Cedar-hawthorn rust is a fungal disease that commonly infects junipers and hawthorns. It is one of the most frequently observed plant diseases in spring and summer. The disease is caused by the pathogen Gynosporangium globosum. Cedar-hawthorn rust requires two hosts to complete its life cycle: a juniper, and a hawthorn. Several other plant species may also be infected.
Distribution & Habitat
Cedar-hawthorn rust occurs globally, wherever susceptible plants are present.
Cedar-hawthorn rust infects most varieties of eastern red cedar, as well as many other junipers. It may also infect numerous alternative hosts, including apples, crabapples, pears, quince, serviceberry, and hawthorns. Hawthorns are especially prone to infection.
Galls on infected junipers overwinter. During rainy periods in spring, gelatinous tendrils on the galls, often referred to as telial horns, erupt and produce spores, which are disseminated by air currents and splashes of rain, or carried by insects to the leaves of nearby hawthorn plants, and other hosts. The spores turn brown, and infect susceptible leaf tissue. Once the fungus has germinated within the leaf tissue, yellow spots form on the leaves. In late summer, spores are produced on the hawthorn leaves, which are borne to needle bases or cracks in juniper twigs. Small galls gradually develop on the juniper twigs and overwinter, completing the disease cycle.
Symptoms on Juniper
The cedar-hawthorn rust fungus produces reddish-brown galls on twigs, and small branches. Galls reach 1/8 to ½ of an inch in diameter. Immature galls are reddish-brown, while mature galls are grayish-brown. Once mature, circular depressions will appear on the galls. In spring, small brown structures begin to protrude from each of the depressions. These structures develop into an orange mass of telial horns, which contain infectious spores. Once the spores have been expelled from the galls, the galls become desiccated, and harden. Hardened galls may linger on junipers for several years.
Symptoms on Hawthorn and Other Alternative Hosts
Cedar-hawthorn rust causes discoloration on the leaves, fruit, petioles, and twigs of infected hosts. Discoloration is characterized by the appearance of tiny yellow spots, or lesions, on sections of the infected plant. As the growing season progresses, the lesions enlarge. They gradually turn orange, with small black dots forming in the center. By mid-summer, the telial horns will become visible on the underside of the mature leaf lesions, or within the lesions on the fruit, petioles, or twigs. On severely infected hosts, the foliage may turn bright yellow, and drop prematurely. Infected fruit will often rot, before being shed.
- Cedar-hawthorn rust seldom kills trees, but it can disfigure plants when twigs are infected. As such, prune out diseased branches. This will limit the spread of the fungus, and help to prevent other sections of the plant, and nearby plants from becoming infected.
- Fungicide applications can be performed during sporulation. Thorough coverage of the plant is required to effectively combat the disease. Hawthorns and fruit trees should be sprayed prior to their bloom period in spring. Fungicides should be administered to junipers three times, at two-week intervals, beginning in mid-July.
- Avoid planting susceptible hawthorns within a two mile radius of junipers.
- Cull severely infected hawthorns, and replace them with genetically resistant varieties.
Photo courtesy of Ohio State University