This article was published originally on 11/11/2009
Bitter rot was found on river apples from Winneshiek County, Iowa last September. Infection was 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 are produced in concentric circles around the point of infection (photo below). 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 lesion. As the lesion 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.
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. Spores of the fungus 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). Amount of infections increase with long periods of wetness duration (up to 60 hours) and disease can spread rapidly if left uncontrolled. Control is based on sanitation and a protectant 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 (burn or bury) 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 interval, from first cover until harvest is the second most important mean of control after sanitation.
Apple Bitter Rot. Photo from Cornell University Extension Tree Fruit and Berry Pathology Group.