By the time the leaf spots become visible, the events that contributed to their development are history and it is too late to apply control measures. Fortunately, the fungi causing leaf spotting in the northeast do not have secondary cycles on leaves. Therefore, the most of the spotting will be limited to those leaves that became infected during the extended wetting periods prior to petal fall or first cover. Leaf spots can be caused by spray injury or by a variety of different fungi that attack leaves during wet weather. The exact causes of leaf spotting are often difficult to determine because spots attributable to various causes all look very similar.
Most leaf spots are uniformly circular and one to four millimeters in diameter.
Individual leaves may have a single spot or as many as 30-50 spots. Severely affected leaves will turn yellow and drop from the tree within the next several weeks, but the number of leaves lost is usually insignificant and does not adversely affect fruit growth. Where spotting is limited to only a few spots per leaf, the leaves will survive and remain green. The black rot fungus is probably the most common cause of leaf spotting. As already noted, other fungi can cause similar symptoms, so the spots themselves are not diagnostic.