|The apple scab fungus overwinters in infected leaves that have fallen to the ground. In these leaves, from autumn to early spring, the fungus mates, resulting in the development of what are termed ascospores, which cause primary infections. Ascospores mature as spring progresses with a few ascospores usually maturing by bud break (green tip). The proportion of ascospores maturing progresses slowly until about the tight cluster stage of blossom development.
|Season-long control is difficult if primary infections develop. Even moderate numbers of primary lesions can produce an extremely large population of secondary spores, conidia, requiring an intensive fungicide program to protect fruit throughout the summer. Conversely, good control of primary infections allows use of fungicides to be reduced or omitted during the summer, once ascospores have been depleted and fruit become less susceptible.
Control of primary infections has traditionally begun at or shortly after green tip, when the first ascospores become mature. The percentage of spores that are mature at this time is low, and the actual number of mature spores may be insignificant during the early stages of bud development if very little leaf scab developed the previous year.