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. According to a recently adopted convention in botany, the asexual form of Venturia inaequalis has a different species name, Spilocaea pomi . This naming convention has been adopted to differentiate the asexual forms of certain groups of fungi because these stages are so distinct from the sexual forms.