Cedar-apple rust (Gymnosporangium juniperivirginianae), cedar-hawthorn rust (G. globosum), and cedar-quince rust (G. clavipes) are closely related rust diseases that require two hosts to complete their life cycle. All three rusts can infect most varieties of eastern red cedar (Juniperus virginiana) as well as many other junipers and an alternate host. Cedar-hawthorn rust, in addition to affecting apples, crabapples, and many hawthorns sometimes infects pears, quince, and serviceberry.

Symptoms and Diagnosis

Symptoms on junipers: The fungus produces reddish-brown galls on twigs and small branches of susceptible junipers. After reaching a diameter of about 1/8 to 1/2 inch, circular dimple-like depressions will appear resembling the surface of a golf ball. In the spring, small chestnut brown structures protrude from each of the dimples. During wet spring weather, these projections greatly expand into an orange mass of spore-bearing, jelly-like tendrils known as telial horns. Blown by the wind, the microscopic spores infect neighboring hawthorn trees. The galls produce spores at least two years in succession after which they dry out and harden. They can remain on the juniper host for several years.

Symptoms on hawthorns: Cedar-hawthorn rust causes discoloring and yellow to nearly black spots on the leaves, fruit, petioles, or new twigs. These spots contain black pimple-like fruiting bodies that produce spores.

Life Cycle

The disease overwinters as galls on infected junipers. During wet spring weather, "horns" extrude from the galls and produce spores. These spores are then windblown or carried by insects to the leaves of nearby susceptible hawthorn plants. The spores then turn brown, infect the leaf tissue, and form yellow spots on the leaves. In the late summer, spores are produced on hawthorn leaves. They are windblown back to needle bases or cracks in juniper twigs. After infection of the juniper, small galls form, thus completing the cycle. It takes two years to produce a spore-bearing gall.

Integrated Pest Management Strategies

1. For both hawthorns and junipers, prune out diseased branches. The spread of cedar-hawthorn rust can be limited by reducing infested plant parts.

2. Live with the disease. The disease rarely kills trees. It can, however, disfigure plants when twigs are infected.

3. Use fungicidal sprays. Both junipers and hawthorns can be protected from infection with fungicide applications during periods of spore production. Spray hawthorns prior to spring bloom with a fungicide. Pesticides registered for use include chlorothalonil (Daconil) and mancozeb. Thorough uniform coverage of plant surfaces is essential for good disease control. Spray foliage to the point of runoff. Repeat on a 7–10 day schedule while cloudy and/or wet conditions favorable for disease development are common. Spray junipers three times, at two-week intervals, beginning in mid-July, with a fungicide such as Acme Bordeaux Mixture.

4. Avoid planting susceptible hawthorns within a two-mile radius of junipers. (This may be impractical in an urban environment.)

5. Replace infected trees with genetically resistant hawthorn varieties.

Organic Strategies

Strategies 1, 2, 4, and 5 are strictly organic approaches.