2025 Disease Update: Scab, Apple Blotch, Fire Blight Risk May 2–7

Although many areas are experiencing petal fall in their apple orchards, there are enough trees with open blossoms for fire blight risk to still be an issue. Coinciding with bloom and petal fall is the peak number of apple scab ascospores that are mature, and apple blotch (Marssonina blotch) ascospores beginning to disperse. In addition, rust galls are active every time it rains. Based on the forecast for late May 1st through May 7th, there is a high probability for thunderstorms and significant rain. It will also be warm with temperatures averaging 67–72°F. The rainfall and warm temperatures will set the stage for optimal infection conditions for many diseases in the orchard, and disease pressure will be high. Growers in Pennsylvania and Maryland need to be on alert and prepare accordingly.

Fire Blight

If you have open blooms on any varieties, these blossoms are still at risk for infection if they have not been pollinated and are not at petal fall. Even if you applied streptomycin earlier in the week, you would need to reapply protection again either on May 1 or May 2, especially if more "straggler" blossoms have opened. In addition, we have had enough heat units since the "clock" was reset earlier in the week if streptomycin was applied to trigger a high EIP infection risk (150 – 200+ EIP). I would also highly recommend growers to consider adding a plant defense elicitor, such as Actigard, as well as the plant growth regulator Apogee/Kudos (prohexadione calcium) to this bloom spray to help control fire blight during this acute period, as well as protect from shoot blight that could possibly sneak up on folks in a few weeks.

For additional management recommendations, please refer to the Disease Alert from April 18, 2025.

Apple Scab, Rust, and Apple Blotch

This will be a large spore dispersal event for scab, rust, and apple blotch. Temperatures will be ideal for minimal leaf wetness hours needed to trigger an infection. Tank mix your rainfast mancozeb with FRAC Group 7 products at this time. Should another spray be needed late next week, rotate with FRAC Groups 3 or 9. FRAC Groups 3 and 7 fungicides work well on scab, rust, and apple blotch. Save FRAC Group 11 fungicides for the summer cover sprays.

For additional management recommendations, please refer to the Disease Alert from April 22, 2025.

Fungicide Application Reminders

If we have a prolonged period of rainfall, reapplication of fungicides may be needed next week. Please keep your spray intervals short, especially if you are using alternate row middle (ARM), and pay attention to the amount of rain that falls in your location during the next seven days. Although many conventional fungicides can penetrate the first few layers of cells (called translaminar fungicides) and are protected from being washed off, there are limits. Also, broad-spectrum fungicides, such as mancozeb, captan, and sulfur, do not penetrate the plant tissue and are on the surface only. Consequently, these can be easily washed off. The typical rule of thumb is that after 2 inches of rain, fungicides need to be reapplied.

Additional Resources

Dr. Tree Fruit and Don Podcast

Apple scab

Fire blight

Rust

Apple (Marssonina) Blotch

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 assist with management decisions (especially infection events), growers can use the NEWA website, which has weather stations all over Pennsylvania. Before chemical products are applied, be sure to comply by obtaining the current usage regulations and examining the product label. Product information can easily be obtained from CDMS, Agrian, or Greenbook.