Any of the three fungal diseases described here -- scab, cedar apple rust and powdery mildew -- can cause serious defoliation that threatens not just the quality of your apple crop but the future health of your trees as well.
Although you could switch over to the new, highly disease-resistant cultivars, maybe you already have well-established trees that you can't bear to give up. Or perhaps you're a fruit connoisseur who simply has to have 'Freyburg', 'Fuji', 'Esopus Spitzenburg' or some of the other highly flavored varieties that unfortunately are also highly disease prone. If so, you will need to spray your trees for good results. But you can still greatly reduce the number of sprays by understanding the life cycle of these diseases and coordinating your spraying with the weather conditions that foster them.
Older fungicides like wettable sulfur (an organically approved fungicide) are protectants. They only prevent spores from germinating and do no good once the infection has occurred. These sprays generally need to be applied after every rain during infection periods. Newer fungicides, such as Neem oil and Streptomyes bacteria, prevent infection and are more environmentally friendly. Because the regulatory status of fungicides changes regularly and varies from state to state, consult your Extension fruit specialist for specific recommendations on the kinds you should use.
Apple Scab (Venturia inaequalis)
Scab can defoliate trees and disfigure fruit. Control the spring infection and there will be little or no secondary one. But once established, scab can be a season-long problem.
When apple buds first open in spring, infected leaves on the ground from the previous season eject spores into the air during humid weather. This can go on for the next two months, and the spores can travel about a mile. Once on the leaves and fruit, the scab spores need an extended peod in wetness (rain, mist or heavy dew) on the leaf to germinate and grow. At 50 degrees F., scab needs 13 hours of near-continuous wetness, but only nine hours when the temperature is between 62 degrees F. and 75 degrees F. Symptoms appear 10 to 20 days later. The first signs are dark olive green spots, which darken and harden as they age.
Later these infections can produce another generation of spores (and secondary infections) repeatedly through the summer whenever conditions are right (temperature in the 60s and 70s and wet periods of six to eight hours). These spores do not travel so far, and secondary infections are usually confined to the parent tree or adjacent foliage.
Controlling scab with sprays is a challenge. You must try to calculate infection periods by recording temperatures regularly. And you'll need to spray after rain, especially if you are using sulfur or other fungicides with no kickback. It's also helpful to rake up apple foliage in the fall and compost it thoroughly.
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.