Apple Diseases (cont)
By: Jack Ruttle
Cedar Apple Rust (Gymnosporangium juniperi-virginianae)
Cedar rust can defoliate trees and disfigure fruit wherever Juniperus virginiana, the red cedar, or Rocky Mountain cedar, J. scopulorum, is established. After the first eight weeks of the season, the rust threat is over.
When the apple buds are opening, small brown galls on the cedars are swelling with rainwater, becoming bright orange and brandishing gelatinous horns that release spores. This occurs both during rain and when the relative humidity more than 85 percent. The spores can travel up to five miles, but the problem is severe when the source of infection is abundant and nearby. After six to eight weeks, the galls have exhausted their spores and infection stops.
On the apple leaves the spores need four to six hours of continuous wetness to germinate. Symptoms (circular yellow spots with red halos) appear two weeks later. Six to eight weeks after that, the rust infections produce different spores that can only infect cedars. The cedar infections establish themselves in the fall and take the entire following year to form the mature galls.
Generally scab sprays will control cedar apple rust, too. It's also very helpful to remove any cedar trees near your apples. Quite a few apple varieties (not just the new disease-resistant ones) can tolerate moderate rust infections.
Powdery Mildew (Podosphaera leuchotricha)
Mildew destroys new shoots on apples and gradually saps the tree's vigor. Buds lose cold hardiness. Damage to the fruit is a cosmetic netting on the skin that doesn't affect eating quality.
Unlike scab and rust, mildew spores will not germinate and grow on wet leaves. They do, however, need 90 percent humidity and temperatures of 65 degrees F. to 75 degrees F. The problem can be severe in the West when dense foliage and transpiration create high humidity in the air immediately around leaves and shoots. The disease overwinters in buds infected the previous season, which open late. The spores grow on the young leaves and shoots. Symptoms appear 48 hours after infection.
Infected leaves are narrow, don't open properly and eventually become brittle. As the infection produces thousands of new spores, the vegetation becomes powdery white. The infection can spread down the shoot to older leaves and infect dormant buds to restart the cycle next year. Spores can travel many miles on the wind.
The infection moves from shoot to shoot and tree to tree, so it is crucial to keep the spore levels in your trees low. A single very mildew-susceptible cultivar can eventually create a problem for varieties that ordinarily are quite resistant. In the fruit garden, you can prune and destroy infected shoots as they appear, and spray only for severe infections.
Jack Ruttle is a former editor at National Gardening.