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Apple Scab

Fruits Affected

Apple

Also See

Fruit Tree Diseases and Pests: An Introduction
List of Common Fruit Tree Diseases and Pests

Apple scab is caused by the fungus Venturia inaequalis. This fungus overwinters on fallen diseased leaves, and in spring, the fungi shoot spores into the air. The fresh spores are carried by water or wind to newly developing leaves, flowers, fruit or twigs. Spores need several hours of moisture on the plant surface in order to start new infections, and the new infections grow into spots that can produce more spores within a week or two. The infection cycle can repeat many times throughout the growing season whenever leaves remain wet for long enough. Warm, rainy weather in the spring and summer creates ideal conditions for apple scab, as the moisture both allows the disease to enter the tree and facilitates spreading of spores.


The symptoms of this disease appear on the leaves and fruit. Leaves develop round, velvety, olive-green spots with fringed borders. These spots can expand and merge. Infected leaves may turn yellow and drop by mid-summer, which weakens the tree. Several years of early leaf loss can result in decreased growth, reduced bloom, and increased susceptibility to winter injury. Infections on fruit are easiest to positively identify; these appear as olive-green spots that can become brown, black, corky, and cracked over time.


The first and best defense against apple scab is to plant resistant varieties. Some varieties, such as McIntosh and Cortland are notoriously susceptible to scab, while others, especially modern cultivars bred for especially for disease resistance, are immune or resistant, e.g. Akane, Freedom, Liberty, and Pristine. The majority of apples occupy a middle ground, and moderate levels of scab will be tolerable for most home orchardists but not for commercial growers. A few simple cultural practices can also significantly diminish scab levels. Dead leaves should be removed from the orchard floor in fall, and trees should be planted with sufficient spacing to ensure airflow. Likewise, annual pruning to keep the canopy open will help the tree dry quickly after rain. Remember: scab needs prolonged moisture to initiate infections.


For further information, see the Cornell Guide to Growing Fruit at Home and the Cornell Cooperative Extension Factsheet on Apple Scab.






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