2026 Disease Update: Scab, Marssonina Blotch, Rust Potential This Week

The whiplash sensation of the 2026 season continues. A week ago, it was summer, and fire blight was on our minds; now we are back to winter with the worry of the ramifications of a freeze event during the early morning hours of April 21. I don't know about you, but I'm already ready for the season to be finished. Across Pennsylvania, tree fruit, especially apples, are in various stages of bloom through petal fall. Peaches and cherries are post-petal fall. What needs to be on your mind this week when it comes to disease management in the orchard?

Apple scab, Marssonina blotch, Rust

If the forecast comes true, there will be an apple scab infection, possibly starting on April 24 and continuing through April 25. In the southern half of Pennsylvania, we are nearing the end of the peak for ascospore availability from overwintering leaves. In addition, cedar apple rust and quince galls are still actively producing spores during rain events. Just as the primary apple scab infection period comes to a close, Marssonina blotch is gearing up. Petal fall is the typical time when the first Marssonina spores emerge from their overwintering locations. The window for infection will be active until about early June. Apple blotch infection favors 68-77°F with as few as 4 leaf wetness hours. For more information about protecting apple trees during bloom through petal fall, please refer to the 2026 Disease Update from April 16, 2026. 

Fire Blight

For those who experienced bloom April 12-18, we had two significant fire blight infection events: April 14-15, and April 18. The EIP (Epiphytic Infection Potential) during these days was 182%, 291%, and 267%. When the EIP is close to or over 200%, this indicates that large numbers of infections are likely should a wetting event occur. Needless to say, 2026 will most likely be a substantial fire blight year. With that said, being proactive to prevent shoot blight should be a priority. The most effective way to limit shoot blight is to use prohexadione calcium (Kudos, Apogee), a plant growth regulator that thickens plant cell walls and hardens off shoots. It takes 10-14 days for the effect to occur in the tree. Consequently, these applications should start now and be repeated. 

Powdery mildew and rusty spot

We have experienced enough days without precipitation to indicate this will be another good powdery mildew year. We are in the middle of managing this disease on both apple and stone fruit. For disease management recommendations, please refer to the 2026 Disease Update from April 7, 2026.

Cherry leaf spot

Don't forget your tart cherry trees and the need to manage for cherry leaf spot. Cherry leaf spot is similar to apple scab in terms of infection conditions. The optimal temperature range for the spread of this fungus is between 60 to 68°F. Later in the week, we may have excellent conditions for cherry leaf spot infections. Many products labeled for apple scab control will work to keep cherry leaf spot in check.

Additional Resources

Dr. Tree Fruit and Don Podcast

2026 - 2027 Penn State Tree Fruit Production Guide 

For commercial fruit growers, please note: When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one's own orchard. To support management decisions (especially during infection events), growers can use the NEWA website, which provides weather data from weather stations across Pennsylvania.  Before applying chemical products, be sure to comply with current use regulations and read the product label. Product information can easily be obtained from CDMS, Agrian, or Greenbook.