Bitter rot management strategies for apple growers

By Blake Jackson

Bitter rot, caused by a fungus, is a serious concern for apple growers in United States, including Pennsylvania. This disease can penetrate intact fruit skin, leading to significant crop losses.

Unlike some other fungal diseases, bitter rot spores can remain dormant within the apple after initial infection. This makes fungicides ineffective after infection occurs. Therefore, prevention is crucial.

Favorable Conditions and Infection Periods

The bitter rot fungus thrives in warm, wet environments. Spores are readily available throughout the season, residing in buds and mummified fruit on trees. Rain disperses these spores, with ideal infection periods occurring between June and harvest when temperatures are high, and moisture is abundant. Research suggests at least 12 hours of leaf wetness are needed for successful infection.

Management Strategies

• Preventative Fungicide Applications: Apply fungicides before rain events to prevent initial infection. Captan (3 lb/acre) remains the most effective option. Ziram (4 lb/acre) with a spreader sticker is an alternative.
• Fungicide Rotation: To manage fungicide resistance, rotate FRAC groups. Consider Merivon as a pre-harvest application for fruit destined for storage.
• Monitoring and Reapplication: Regularly monitor orchard conditions and reapply fungicides after heavy rain events, considering the amount of rain and the last application date.

Additional Considerations

• Mixing for Enhanced Control: During frequent rain, tank-mixing Captan or Ziram with fungicides like Aprovia, Omega, Flint Extra, Luna Sensation, or Merivon can provide extended protection.
• Organic Options: Organic options are limited. Sulfur offers some control but can cause fruit russeting at high temperatures. Repeated applications of bacterial products like Serenade and Double Nickel may offer limited protection.
• Promising Research: Studies suggest adding phosphorous acid-based products (like Rampart or ProPhyt) and tank-mixing Regalia with conventional fungicides might enhance control, particularly during wet seasons.

Researchers are actively exploring new and improved methods to manage bitter rot in Pennsylvania. By understanding the disease cycle, implementing preventative measures, and utilizing effective fungicide strategies, growers can minimize bitter rot damage and ensure high-quality apple harvests.

Photo Credit: istock-rixipix