By Blake Jackson
With summer finally arriving in 2025, apple growers must prepare for the heightened risk of bitter rot. If rain falls in June as forecasted, conditions will be ideal for the season's first significant infection period. Proactive management is crucial to prevent this damaging fruit disease.
Bitter rot has become a major concern for apple producers in the Eastern U.S., especially for cultivars like Honeycrisp, which are highly susceptible. What makes bitter rot especially difficult is that its spores can penetrate the fruit’s skin without a wound.
After entering, the spores may remain dormant and unaffected by fungicides, making early preventive applications essential. Once visible symptoms appear, it's already too late for effective treatment.
Research from Penn State has identified key insights for managing bitter rot. Spores are present year-round, hiding in buds and leftover fruit.
Rainwater spreads the spores, and warm temperatures above 75°F combined with at least 12 hours of moisture create high-risk conditions. These typically begin in June but have recently been occurring as early as late May.
Growers should apply fungicides before rain events. Recommended combinations include captan or ziram with products like Aprovia, Omega 500F, Flint Extra, Luna Sensation, or Merivon.
Topsin and Fontelis can also be used, though they offer less control against common bitter rot strains in Pennsylvania. Rotating FRAC groups is critical to prevent resistance, and growers must track previous applications, especially those targeting apple scab.
Saving a Merivon spray for near harvest is advised for those storing or packing fruit, as it can reduce postharvest rot. Additional products like potassium phosphites (e.g., Rampart) and Regalia have shown promising results, particularly in wet seasons.
Organic growers have fewer options. Sulfur provides some control but can cause russeting in high temperatures. Products like Serenade, Double Nickel LC, and OSO 5%SC have limited effectiveness and require frequent applications.
To learn more, growers are invited to the Penn State Fruit Research and Extension Center’s Grower Field Day on June 25, 2025. The event features demonstrations, research updates, and educational sessions. Details and registration are available on the FREC website.
Photo Credit: istock-rixipix
Categories: Pennsylvania, Crops, Fruits and Vegetables