There are 36 species of the genus Gymnosporangium that are known in North America. The genus Gymnosporagium is responsible for the fungus diseases that are commonly referred to as “cedar-apple rust.” Of the 36 species there are only three that are significant enough in the Northeast to warrant our concern.
Infected red cedars and other junipers (members of the genus Juniperus) form reddish brown, round to kidney shaped galls of woody tissue, up to 2 inches in diameter, in response to infection by this pathogen.
During moist spring weather, bright orange, cylindrical gelatinous tendrils are extruded from indentations on the galls. It is from the tendrils that infectious spores are produced for about two to three weeks. The spores are thin-walled and succumb easily to drying, a feature that causes many to be lost as they waft through the air. However, the successful ones land on leaves (occasionally fruit or twigs) of Malus sp. (apple, crabapple) where they germinate, penetrate into the tissue, and continue the disease cycle.
Symptoms on apple (and crabapple) appear as small greenish-yellow spots on upper surfaces of leaves in late spring. During the summer, these spots gradually enlarge and change to yellow-orange surrounded at the border by concentric red bands. Later, the undersides of these spots produce fringed, cup-shaped structures with short protruding “fingers” of spore bearing fungal tissue. Spores from these must find their way back to junipers for infection of foliage in late summer and fall.
Galls on junipers may grow for up to 5 years after infection, producing spores for all but the first of those years.
Damage on junipers is generally minor and involves growth of the galls and some twig dieback. On apples and crabapples, fruit infections and leaf drop also can occur. If infection is severe, enough defoliation may occur to weaken the plant.
Quince rust is the most lethal but least noticeable of these diseases on juniper. Infected areas on juniper are much less spectacular than those caused by cedar-apple rust. The pathogen causes junipers to produce perennial cigar-shaped galls that may eventually cause death of larger branches. Bark on infected twigs is usually cracked and rougher than surrounding healthy tissue. In late April and May, short, gelatinous “cushions” of spore-bearing tissue emerge from these swollen areas. As with cedar-apple rust, the spores from these must find their way to leaves of twigs of the alternate host.
On the broad-leaved hosts (hawthorn, quince, apple, and crabapple) this fungus causes distortion of twigs, buds, and fruit, but leaf spots are less commonly produced. On hawthorn, the most common host in the Northeast, fruits become shrunken and often die and twigs may become swollen and cankered. White tubes, about the size of pencil lead, protrude as far as 1/4 inch from the surface and bright orange spores are shed from these tubes and carried by wind back to junipers for fall infections.
Round galls on juniper twigs, seldom over 1/2 inch diameter, form in response to this disease. The galls are usually smaller and more irregular in shape than the cedar apple rust galls, and they produce shorter spore horns. They are mahogany-red with elevated areas on the surface from which tongue-shaped gelatinous tendrils protrude in moist weather. The galls remain functional for 3 to 5 years, producing tendrils in the spring from which spores may infect nearby rosaceous hosts. Yellow-orange leaf spots, primarily on hawthorns rather than crabapples and apples, are most common symptoms. On hawthorn fruits, light brown cups protrude. The cups produce orange spores that infect junipers.
These diseases are typically more harmful to broad-leaved hosts than to the junipers. They can cause reduced fruit size on apples and premature defoliation of hawthorn, apple, crabapple or mountain ash. G. clavipes also causes twig blight.
The best method of avoiding cedar rust diseases is to keep susceptible junipers far enough away from rosaceous hosts to minimize chances for successful exchange of spores.
Another valuable strategy is to use resistant cultivars when choosing new trees. Much progress in identifying resistant crabapple selections has been made, but one should not focus solely on rust resistance. Crabapples have several other important diseases and the best selections should be resistant to them, as well.
Rust on broad-leaved hosts can also be reduced with use of appropriate fungicides, but applications must be made before infection occurs. When you see one of the cedar rusts on infected hawthorn, apple or crabapple fruits and leaves, its far too late to spray that host for that year.
Control on Fruit Bearing Apple Trees (cedar-apple rust): Especially a problem during wet seasons. If wet weather prevails, apply myclobutanil.
Control on Flowering Crabapple (cedar-apple/cedar-hawthorn rust): Apply chlorothalonil, mancozeb, propiconazole, thiophanate-methyl, or myclobutanil when the sphere-shaped galls on nearby junipers produce jelly-like, and sometimes stringy, orange masses. Use according to label directions.
Control on Hawthorn (cedar-apple/cedar-hawthorn rust): Apply chlorothalonil,or mancozeb according to label directions. Begin when orange masses appear on juniper.
Control of Rust on Quince (Cydonia sp.): Apply sulfur (Bonide) according to label directions.
Control of Rust on Junipers (cedar-apple/cedar hawthorn rust): Spray with mancozeb, thiophanate-methyl, Heritage, Kop-R-Spray, or Camelot beginning in early August and according to label directions.
The information for the leaflet was obtained from: “Gymnosporangium Rusts: The Three Common Cedar Rust Diseases” by George Hudler and Dawn Dailey O’Brien, Branching Out, Volume 4, No. 2, 4/25/97.
Pesticide and management recommendations obtained from: Part I Guide to Pest Management Around the Home, Cultural Methods and Part II -- Pest Management Around the Home, 2005-2006 Pesticide Guidelines, Miscellaneous Bulletins 139S74I and 139S74II, Cornell Cooperative Extension Publications.
This publication contains pesticide recommendations. Changes in pesticide regulations occur constantly and human errors are still possible. Some materials mentioned may no longer be available, and some uses may no longer be legal.
TK: 5/2006 #117