Black rot is a fungal disease, caused by Diplodia seriata on apple. This disease can cause defoliation, fruit rot, and limb cankers. Leaf symptoms start appearing about one to three weeks after petal fall. Initially, the infections start as tiny purple spots. Eventually these spots form circular, dark brown lesions with purple margins that are about a quarter inch in diameter. These foliar lesions can also become invaded by other pathogens, which cause the formation of enlarged, irregularly-shaped lesions. When infections are severe, leaves become chlorotic and fall to the ground. The optimum temperature for infection is 80°F with 4.5 hours of moisture on the foliage.
Sepals of flowers can also become infected early in the growing season with symptoms of minute red specks. The optimum temperature range for fruit infection is 68 to 75°F with nine hours of wetting. Severely affected fruit becomes mummified and remains on the tree. However, dark pimple-like lesions appear on the peel of small developing fruit when the initial infection is less severe. However, as the fruit matures, lesions enlarge, forming concentric brown rings of rot usually at the calyx end of the apple.
Limb cankers are first visible as reddish-brown areas on the bark. Cankers can remain small or enlarge to more than 18 inches-long. These cankers often begin as superficial bark damage but can become severe with cracked, dead bark. The black rot fungus can also colonize fire blight cankers and limb tissue damaged by low temperature injury.
Apple cultivars vary in their susceptibility to black rot. Orin, Pristine, and Red Delicious are very susceptible to this disease. Infected Red Delicious apples often develop core rot about three to six weeks before their normal harvest date and fall from the tree before disease symptoms are visible on the fruit surface. Ginger Gold, Cameo, and Yataka are moderately susceptible to black rot, while Golden Delicious, Gala Supreme, Braebrun, GoldRush, and Fuji are more resistant.
Black rot can be reduced by removing prunings from the site during late winter. Also, pruning out young shoots infected with fire blight during late spring and removing mummified fruit helps minimize sources of black rot inoculum. When this disease becomes problematic, fungicides can be applied from bloom until harvest. For backyard trees, Captan and Mancozeb may be used. Captan can be applied up until the day of harvest. In contrast, Mancozeb cannot be applied within 77 days of harvest. Also, no fungicide is effective in controlling core rot phase of this disease in susceptible cultivars. For commercial apple production, recommended products for black rot control are listed in the Midwest Fruit Pest Management Guide, which is available online.
REVISED: June 19, 2020