly advised to avoid using FRAC Group 7 fungicides early in the season and instead reserve them for peak disease pressure periods—late pink to petal fall for apple scab and petal fall to second cover for apple leaf blotch (Marssonina).
• Avoid back-to-back sprays of the same single-site fungicide group (e.g., QoIs – FRAC 11).
• Tank-mix systemic fungicides with a protectant, when possible, particularly under high pressure.

To reduce inoculum for this spring, consider incorporating leaf shredding or urea sprays. These strategies help accelerate leaf decomposition and reduce ascospore release.

Key Takeaways for Growers This Season:

• Start strong with protectants at green tip.
• Use NEWA forecasts to guide application timing.
• Assume elevated scab pressure due to last year’s high disease levels.
• Tighten intervals and rotate fungicides to reduce risk and maintain efficacy.

Resources
NEWA: https://newa.cornell.edu/apple-scab/
Midwest Fruit Pest Management Guide: https://ag.purdue.edu/department/hla/extension/_docs/id-465.pdf
 

This article was posted in Apples, Disease and tagged Apple Scab, apple scab management, Apples, disease, high-inoculum year, Leslie Holland.