Recent rainfall across Wisconsin has created favorable conditions for several important summer fruit rot diseases. As we move into the warm, humid months of the growing season, growers should begin shifting their focus from primary apple scab management toward protecting developing fruit from the rot pathogens that can cause significant losses at harvest and during storage.

Common Summer Apple Rot Diseases

The most common summer fruit rots in Wisconsin include black rot, bitter rot, and white rot. While these diseases differ in biology and symptom development, they share one important characteristic: prolonged periods of moisture and warm temperatures greatly favor infection and disease development. Orchards that experienced hail, winter injury, fire blight, pruning wounds, or other sources of dead wood are often at elevated risk because many of these pathogens survive and produce spores on cankers, dead branches, mummified fruit, and other decaying tissues.

Black rot can cause both fruit rot and frogeye leaf spot. Fruit infections often begin as small brown lesions that expand into firm, dark rots with characteristic concentric rings. The pathogen overwinters in dead wood, cankers, and mummified fruit.

Bitter rot has become an increasing concern in many apple-growing regions and is favored by extended periods of warm temperatures and high humidity. Symptoms typically appear as sunken lesions with concentric rings of salmon-colored spore masses. Once established, bitter rot can spread rapidly within orchard blocks during wet weather.

White rot is often associated with heat stress and wounded fruit. Infections typically develop during warm weather and can progress rapidly as fruit approach maturity.

Cultural Pest Management Practices Are Important

Fungicides are important tools, but they work best when combined with sound cultural practices.

Growers should:

• Remove dead limbs, cankers, and mummified fruit whenever possible.
• Prune to improve air movement and canopy drying.
• Manage vigorous growth that contributes to dense canopies.
• Maintain good fire blight sanitation, as dead and declining wood can serve as a reservoir for rot pathogens.
• Minimize mechanical injury to fruit during summer operations.
• Scout regularly for developing symptoms, particularly following hail events or prolonged wet periods.

As fruit continue to size, maintaining fungicide coverage through periods of wet weather and reducing inoculum sources within the orchard will be essential for protecting fruit quality and minimizing losses at harvest.

Growers should pay particular attention to blocks with a history of black rot, bitter rot, or white rot, as these orchards are likely to experience the greatest disease pressure if wet conditions persist throughout the summer.

A future article later this season will discuss fungicide management strategies for summer rots and SBFS.