Wet weather in April and May builds potential for apple scab, a fungal disease of apples.
The apple scab disease fungus overwinters on dead apple leaves and fruit left on the ground, explained Ross Penhallegon, horticulturist for the Oregon State University Extension Service. During spring moisture, scab spores are forcibly discharged and ride air currents to infect developing leaves and fruit of apples. All outer parts of unopened fruit buds are highly susceptible to scab. As the fruit matures it is much less susceptible.
The first visible symptoms of apple scab in the spring are pale, water-soaked spots the size of a pinhead on the new leaves. These spots enlarge, become darker and smoky colored. Later, the spots turn brownish-black color. Spots may be any shape, but tend to be circular, said Penhallegon. Diseased leaves may be curled, distorted and drop off early. Heavy infections can defoliate and weaken your apple trees.
On the fruit, the symptoms of scab include small raised brown or black circular areas (scabs). The skin breaks later in the season and the exposed tissue turns velvety brown or black. As the fruit enlarges, the scab spots become brown and corky. To help control apple scab, Penhallegon recommends:
- Grow scab resistant cultivars of apples. Apples with good resistance include Akane, Chehalis, Liberty, Prima and Tydeman Red. - Apply nitrogen to leaves in the fall to enhance decomposition of fallen leaves and make them more palatable to earthworms. - Shred fallen leaves in the fall with a mower to help speed up decomposition. - Prune your apple trees to open up branching and allow more air circulation. - When watering your apple trees, avoid getting foliage wet. - Apply dolomitic lime in the fall, after leaf drop, to increase pH and to help reduce fungal spores in the spring. - For chemical control, consult the current "Pacific Northwest Plant Disease Control Handbook," on file at your local county office of the OSU Extension Service.
For more information about scab resistant varieties, the OSU Extension Service offers a circular, EC 1334, Scab-immune apple varieties for new orchards, by Robert L. Stebbins, Extension horticulture specialist emeritus. The publication talks about prevention of apple scab, origin of scab-immune varieties, and detailed descriptions of 11 apple varieties found to be immune to apple scab in Corvallis. Visit our on-line catalog. Our publications and video catalog at: https://catalog.extension.oregonstate.edu/ shows which publications are available on the Web and which can be ordered as printed publications.