Diseases of Apple Trees - Black Rot
The most-common disease pests of apples are fireblight, apple scab and black rot canker. Black rot canker has been the most common issue, and the rest of this article will cover this disease and the associated problem known as frogeye leaf spot. For more information about fireblight and apple scab, see the NDSU Extension publication “Insect and Disease Management Guide for Woody Plants in North Dakota” (http://www.ag.ndsu.edu/pubs/plantsci/trees/f1192w.htm).Black rot is a canker disease caused by the fungus (Botryosphaeria obtusa). When the fungus infects stems or branches, it causes cankers which tend to grow more quickly along the length of the branch, compared to going around the branch. A canker will typically be sunken, have darkened bark, and have small bumps that are the fruiting bodies of the fungus. As the canker develops and expands around the branches, the leaves on the girdled branches will turn bright yellow and fall to the ground, even in the middle of summer (Figure 1). As the canker continues to develop, entire branches or stems will be girdled and killed. Infections can also occur in the outer bark, which is dead. Outer bark infections are not sunken and cause no damage, but contain fungal fruiting bodies that can serve as a source of spores that cause new infections. Infections in the outer bark can develop into cankers if the tree is wounded or stressed.
The fungus also causes a leaf disease called frogeye leaf spot (Figure 2). Frogeye leaf spots are typically chocolate brown with a dark ring around the edge of the spot, and the margin of the spot is sharply defined. The infections on leaves typically occur early in the season and take place during cool, wet weather. If there are several leaf spots, or if leaf spots occur near the leaf petiole, the leaf will turn yellow and drop.
New infections on branches and stems occur through wounds – pruning cuts, hail damage, or tissue damaged during the previous winter. While old infections cannot be cured, there are several steps that we can take to prevent new infections or slow development of cankers. Avoiding stress to the tree allows the tree to resist initial canker infection and expansion of existing cankers. Be careful to not wound the tree with mowers or weed trimmers and minimize the use of herbicides – both regular herbicides and those used in weed-and-feed formulations with fertilizer – within the dripline of the tree. If the cankers are confined to a manageable level, branches with cankers should be pruned out, several inches below the most basal portion of the canker. Do not leave branch stubs that can serve as an entry point for the fungus. Pruning apple trees is best accomplished during the dormant season to minimize chances of infection by fireblight. If apple trees are pruned during the growing season, applying streptomycin or a copper-based fungicide will help reduce the risk of fireblight infection. If branches are broken during a windstorm or if bark is damaged by hail, treating the wounds with a copper-based fungicide will help reduce the risk of infection by both black rot and fireblight. Pruning of outer bark infections is not practical because they are usually too numerous and are not yet causing damage; instead, outer bark infections are best taken as an indication that care should be taken to avoid wounds and stress to the tree. If the fruit load is very heavy, consider thinning out the number of fruits to about one apple per six inches of branch.
Joe Zeleznik - Extension Forester
Jim Walla - Forest Pathologist