Dirty Film on Fruit
For the past two years our Red Delicious and Cortland apples have been covered with a dirty film. What causes this and what can be done about it?
Sooty blotch and flyspeck are two fungal diseases that can cause the problem you describe. These diseases make the fruit unsightly but do not affect its flavor. Wash off the sooty covering before storing or eating. Infected skins can be removed before eating. Prune the trees to improve air circulation and reduce disease problems. If the problem continues you can apply a fungicide labeled for fruit trees in spring, and through the cool periods of fall. Be sure to read and follow all label directions before using these or any chemicals.
Photo: Clemson University - USDA Cooperative Extension Slide Series