The disease "Apple scab", which is caused by the fungus Venturia inaequalis (Pleosporales; Venturia) is one of the most economically important diseases worldwide. Apple scab occurs in every apple growing area worldwide and has a faculative parasitc life cycle (also saphrophytical growth possible).
Under unfavourable conditions (no vegetation, during winter time) it survives in a stage of dormancy (fruit (sexual) form, called ascocarp (pseudothecium) on leaves on the soil surface. Another form of hibernation of the fungus is in dormant pustules on shoots or buds.
Pseudothecium (sexuell form) are formed in infected plant tissue at the end of the saison. They are 90-160 µm in diameter and can be seen with the naked eye or with a hand lens. After wintertime the diploid pseudothecium starts to form numerous asci each containing 8 ascospores. These ascospores are discharged by rain, wind and leaf wetness and are responsible for the infection of the green tissue of the apple trees. Water is one of the most important factors for fungal growth and reproduction, ascospores and conidia are germinating in free water. The germination tube forms an appressorium (pressure, enzyme) and enters the cuticle of the plant. During the summer season the fungus get dispersed by conidia, called sencondary inoculum, that cause secondary infections on leaves, fruit and shoots during wet weather.
Venturia inaequalis colonizes only species of the genus Malus. Within species, varieties and cultivars different levels of resistances are shown. Leaves and fruits become more resistant as they mature.
For information about the discharge and infection by primary and secondary spores refer to the pages.