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Apple ScabApple scab occurs in most areas of the world where apples are grown and is one of the most serious diseases of ornamental crabapple and fruit trees. It is caused by the fungus Venturia inaequalis. Apple scab occurs on the leaves, petioles, blossoms and fruit. Infections usually develop first on the undersides of leaves. Once the leaf has unfolded, both sides may be infected. Apple scab rarely kills a tree, though repeated infections will weaken the tree and increase its susceptibility to other problems. Unless severly infected, the apples are edible, though perhaps not aesthetically pleasing.

Symptoms appear as olive to velvety brown spots that turn black with age. At first, the margins of lesions are feathery and indifinite, but later they ar distinct. Severe infection can cause extensive defoliation. Trees defoliated 2-3 years in a row are weakened and susceptible to other issues. Fruit infections resemble leaf infections when young but become brown and corky with age. Scab infections result in uneven growth of fruit and cracking of the skin and flesh.

Apple scab spores are produced on fallen leaves that were previously infected and on the tree itself during the growing season. The spores spread by wind and rain and provide inoculum for thedisease the following year. The disease is most severe in years with cool, wet spring weather.

The apple scab fungus overwinters on fallen leaves. Any infected leaves that are left under the tree are sources of disease for the following growing season. Though it may be impossible to remove all of the leaves from the area removal of th majority of fallen leaves will greatly decrease the amount of inoculum in the area. To help prevent recurrence of the disease next year, remove and destroy leaf debris and infected fruit in fall. We control apple scab with a fungicide spray. The fungicide needs to be applied 3 times per year with the first 1 done at bud break and then followed up 3-4 weeks later for each additional spray. Like most fungal diseases, timing of the fungicide applications is important for control of the disease. Apple scab can be prevented but once it is noticed it can not be removed and you will only be able to prevent it from spreading further, the next year.