Venturia inaequalis is an ascomycete fungus that causes apple scab disease
Life cycle and appearance of Apple scab
Venturia inaequalis overwinters in the form of unripe fruiting bodies in infected fallen leaves or in lesions on twigs. These unripe fruiting bodies, or pseudothecia ripen in winter and form ascospores. Depending on the temperature, these ascospores mature and are subsequently released when they become wet. The ascospores are wind-dispersed to the fruit trees and will germinate if free water is available. They infect the plant by direct penetration of the cuticula and then grow between the cuticula and the epidermis of the plant cells. This causes lesions, on which conidiophores, structures which carry conidia, are formed which extend through the cuticula. The asexual conidia are released and repeat the cycle of infection of new plant parts and spore production. In autumn, the fungus produces new pseudothecia in leaves that have already fallen off the trees.
In response to the infection, the plants produce cork as a defence against further growth of the fungus. In cultivars that are more resistant, this happens faster so the lesions remain smaller.
Based on the research that has been done on the influence of climatic conditions on release of ascospores and risk of infection, a decision support systems has been build. This system warns growers about conducive periods and when they are to be expected. Ascospores are released at heavy rainfall. They germinate between 6 and 26 °C. At optimum temperatures, between 18 and 24 °C, only 9 hours of leaf wetness is required, at lower temperatures this increases to 12-28 hours of leaf wetness for successful infection. Conidia also need water for germination.
Venturia inaequalis causes apple scab disease, which at first presents itself as lesions on blossoms, leaves, petioles and fruits and sometimes on twigs and bud scales. The lesions are olive-green. When spores are produced, the lesions become grey and fluffy. The lesions caused by ascospores are more restricted, whereas the lesions caused by conidia extend faster and may merge. Infected fruits and leaves may fall off prematurely. The plants’ defence mechanism is to produce corky rings around the lesions to prevent further invasion of tissue by the fungus. Late fruit infection only results in small dark lesions which may not be noticed before harvest.