crap apple scum

Asked October 21, 2015, 7:44 PM EDT

we always get a scum on our crabapples. our kids eat them as soon as they start becoming apples, and we havent noticed anything there, but still curious as to whats going on as im having many people tell me they havent seen that like that before.. in the photo, the apple on the left has been scrubbed, the apple on the right is one of the more bad examples, most have less than this.

St. Louis County Minnesota

1 Response

Thank you for the question. From your description and photo, it looks like your crabapples have sooty blotch, a fungal infection that infects the surface of the apple and can be vigorously rubbed off. This is mainly a cosmetic problem and the fruit is still edible. This disease develops during the summer during cool rainy weather, particularly in dense, unpruned trees with poor air circulation.
To control this disease, prune your trees to keep them from becoming too dense. The best time to do this is late winter/early spring.
Here is our publication on proper pruning of trees and shrubs:

Thank you for contacting Extension.