Spring certainly seems unresolved in Wisconsin this year. With warmer temperatures felt across much of the central and southern parts of the state, buds on apple trees (and many other plants) broke and green tissues emerged. Despite the return of winter temperatures and snow in some locations, Apple Scab management remains critical in the early season. Many growers have already made a fungicide application for apple scab prevention this season, and protection will continue to be warranted especially with forecasted rain events.
Ascospores (responsible for primary infections) of the apple scab fungus mature in overwintering leaves and spread early in the growing season to green tissue on the trees. However, from tight cluster through bloom, ascospores are very active and can result in significant infections. Protection is needed during this period. Between pink and bloom stages, ascospore discharge is at its highest and declines 1 to 2 weeks following petal fall. If primary infections go unmanaged, then season-long control of the pathogen becomes very difficult.
The current Wisconsin weather conditions may impact the timing of apple scab spore discharge. If frost occurs, apple scab lesions may appear dark and crunchy instead of soft and fuzzy. This does not mean that the fungus froze to death. Once temperatures warm up again, lesion development can resume. Some apple scab infections may be delayed by the current weather conditions but that does not mean they disappeared. Additionally, rainfall at nighttime may delay the release of ascospores until the following day. Keep an eye on the Apple Scab Disease Forecast model on NEWA to evaluate possible infection events in your area.
**According to the NEWA model, apple scab infection events are forecasted for this week in various parts of the state. Check NEWA to see predictions in your area.
Several growers dealt with Apple Leaf Blotch towards the end of last season. Like the apple scab pathogen, the apple blotch fungus overwinters in fallen leaves and in the spring, spores release during rain events or in high humidity. This pathogen can be managed with the conventional fungicides used to manage other early- and late-season diseases. Like apple scab, early season control of the apple blotch fungus will help reduce secondary infections. Fungicides can wash off during rain events, therefore fungicide reapplication may be necessary.
With last week’s warm weather some orchards are entering tight cluster which is also a critical time for Powdery Mildew control. Remember that conditions for powdery mildew development and apple scab development are different, where powdery mildew does not require rain for spore dispersal, but apple scab does. During very rainy seasons powdery mildew infections may be low as these conditions do not favor the pathogen. If your foliage remains wet for extended periods of time it is unlikely that you will see powdery mildew develop. Alternatively, if we experience prolonged dry and warm periods, powdery mildew infections are more likely to occur.
- (New version!) 2023-2024 Midwest Fruit Pest Management Guide – https://ag.purdue.edu/department/hla/extension/_docs/id-465.pdf
- Network for Environment and Weather Applications (NEWA) Apple Scab Disease Model – https://newa.cornell.edu/apple-scab
- New to NEWA? Check out their brief tutorial for setting up your user profile – https://newa.zendesk.com/hc/en-us/articles/4417360459799-Create-and-Configure-Your-User-Profile-3-minutes-
- Managing Apple Powdery Mildew – https://fruit.wisc.edu/2022/07/14/managing-apple-powdery-mildew/
- Apple Blotch Disease in Wisconsin Apple Orchards – https://fruit.wisc.edu/2022/10/06/apple-blotch-disease-in-wisconsin-apple-orchards/
- Scab, Frost… Now What? – https://fruit.wisc.edu/2016/06/10/scab-frost-now-what/