Bitter rot is caused by the conidial type of the fungus Colletotrichum gleosporioides. Fruit infections that occur a month after petal fall begin as small, slightly sunken brown lesions that may be surrounded by a red halo on mature fruit. Lesions become sunken as they enlarge (1-3 cm in diameter), and fungal structures form concentric circles around the point of infection. These fungal structures, called "acervuli," produce large masses of spores which look crystalline when dry or creamy salmon to pink under humid conditions. These spore masses can later turn dark brown to black on older lesions. As the lesions enlarge, the rot progresses to the core of the fruit in a V-shaped pattern and has a bitter smell. Fruits are susceptible during all stages of development. Leaf infection is rare. If present, it starts as small red flecks that enlarge to irregular brown spots, and leaves may drop prematurely. A canker phase of the disease has also been reported.
Management
The pathogen survives one season to the next on the tree in mummified apples and fungal mycelium in colonized dead wood and cankers. Apples mummified as a result of chemical thinning and stems left on the tree are good sites for this fungus to overwinter. Fungal spores move with air and water during rainfall throughout most of the growing season. Infections can occur in as little as 5 hours at optimum temperatures (80 degrees F). The amount of infections increases with long periods of wetness duration (up to 60 hours), and the disease can spread rapidly if left uncontrolled. Control is based on sanitation and a protective fungicide spray program.
Removal and discarding of mummified fruit, cankers, or dead wood and branches affected by fire blight is important in managing bitter rot effectively. In addition, removing and discarding (burning or burying) infected fruit throughout the growing season is also important in reducing the amount of fungal inoculum and therefore the rate of disease increase. No commercial cultivar is sufficiently resistant, so application of fungicides such as Captan, Mancozeb, or Topsin-M at 10-14 day intervals, from first cover until harvest, is the second most important means of control after sanitation.
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