Apple IPM - Bitter Rot

Overview

• Bitter rot of apple is more common in the warm, humid climate of the southeastern U.S., and occurs sporadically in the Northeast. Extensive damage can develop rapidly in New England orchards during periods of prolonged hot, wet weather if inoculum sources are present.
• Bitter rot caused by fungi in the same genus, Colletotrichum, with many different species shown to infect apple fruit. 
• Colletotrichum spp. infect during extended warm rainy periods after fruit set, continuing through the summer. Infection risk increases as fruit matures.
• Bitter rot is more common on light or bicolored fruit such as Empire, Honeycrisp, Mclntosh, Sunrise, Paulared, and Jonagold.

Symptoms

Bitter rot is usually first seen from July on, depending on weather and the available inoculum. Warm, wet weather favors infection, and blocks that have had bitter rot in the previous growing season will tend to show symptoms earlier. They appear as small, light brown, circular spots on the fruit. One or many spots may appear. When temperatures are warm, around 80 to 85ºF, these spots expand rapidly and become light to dark brown in a target pattern of concentric rings. Initial spots are distinctly sunken or saucer-shaped. Later, small, dark raised bumps, the fungal fruiting bodies, develop in the spots. The fruiting bodies produce salmon-pink spore masses.

Beneath the surface of an infection site, the flesh is light brown and watery. The rotten spot forms a cone under the surface of the skin, so that when the fruit is cut, a clear triangular shape is seen.  As the fruit ripens, it decays rapidly and finally shrivels into a mummy, a combination of the fungus and dead fruit tissue.

Disease Cycle

The bitter rot disease cycle in apple is only partially understood. Bitter rot fungi overwinter in dead wood, including areas damaged by fire blight or other diseases, and on mummified fruit in apple trees. They may also live on trees surrounding orchards or on broad-leaf weeds in orchards. The fungi produce spores in spring and summer, which are released by rain and dispersed throughout trees. The optimum temperature for spore germination is 79 to 80 F, when it takes only 5 hours to infect. Infections commonly start during prolonged warm, wet weather. As soon as infected fruit produce spores, these can cause new infections.

Infected fruit are susceptible to decay before and after harvest. Some infections may show no symptoms in the field, but develop after harvest. 

Management Strategies

Cultural/Biological

• Remove sources of inoculum like mummified fruit, dead wood, and fire-blight damaged wood from trees.
• Eliminate wild or unmanaged apple trees from areas adjoining the orchard.
• Remove newly infected fruit during the growing season. 
• Chopping leaves and other debris on the orchard floor reduces inoculum.
• Prune trees to open the canopy to light, air, and spray penetration.

Chemical

Fungicide applications targeting bitter rot should start at fruit set if weather and previous disease history favor infection at that time. For the rest of the summer in orchards with a history of the disease, fungicides to manage bitter rot should be applied when prolonged warm, wet weather is predicted, particularly as fruit matures.  Among the most effective fungicides against bitter rot are the EBDCs. Unfortunately, they have a 77-day pre-harvest limit.

• See New England Small Fruit Management Guide for currently recommended spray materials for controlling Bitter Rot in Apples.
• Apply recommended protectant fungicide at highest labeled rate before rain if infections are found and wet, warm weather is forecast.
• Make follow-up applications as indicated by scouting and weather information.
• Rotate fungicide materials from different FRAC groups to avoid promoting the development of resistant strains of this disease.

See current New England Tree Fruit Mgt Guide Version of this Fact Sheet: https://netreefruit.org/apples/diseases/bitter-rot

Date: March 2020
Author(s): Dan Cooley & Liz Garofalo, UMass Extension, Heather Faubert, URI Extension

Additional information available on the MYIPM app: https://apps.bugwood.org/apps/myipmseries/

This work was supported in part by funding provided by USDA NIFA Extension Implementation Program, Award No. 2017-70006-27137