Black rot of apple
Fig. 7. Black rot canker.
Photo: G. Sundin.
Fig. 8. Black rot infection on fruit.
Photo: G. Sundin.
Fig. 9. Black rot infection on fruit.
Photo: UGA, Bugwood.org.
Fig. 10. Branch dieback due to black rot cankers.
Photo: G. Sundin.
Black rot is occasionally a problem on Minnesota apple trees. This fungal disease results in leaf spot, fruit rot and cankers on branches. Trees that are not fully hardy in Minnesota, are infected with fire blight or are under stress due to environmental factors like drought are more susceptible to black rot infection than well maintained trees. Many cultural control practices will reduce the severity of this disease.
Although infected fruit remains firm, rotten tissue with brown and black concentric rings can cover large areas of the fruit. Fungal spore producing structures, called pycnidia, can be seen as small black spots on older infections. Some fruit mummify and remain attached to the tree. Occasionally fruit are infected early in the season resulting in fruit that ripen weeks before the typical harvest date and are rotten at the core.
Infected leaves develop "frog-eye leafspot:” circular lesions with purplish or reddish outer borders and light tan interiors.
Cankers appear as a sunken, reddish brown area on infected limbs often with rough looking or cracked bark. These cankers may not be readily apparent. If rotten fruit or frog-eye leafspot is present in a planting, inspect the trees for cankers.
The fungus that causes black rot can infect dead tissue as well as living trunks, branches, leaves, and fruits. Branches or portions of the trunk that are dead or that have suffered winter injury or other damage are most likely to be infected.
Minnesota’s severe cold often injures apple trees. Although the trees may not be killed outright, damaged tissue is susceptible to infection by black rot. Sunken black rot cankers may appear on the southwest side of young trees, where winter injury is common, or at crotches where branches join the main trunk, since these areas are typically the last to harden off in fall. Similarly, apple trees grown on sandy soils without supplemental irrigation, trees that are not irrigated during particularly dry spells, and trees grown in poorly drained soils, are all more susceptible to black rot. This is because both drought stress and the low oxygen of waterlogged soils lead to death and damage of the above-ground parts of trees. Fire blight, because it leads to dead wood easily infected by the black rot fungus, can make a planting more conducive to black rot infections.
Infected wood and mummified fruit are a source of infectious spores. The black rot fungi survive Minnesota winters in branch cankers and mummified fruit attached to the tree. In wet weather, spores are released and start new infections within the tree. Most new infections occur in spring.
Leaf spots do not seriously affect the health of the tree unless many leaves turn yellow and fall off as a result of the infection (this is rare). In addition, leaf spots do not release fungal spores that start new infections on fruit or branches. Leaf spots are therefore of minor concern and management practices focus on reducing fruit and branch infections.
To control this disease, remove dead material from the planting. Prune out dead or diseased branches, and pick all mummy fruits remaining on the trees, as these are sources of spores for future infections. All infected plant parts should be burned, buried, included it in household trash, or sent to a municipal composting site. Be sure to remove the stumps of any apple trees you cut down, as dead stumps can be a source of spores.
All winter pruning, of healthy or dead tissue, must be completed during freezing weather, as the fungus is not active until spring. Pruning cuts made in winter will have dried out and will not be susceptible to the disease by the time black rot spores are available to infect them.
Maintain tree health by reducing stress from environmental conditions and cultural practices. Choose an appropriate site for all new apple trees and plant only cultivars that are truly hardy for your area. Provide trees with adequate water. Keeping fire blight in check and removing any limbs or trees killed by fire blight can discourage black rot.
Fungicide sprays are typically unnecessary for black rot management in Minnesota. Use fungicides only if the disease has persisted after cultural control practices have been implemented.
Captan and sulfur products are labeled for control of both scab and black rot, so a scab spray program including these chemicals may help prevent the frog-eye leafspot of black rot, as well as the infection of fruit. These sprays will not control or prevent infection of branches.