Disease: Apple Scab Pathogen: Venturia inaequalisHost: Apples (Malus X domestica)
Apples are popular backyard trees for homeowners who wish to grow their own fruit. This is the time of year that apples are being harvested and homeowners may begin to notice the blemishes and blotches on their fruits. Apple scab is one of the most important, if not the most important, disease of apples world wide, and it is caused by the fungus Venturia inaequalis. Symptoms of apple scab are shown in the figure and include the scabby lesions found on leaves and on fruits.
The fungus has two spore stages, one forming over the winter on the infected leaves that fall and accumulate beneath the trees. This is the stage that allows the fungus to infect newly expanding leaves in the spring. The second spore stage develops from the early infections. It is this second stage that causes the disfiguring lesions on leaves and fruits. Although unappealing, it is not dangerous to eat scabby apples. They are commonly used for cider and applesauce because they are vulnerable to decay in storage.
The disease is managed in several ways. First, some resistant cultivars are available. Second, fungicides are effective in reducing damage when applied regularly, and are most effective when begun when leaf buds open and continuing through the flowering stage. Third, infection of isolated individual trees can also be reduced by destroying all infected leaves beneath the tree.
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