Paul Pecknold, Extension Plant Pathologist
Cedar apple rust is a common fungal disease of junipers, apples and crabapples. Other similar rusts cause problems on hawthorns. There are three separate rust fungi, all of the genus Gymnosporangium, with very similar life cycles. Each fungus requires two different tree species to complete its life cycle.
In early summer, small yellow spots appear on the upper-leaf surface of infected apples and crabapples. The spots rapidly enlarge, becoming a brilliant red-orange color. These spots can cause loss of leaves (defoliation) if infection is severe. With time, usually later in the summer, the undersides of these spots produce short protruding "fingers" of fungal tissue that bear microscopic spores.
Infected red cedars and junipers form chocolate brown galls, generally the size of a half-dollar, on infected twigs. In early spring, finger-like tendrils (spore horns) emerge from the galls. The horns have a gelatinous (jelly-like) texture, and it is from these structures that spores are produced. These galls maintain their bright color for two or three weeks, then dry and wither.
Junipers are considered the primary host, and it is here where the fungus overwinters. Galls are produced two years after infection by spores produced on the apple or crabapple. During spring, with emergence of spore horns on the junipers, a different type of spore is produced that infects the alternate host--apple or crabapple. In midsummer, infected apple leaves produce spores which in turn infect junipers, completing the life cycle. The most damaging phase of the disease is on the alternate host: apples and crabapples. Severe defoliation may occur, weakening the plant. This disease generally does not cause significant injury to junipers.
Infected juniper twigs look slightly swollen and may be cracked. In late April and May, orange spore masses emerge from these swollen areas, but no tendrils are produced. Infected twigs may develop cankers (dead areas); twig and branch dieback can result. On the alternate hosts (hawthorn, quince, apple and crabapple) leaf spots are rare. Instead, this fungus causes distortion of twigs, buds, and fruit. The most common symptom is on hawthorn. Fruits become shrunken and often die; twigs become enlarged and woody. Pink tubes, about the size of a pencil lead, protrude from affected fruits and twigs and shed orange spores.
Galls are produced on junipers. These galls are usually smaller and more irregular in shape than the cedar apple galls, and produce slightly different spore horns. Yellow leaf spots are produced primarily on hawthorns rather than crabapples and apples.
The life cycle is similar to that described for cedar apple rust.
Control methods are similar for the three fungi.
The best method of avoiding cedar rust diseases is to use resistant plants when installing new trees. However, the cedar rusts are not the only diseases to attack crabapples, apples and hawthorns. Apple scab and fire blight are two other important diseases of crabapples and apples. Fabraea leaf spot is common to hawthorn. On junipers, Phomopsis and Kabatina tip blights are recurring problems almost every year.
Therefore, when choosing a cultivar to plant, consider total disease resistance, not just resistance to one particular problem.
The rust fungi are dependent upon both the primary (juniper) and alternate (apple, crabapple, quince, or hawthorn) hosts for survival. Removal of one or the other breaks the life cycle of the fungus, thus preventing disease. A distance of 1/4 mile between junipers and alternate hosts is helpful, but this is often not practical. Whenever possible, at least avoid planting the two different host types right next to each other.
In late winter remove and destroy all galls from junipers. This is a practical control if infection is light and there are not many susceptible junipers in the area. However, even if you are successful in removing all galls, infectious spores can be blown in from other trees.
Rust does not kill apples, crabapples, or hawthorn, and generally does not cause sufficient injury to warrant use of fungicides. If rust is a chronic problem causing leaf drop and poor tree vigor, registered fungicides may be used on the broadleaf host.
These fungicides are preventive and must be applied several times during early spring to maintain a protective coating on developing leaves, twigs and fruit. When spring weather is dry fungicide applications are generally not required. Read and follow label instructions regarding amounts of fungicide method of application, and safety precautions.
Registered fungicides for rust control are subject to change. For current control recommendations, consult your local Purdue Cooperative Extension office or Purdue University's Plant and Pest Diagnostic Laboratory (1155 LSPS, Purdue University, West Lafayette, IN 47907; (765) 494-7071).
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