The Rotten Truth about Bitter Rot

The season for apple harvest has arrived, bringing with it the possibility of late season fruit rots (summer rots). While fruit rots have a variety of causes, the most common fungal fruit rot of apple in Kentucky is bitter rot. This disease results in rotten, inedible fruit. Fungicides are available for management; however, sanitation is critical for disease prevention. Ongoing research at the University of Kentucky is providing new insights and understanding of the pathogens that cause bitter rot.

Bitter Rot Facts

  • Symptoms include small, slightly sunken lesions that eventually develop a bull’s-eye pattern (Figure 1). Cutting into infected fruit reveals an internal rot with a V-shaped pattern (Figure 2).
  • Symptoms may not appear immediately after infection and may take several months to become visible.
  • Initial infection begins as early as bloom and may continue through harvest.
  • The pathogen overwinters in fallen fruit, dried fruit (mummies), and in crevices in bark and dead wood.
  • Caused by multiple species of the fungus Colletotrichum.
Figure 1: Sunken lesions with bull’s-eye appearance are symptoms of bitter rot on apple. (Photo: Nicole Ward Gauthier, UK)

Figure 1. Sunken lesions with bull’s-eye appearance are symptoms of bitter rot on apple. (Photo: Nicole Ward Gauthier, UK)

Figure 2. Internal V-shaped rot in apple caused by bitter rot. (Photo: Nicole Ward Gauthier, UK)

Figure 2. Internal V-shaped rot in apple caused by bitter rot. (Photo: Nicole Ward Gauthier, UK)

Management Options

  • Remove and discard diseased fruit immediately.
  • At the end of the season, remove fruit from the ground, as well as cankers and dead wood that could harbor fungi.
  • Plant cultivars that are less susceptible to bitter rot, including Rome Beauty, Winesap, and Red or Yellow Delicious.
  • Homeowners can apply fungicides that contain captan or mancozeb beginning soon after petal fall and continuing every 10 to 14 days until harvest. Always follow label directions when utilizing fungicides.

Research Update

  • Five species of Colletotrichum have been documented as causal agents of bitter rot in Kentucky.
  • More than one species has been documented within the same orchard and within a single tree.
  • Aggressiveness and fungicide sensitivity varies across species.
  • Continued research is needed to provide growers with more targeted management recommendations.
  • Samples of bitter rot are needed to continue with this research. Please contact Dr. Nicole Ward Gauthier (nicole.ward@uky.edu) for more information.

Additional Information

  • Apple Fruit Diseases Appearing at Harvest (PPFS-FR-T-02)
  • Fruit, Orchard, and Vineyard Sanitation (PPFS-GEN-05)
  • Simplified Backyard Apple Spray Guides (PPFS-FR-T-18)
  • Effectiveness of Fungicides for Management of Apple Diseases (PPFS-FR-T-15)
  • Midwest Commercial Tree Fruit Spray Guide (ID-92)
  • Characterization of Colletotrichum species causing bitter rot of apples in Kentucky orchards (M.S. Thesis by Misbakhul Munir)

 

By Kimberly Leonberger, Extension Associate, and Nicole Ward Gauthier, Extension Plant Pathologist

 

 

Posted in Fruit