Apple Scab - Life Cycle

Life Cycle

The infection cycle begins in the springtime, when suitable temperatures and moisture promote the release of V. inaequalis ascospores from leaf litter around the base of previously infected trees. These spores rise into the air and land on the surface of a susceptible tree, where they germinate and form a germ tube that can directly penetrate the plant's waxy cuticle. A fungal mycelium forms between the cuticle and underlying epidermal tissue, starting as a yellow spot that grows and ruptures to reveal a black lesion bearing asexually as the conidia are released and germinate on fresh areas of the host tree, which in turn produce another generation of conidial spores. This cycle of secondary infections continues throughout the summer, until the leaves and fruit fall from the tree at the onset of winter.

Over the winter, V. inaequalis undergoes sexual reproduction in the leaf litter around the base of the tree, producing a new generation of ascospores that are released the following spring. Scab lesions located on the woody tissues may also overwinter in place, but will not undergo a sexual reproduction cycle; these lesions can still produce infective conidial spores in the spring.