Cedar-apple fungal disease, called cedar-apple rust (Gymnosporangium juniperivirginianae), attacks numerous varieties of apples plus pears and other fruit in addition to cedar trees and other members of the juniper family. The disease winters on cedar trees and juniper hosts; it summers on apple trees and other susceptible fruit trees.
Cedar-Apple Rust Hosts
Numerous commercially grown varieties of apples are susceptible. Crabapple, quince, pears and serviceberries also host the disease. Hosts in the juniper family include the eastern and southern red cedars, Chinese juniper and Rocky Mountain juniper.
Symptoms appear on the leaves more than the fruit of apple, pear and other fruit trees. Bright orange, glistening lesions form on the leaves. Tufts of spore-producing structures appear on the bottoms of the leaves in late summer. Bright orange lesions appear on the fruit. These may look brown and cracked as the fruit gets larger. Some harvested fruit will still have the orange color. Lesions on the fruit produce fewer spores than those on leaves. Both the leaves and fruit may drop prematurely.
On cedar trees, cedar-apple rust forms brown, round galls 1/4 to 2 inches in diameter. They are dimpled like a golf ball when the tree is dormant, but produce gelatinous, orange spore horns during spring rain.
Cedar-apple rust spends the winter as galls on cedar trees. When the spring rains come, the galls produce spores that the wind spreads to apple and other susceptible fruit trees. Lesions begin appearing on apple trees 10 to 14 days after they are infected. In late summer, the leaves of infected trees produce spores that infect cedar trees and the cycle of disease repeats itself.
Contact your agricultural extension service for varieties of trees in your area that resist cedar-apple rust. Because the wind can blow spores for several miles, it is not always practical to remove galls from juniper hosts before they release spores. You can remove host junipers close to your fruit trees
Severe infestations of cedar-apple rust may be managed with fungicides. These fungicides include those that contain active ingredients of chlorothalonil, copper, mancozeb, myclobutanil, propiconazole or sulfur. Check the labels of fungicides to determine which ones are registered by the EPA for treating cedar-apple rust. Follow the directions on the label.
Spray fruit trees at seven-day intervals beginning at blossoming and continuing until the cedar galls stop spreading spores. Spray cedars every two weeks from June through September, a period when the leaves are being infected by spores from fruit trees.