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Gymnosporangium rusts infect trees and shrubs in the cypress family alternately with those in the rose family. On cypress hosts they cause galls, stem swellings, and twig dieback. Symptoms on Rosaceous hosts include colorful spots on leaves and fruits. Of the 36 species of Gymnosporangium in North America, there are only 3 of concern in the Northeast. Cedar Apple Rust (pathogen: Gymnosporangium juniperi-virginiae) Alternate hosts are eastern red cedar and juniper, and apple and crabapple trees. Junipers become infected in late summer or autumn. Galls, formed from a combination of plant and fungus tissue, are first evident as green swellings on leaves. When mature, the kidney-shaped brown galls are up to 2 inches in diameter. During spring rains, the galls swell and extrude bright orange gelatinous horn-like projections called telia which are comprised of fungus spores infectious to apple and crabapple leaves and fruit. Galls on junipers die after producing spores for one year, but remain attached to the twig for several years. Some twig dieback may result, although damage to junipers is generally minor. Small, yellowish-orange spots appear on infected apple and crabapple leaves in the spring. These spots continue to enlarge and turn more orange, often with a red border. The fungus grows through the leaf, and fringe-like structures arranged in a ring appear on the underside of the leaf in midsummer. These structures produce spores which are infectious to junipers. Leaf drop may result from a heavy infection. Fruit may also be susceptible to infection, evidenced by a general russetting and the same symptoms as leaves. Hawthorn Rust (pathogen: Gymnosporangium globosum): Hawthorn rust is very similar to cedar apple rust, except that it attacks a wider host range in the Rosaceae family. Hawthorn rust alternates between eastern red cedar, common and prostrate juniper and apple, crabapple, hawthorn, and sometimes pear, quince and serviceberry. On juniper, the reddish-brown galls are smaller, usually less than 12 mm (1/2 inch), and more spherical than those caused by cedar apple rust. The telia are chestnut brown. Galls produced by hawthorn rust may produce spores for 3-5 years. Significant twig dieback can result from this disease. On hawthorn, leaf spots are similar to those caused by cedar apple rust. If infection is heavy, hawthorn leaves may yellow and drop prematurely. Fruit and green wood may also be affected. Quince Rust (pathogen: Gymnosporangium clavipes): Quince rust is the most damaging of the three rust diseases, with the widest host range in the Rosaceae family-- 480 sp. in 11 genera (apple, mountain ash, chokeberry, cotoneaster, hawthorn, medlar, pear, photinia, quince, flowering quince, and serviceberry), although it is most often seen on hawthorns in the Northeast. Quince rust generally affects stems and fruit rather than leaves, unlike cedar apple rust and hawthorn rust. It is also a perennial disease, as the fungus overwinters in cankers on twigs. On junipers, quince rust infects leaves and young wood, causing swellings that girdle twigs and branches. From these swellings, spores are produced which are infectious to Rosaceae hosts. Diseased twigs and branches often die. Some twigs survive and a perennial infection ensues in which spores are produced annually. Heavy infection results in dieback and loss of vigor. On Rosaceous hosts, infection is usually characterized by a spindle-shaped swelling on twigs. Fungal mycelium grows from the site of infection through the stem to the buds, causing them to open prematurely. Leaves emerge stunted and misshapen. Hawthorn leaves yellow and drop when infected. Quince rust girdles most infected stems in the second year, causing death of the twig. The fungus overwinters at the edge of the canker. Infected fruits of hawthorn and serviceberry shrivel and die. White tube-like structures up to 6 mm (1/2 inch) in length emerge from cankers on twigs and branches, as well as from infected fruit, releasing masses of bright orange spores which are infectious to junipers. Control: Since rust has an alternate host cycle requiring both susceptible junipers and Rosaceous hosts to be present, the most obvious method of control is to eliminate one of the host groups. This may not be a practical option, since junipers are widespread and spores are wind-dispersed over long distances. Junipers can be examined and galls of cedar apple rust or cedar hawthorn rust removed to prevent infection of Rosaceous hosts. The most effective means of control is to plant cultivars with proven resistance to rust disease. Resistant juniper (Juniperus) cultivars and varieties: J. chinensis
'Foemina' Resistant
hawthorn (Crataegus) cultivars: Resistant crabapple (Malus) cultivars: Adams
Adapted
from Ohio State University Cooperative Extension, 1999.
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