What is bitter rot?
Bitter rot is a common fruit rotting disease of apple (and pear). It is caused by the fungi Colletotrichum gloeosporiodes and C. acutatum. The same pathogens also cause anthracnose fruit rot in peach, blueberry, and strawberry. The sexual phase of C. gloeosporides is called Glomerella cingulata and can also cause fruit rot and leaf spot disease on apples. Disease development is favored by high temperatures (80–90°F) and high relative humidity (80–100%). The fungus can directly penetrate into fruit skin without prior injury.
What does bitter rot look like?
- Symptoms first appear as small, slightly sunken circular areas arranged in concentric rings that are light to dark brown in color and advance to dark brown with small black dots.
- Symptomatic areas in fruit ooze a gelatinous, salmon-pink mass of spores.
- As the lesions enlarge, the rot progresses to the core of the fruit in a V- or cone-shaped pattern.
- As the fruit ripens, it decays rapidly and finally shrivels into a mummy.
- Bitter rot usually appears on the sun-exposed surfaces of fruit, and heat injury or sub-lethal sunburn may increase susceptibility of fruit to this disease.
Where does bitter rot come from?
The fungus survive the winter in dead wood, cracks in the bark, or mummified fruit that were infected during the previous season. Conidial spores are spread by splashing and wind-blown rain, insects and birds while ascospores are released into the air during rainy periods. Insects can spread the fungus from infected fruit to healthy fruit.
How do I prevent bitter rot?
- Remove and properly dispose of dead, jagged limbs, and fruit mummies.
- Maintain healthy, vigorous growth conditions for apple trees.
- Avoid growing highly susceptible varieties.