Occurrence: Spots usually become noticeable from late June through August. The fungi is present throughout the growing season.
Leaf spot are caused by a fungus infections that may start early in the growing season and can lead to premature defoliation. The disease is spread by the production of microscopic spores from early spring into summer and move by air currents, splashing rains, and insects. A few spots on the leaves do little harm to a tree, but two or more successive years of activity can seriously weaken a tree, reduce its growth, and increase its susceptibility to bark borers, winter injury, and other diseases.
Most leaf spot diseases develop as small, circular to oval dead areas scattered in the leaves. These spots are yellow, tan, dark brown, gray, purple and/or black. Some spots are raised, shiny, and coal black, others may drop out, leaving ragged holes; some are light and dark concentric zones. Heavily infected leaves may turn yellow to brown, wither, and drop early, weakening the tree.
Conditions Favoring Disease Development: Cool weather, light and frequent rains, fog or heavy dews, high humidity, and crowded or shady plantings.
Protective control measures are not generally warranted for most leaf spots. Picking up fallen leaves and disposing of them is a good practice, but there is little evidence that this will prevent future occurrences of the disease. In early spring, properly fertilize trees that have been severely defoliated in previous years to help stimulate vigorous growth. Water thoroughly to a depth of 12 inches at weekly intervals during extended dry periods. Prune trees regularly to thin out dense crowns. Remove weak, diseased, or dead wood and crossing or rubbing branches. Proper pruning will promote air movement and stimulate vigorous growth. Preventative fungicide sprays are generally not recommended unless the health of the tree is in danger. Although some leaf spot diseases can be controlled by using fungicides. When treating with fungicides, follow the label instructions particularly relating to timing, number of treatments, and rates.