This is a fungus (Gymnosporangium juniperi-virginiae) that causes a common disease called cedar-apple rust. This disease must pass from junipers to apple and then from apple to juniper. The cedar-apple rust galls on the junipers release millions of spores during rainy periods in the spring. These spores can infect apples and crab apples and cause leaf spots. However, many of the newer crab apples and some fruiting apples are resistant. Though they may show some signs of the disease, they won't defoliate (drop leaves) like susceptible varieties. Even susceptible varieties that defoliate will produce a new set of leaves if they were healthy before infection. Significant damage to crab apples is rare. However, fruiting apples pour a great deal of energy into the fruit and may be stressed more severely. Infections can also be prevented on apples and crabapples by applying fungicides during the spring when the galls on the junipers are active (jellylike). Contact your local Extension office for fungicide recommendations. If you don't want to apply fungicides, use flowering crabapple or apple varieties that are resistant.

Browse related Faqs by tag: horticulture, ornamental trees, plant dieases and disorders, cedar, junipers, cedar-apple rust, gymnosporangiumjuniperi virginiae