Sooty blotch and flyspeck
Fig. 1. Flyspeck and sooty blotch.
Photo: Mark Gleason, ISU.
Sooty blotch and flyspeck are sometimes called the "summer diseases” of apples, because the fungi that cause this problem typically appear in August or September. The damage these fungi cause is strictly cosmetic and does not affect the quality of the fruit.
Sooty blotch appears as grayish, greenish, brownish, or black smudges on the apple’s surface. It can be rubbed or washed off the fruit fairly easily. Flyspeck looks like clusters of tiny black dots. Flyspeck cannot be easily cleaned off the fruit.
Flyspeck and sooty blotch are caused by a complex of several different fungi. These fungi do not infect the live cells of the fruit and therefore do not cause fruit rot. The flyspeck and sooty blotch fungi only grow in the waxy cuticle on the fruit surface, resulting in minor blemishes. These fungi overwinter on the fruit, stems, leaves, and bark of many types of woody plants, including apples, pears, and raspberries. Spore development and germination are encouraged by warm, humid weather. Rain and wind spread the spores onto developing fruit. In Minnesota, flyspeck and sooty blotch occurs more commonly in the southern half of the state.
Since these fungal infections do not affect fruit quality, and sooty blotch can easily be cleaned off the apples, the home grower need not take any action against them. However, pruning that allows good air circulation in the tree canopy will help minimize the severity of infection. Similarly, thinning fruit to only one per cluster allows good air circulation around each individual fruit. After a rain, if the fruit can dry off quickly enough, the spores may not be able to germinate and cause infection.