The yellow-orange spots on your crabapple tree are a fungus called cedar-apple rust.

Cedar-apple, cedar-hawthorn and cedar-quince rust are a series of fungal diseases that depend on two host plants to complete their life cycle. The rust disease spends part of the year on a susceptible juniper and part on a susceptible apple, crabapple, hawthorn or quince, on which they cause premature defoliation, distorted or deeply pitted fruit and fruit abortion.

Cedar-apple rust is by far the most serious and widespread of these rusts in this area.

The symptoms of cedar-apple rust on flowering crab and apple are very easy to spot:

In late spring or early summer, bright, yellow-orange spots about 1/8 to 1/4 inch in diameter appear on the upper surface of the leaves. The spots slowly enlarge and turn orange. Eventually an orange cup-like fungal structure appears on the lower portion of the leaf directly beneath the lesion on the upper surface. Leaves with numerous spots drop during the summer. Cedar-apple rust also can cause fruit lesions. The affected fruit usually develops deep pits or becomes distorted, and normally drops before harvest.

Premature defoliation weakens the tree and reduces fruit set and yields the following year. Trees with severe defoliation are at risk for other diseases.

Cool temperatures (from 50 to 75 degrees) and prolonged leaf wetness (4-6 hours) favor infection of flowering crabs and apples with the rust. The lesions begin to develop about 3 weeks after infection. As with many fungal diseases, the weather conditions determine the severity of infection.

To control cedar-apple rust avoid planting apples of flowering crabs adjacent to junipers. Keeping apple and cedar trees sufficiently distant from each other can accomplish control of cedar apple rust on apple, so that the fungus cannot complete its two-year life cycle. But this is hardly practical or even possible, as the fungal spores can be carried by wind for up to 2 to 3 miles, and junipers are heavily planted in urban and rural areas. Plant apples or crabapples that are resistant to the disease, and apply fungicides in the spring as soon as the bright orange gelatinous tendrils of the galls are seen on junipers. Continue spraying every 7 to 10 days for as long as the galls remain active (usually until the end of May). The same spray schedule is used for control of cedar-quince rust and cedar-hawthorn rust. Call your Cooperative Extension Service for current information on the best fungicides to use, and for a list of resistant or tolerant flowering crabapples, apples, hawthorns and quinces.

For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service