Black rot canker
Black rot, caused by the fungus Botryosphaeria obtusa, causes a canker, leaf spot, and fruit rot on a large number of hosts including apple, crabapple, and mountain ash. This disease can weaken trees and kill affected limbs. The fungus overwinters in cankers and occasionally on mummified fruit. Spores are produced during moist periods throughout the growing season and are wind blown or rain splashed to susceptible tissues. Spores gain entry into branches or the trunk through an injury such as a frost crack, lawn mower damage, or sunscald. Black rot canker initially appears as an area of loose bark or as a slightly sunken, reddish- brown region. Areas beneath the cankered bark are tan to black colored, in contrast to the greenish-white tissue normally found under the bark on healthy limbs. Black rot causes cankers, localized diseased areas, which vary in size from a few inches to several feet. The rough, pimple-like structures on the surface of the canker are the fruiting bodies of the fungus. These fruiting bodies contain spores which spread the disease.
Frogeye leaf spot is caused by spores from the same fungus. Infected leaves show symptoms shortly after they unfold. Small, purple specks enlarge to become spots with distinctive tan to brown centers and purple margins. Severe infections cause leaves to yellow and fall from the tree. In Minnesota, the leaf spot phase of the disease is seldom important.
Fruit infection, occurring occasionally in Minnesota, begins after petal fall. Symptoms on fruit are reddish flecks which enlarge into purple colored, pimple-like structures. Infected areas coalesce and turn dark in color, often with alternating dark and light bands. Infected areas are firm and not sunken. Infected fruit may eventually shrivel into mummies and remain attached to the tree.
Black rot management includes removal and disposal of cankered limbs and dead branches. Dispose of fallen fruit at the end of the season. Stress can predispose trees to infection, so keep trees growing vigorously. Properly water, fertilize, and mulch trees. Avoid injury to the trunk caused by lawn mowers, weed whips, or animals. Wrap young trees to avoid sunscald. If frogeye leaf spot is a problem, it can be controlled by following the same spray program recommended for control of apple scab (See University of Minnesota Extension Service Publication #FO-0675-B Home Fruit Spray Guide).
Chad Behrendt, Crystal Floyd