Apple: Cedar-Apple Rust

Problem
Pale yellow spots appear on the upper surfaces of leaves and on fruit in mid- to late spring. These spots gradually enlarge, turn orange, and develop minute black dots. Small (1/6 inch) cups with fringed edges form on the lower surfaces of the leaves. Infected leaves and fruit may drop prematurely; the fruit is often small and deformed.

Analysis
This plant disease is caused by a fungus (Gymnosporangium juniperi-virginianae) that affects both apples and certain species of juniper and red cedar. This fungus cannot spread from apple to apple, or juniper to juniper, but must alternate between the two. In the spring, spores from brown and orange galls on juniper or cedar are blown up to 3 miles to apple trees. During mild, wet weather, the spores germinate and infect the leaves and fruit, causing spotting, and eventually premature leaf and apple drop. During the summer, spores are produced in the small cups on the underside of the leaves. These spores are blown back to junipers and cedars, causing new infections and starting the cycle over again. For more information about cedar-apple rust on junipers, see Cedar-Apple Rust.

Solution
Cedar-apple rust cannot be controlled on this season's apples and leaves. Next spring, spray apple trees with a fungicide when the flower buds turn pink, again when 75 percent of the petals have fallen from the blossoms, and once more 10 days later. When practical, do not plant apples within several hundred yards of junipers or red cedar.