Apple Scab, caused by Venturia inaequalisThis species overwinters among fallen, infected leaves on the ground, in the state of immature pseudothecia, which constitute the fungal reproductive structure. When spring comes the pseudothecia mature and produce ascospores, which are ejected when the temperature and rainfall are just right. The ascospores can infect tissue of leaves, petioles, stems, blossoms and fruit.

As the fungus grows through the tissue it produces a new kind of spore, called conidia. The infected tissue then becomes covered with brown to olive-brown lesions covered with more conidia. The apple in the picture shows the disease's typical cracks in a brown "scab" area. Once the conidia mature they are blown or otherwise spread to new tissue and can cause a new infection cycle before the growing season ends. The fungus overwinters in infected material that falls to the ground, and the next spring the cycle starts all over again.