Fruits Affected

Apple

Also See

Cedar-apple rust (CAR) is the most common of the three rusts that infect apple trees (the other two being quince rust and hawthorne rust). CAR is a fungus that requires plants from both the Cupressaceae family (red cedar, juniper) and the Rosaceae family (crabapple, hawthorn, serviceberry) to complete its lifecycle.

Symptoms

Red-black spots and visible spores on undersides of leaves; defoliation.

Organic Treatments

Some apple varieties are very susceptible to CAR and infections may result in severe defoliation and crop loss. The best approach to CAR is prevention. Growers who live in areas where pressure is high because there are many juniper or red cedar trees should plant CAR-resistant cultivars, such as Williams Pride, Liberty, and Sundance. There are plenty to choose from. Another sensible preventative measure is to inspect nearby juniper and red cedar trees in spring, and to prune out any galls before they form their crazy spore-bearing growths. Dead leaves and debris should routinely be removed from the orchard floor; this is the primary preventative step for many fungal infections.

Conventional Treatments

Mankozeb (Penkozeb) is the most effective chemical for controlling cedar-apple rust.

Disease Cycle

Cedar-apple rust (CAR) is the most common of the three rusts that infect apple trees (the other two being quince rust and hawthorne rust). CAR is a fungus that requires plants from both the Cupressaceae family (red cedar, juniper) and the Rosaceae family (crabapple, hawthorn, serviceberry) to complete its lifecycle. CAR over-winters on infected branches and galls on juniper and red cedar trees. In wet spring weather, the galls produce truly bizarre-looking, gelatinous, orange fungal growths that open up and release spores. The spores are carried by wind from infected junipers to susceptible apples, and apple trees over a mile away can be infected. The infections on the apple tree grow slowly over the summer, and in mid-to-late summer the infected leaves and fruit will release powdery orange or brown spores. These spores cannot infect apple trees, and they must be carried by wind to start new infections on young needles and shoots of juniper or red cedar trees. It then takes up to two years for new galls to form on the juniper or red cedar tree.

The first symptoms on an apple tree will be bright red spots on the leaves. Small black dots will eventually appear inside the spots, and on the underside of the leaves, tiny finger-like fungal tubes will protrude from the dark areas. These tubes, which are a mini version of the orange space monsters that the junipers produce, will open to release the spores that will infect nearby red cedars or junipers.

For more information, see the Cornell Guide to Growing Fruit at Home and the Cornell IPM Factsheet for Cedar-Apple Rust.

Photo courtesy of Dr. David Rosenberger, Cornell University.