The photomicrograph below illustrates a stained thin section of apple tree stem infected with apple scab, a destructive disease of apple trees caused by the fungus Venturia inaequalis .
Apple scab is one of the most serious diseases of apple trees and occurs wherever apples are grown.
It can also infect crab apple and mountain-ash trees. Losses result from scab-like spots on fruit or leaves. If enough leaves are damaged, the tree is left nearly defoliated and unable to grow or produce fruit. A fungus, Venturia inaequalis , causes Apple scab. This fungus reproduces both sexually and asexually and the spores from both types of reproduction can infect plants.