Apr 27, 2023

Pennsylvania growers: prepare for fungal diseases

Significant rains forecast for Pennsylvania could favor fungal diseases, including Marssonina blotch, apple scab and cherry leaf spot.

Cool temperatures, however, are not expected to spur fire blight risks.

Trees will need to be protected before the weekend to prevent diseases, according to a news release from Penn State Extension.

April 28-May 1 will be an extended wetting period favored for many fungal diseases. There is no fire blight risk due to the cooler temperatures, according to a Penn State Extension office.

Trees will need to be protected prior to the weekend to prevent disease, according to Penn State Extension.

Apple Scab

All overwintering spores will be mature by this point. We most likely saw the biggest scab spore release on April 15 and April 22; however, available spore numbers are still high enough to make this a significant infection event this weekend.

Marssonina Blotch

The primary infection period for Marssonina Blotch is underway, and April 28-May 1 will probably be an important infection event. Consequently, control is critical to disrupt the disease cycle now to limit premature defoliation later in the season. Marssonina has become pervasive throughout Eastern apple orchards. Everyone has Marssonina blotch in their orchard, whether they realize it or not, so don’t let up off the gas pedal right now. We might be on the downside for apple scab, but Marssonina is picking up speed right behind it.

Fire Blight

There is no fire blight infection risk this weekend (April 28–May 1) because conditions have been too chilly this week. The southern part of Pennsylvania had fire blight infection events April 14-16 and April 21–22. It is important to begin scouting orchards for infections.

Diseases on Stone Fruit

Growers should be thinking about their cherries, peaches, and nectarines. Cherry leaf spot is like apple scab when it comes to infection conditions. Rusty spot is powdery mildew on peaches and nectarines. Captan or sulfur is sufficient to keep brown rot in check right now. Current conditions are too cool for the disease to be problematic for bacterial spot.

— Penn State Extension

Photo: Honeycrisp trees defoliated in September due to Marssonina infection. Stop primary infection of Marssonina now to limit defoliation of apple trees later in the season. Photo: K. Peter, Penn State