Disease Cycle and Epidemiology

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Disease Cycle

In living leaves, the growth of the apple scab fungus is confined to the area between the host cuticle and the epidermis. It is not clear where the fungus obtains nutrients since it does not grow into the leaf cells, but it is generally believed that it degrades and utilizes the cuticle itself as well as some host cell wall and pectic material. Once infected leaves have fallen and died, the fungal mycelium colonizes them completely. Venturia inaequalis usually overwinters in the fallen leaves as mycelium and pseudothecial initials. In milder climates, the fungus may overwinter in infected buds and produce conidia which serve as primary inoculum. In leaf debris, most of the pseudothecial initials form within a month after the leaves fall, and then enter a period of dormancy.

In the spring, mating takes place in the leaves between two mating types. When the mature pseudothecia become wet, the asci expand through the top of the pseudothecium. The ascospores are forcibly discharged into the air where they are spread by the wind and by wind-blown rain. If there is sufficient moisture from rain or dew, the ascospores infect and produce lesions on apple blossoms and young unfolding leaves. In most years, ascospore release coincides with the several-week period from budbreak through the end of bloom.

From 9 to 30 days later, the primary lesions produce conidia, which are dispersed to other leaves and developing fruit where they establish secondary infections. Up to 100,000 conidia can be produced by a single lesion. Lesions from ascospore or conidial infection expand at a rate which is determined partly by temperature and partly by characteristics of the host tissue, including genotype and age. The rate of lesion expansion in turn affects the speed with which new spores are produced. During a cooler season, or on more resistant apple trees, lesions expand more slowly and may be smaller in size. As a result, the number of secondary cycles will be fewer.

The production of conidia is affected by humidity: levels of at least 60-70% are required for spore production. Infection of leaves by ascospores and conidia is highly dependent on how long the leaves or fruit stay wet, and on the average temperature. The Mills table relating leaf wetness duration and temperature is used to determine the likelihood that infection will occur if conidia are present (Click here for Figure 14, the Mills table). For example, at an average temperature of 18° C (65° F), light infection will result if leaves remain wet for nine hours, but if leaves are wet for 18 hours or more heavy infection will occur. Lesions will produce conidia after nine days if the temperature averages 18° C (65° F), but not until 17 days if the temperatures are lower, averaging only 8° C (49° F). The Mills table continues to be revised as more data are gathered from different regions.

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