Fungi in the genus Venturia are microscopic members of the class Ascomycetes, spore-sac fungi.
They produce thread-like tubular mycelia that form between the leaf cuticle and epidermis. They form two types of spores: conidia (asexual) and ascospores (sexual). The latter are usually arranged in groups of eight in an ascus, a small sac inside which meiosis occurs.
They overwinter in the form of immature ascospores deep inside fallen leaves and fruit or hidden between the scales on buds.
In spring, the mature ascospores are released and dispersed by wind and rain. Given favourable weather conditions, they quickly form fructifications and the mycelia proliferate, producing scattered spots on the surface of wet leaves and fruit. This is the primary infection period, which lasts for six to ten weeks, from the time when the buds burst until late June.
Later, the scabs become downy looking, indicating that the mature mycelia have produced fructifications and summer spores (conidia). As these are dispersed, they spread the disease to other tissues. These secondary infections continue as long as weather conditions remain favourable.
In fall, the Venturia fungi invade the tissues more deeply, where they form overwintering reproductive structures that release ascospores the following spring.