Apple scab is a fungal disease that affects apple trees. It causes olive to brown spots on leaves, petioles, blossoms, and fruit. If left untreated, it can cause extensive defoliation and weaken trees over multiple years. The fungus overwinters in fallen leaves on the ground, so removing and destroying infected leaves and fruit in the fall helps prevent the disease the following year. An effective treatment is to apply fungicide sprays three times per year at bud break and again 3-4 weeks later for follow up applications, as fungal diseases require properly timed prevention. Once apple scab appears, it cannot be removed but future spreading can be prevented through ongoing fungicide use.

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Apple Scab
Apple 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 the
disease 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.