Overview of cedar-apple rust
Cedar-apple rust is an interesting disease. It requires both an apple and cedar or juniper to complete its life cycle. On the cedar, the fungus produces reddish-brown galls that are up to golf-ball size on young twigs. During wet weather these galls swell and begin to push out bright orange gelatinous tubular structures. Wind carries fungal spores from these gelatinous structures to susceptible apple or crabapple cultivars.
Signs and symptoms of cedar-apple rust
During warm rainy days in late April and early May, cedar trees infected with the cedar-apple rust fungus (Gymnosporangium juniperi-virginianae) will develop bright orange, gelatinous galls. Apple trees will present small, yellow spots that appear on the upper leaf surface shortly after bloom. Spots gradually enlarge and become a bright yellow-orange color, which are easy to identify.
Disease cycle of cedar-apple rust
It requires both an apple and cedar or juniper to complete its life cycle. On the cedar, the fungus produces reddish-brown galls that are up to golf-ball size on young twigs. During wet weather these galls swell and begin to push out bright orange gelatinous tubular structures. Wind carries fungal spores from these gelatinous structures to susceptible apple or crabapple cultivars. Infection occurs when these spores land on a susceptible apple cultivar and moist conditions prevail. Small, yellow spots begin to appear on the upper leaf surface shortly after bloom. Spots gradually enlarge and become a bright yellow-orange color. These brightly colored spots make the disease easy to identify on leaves. Heavily infected leaves may drop prematurely. In late summer small tube-like structures develop on the underside of the apple leaves. Spores are released from these structures and are blown by wind back to susceptible cedars or junipers, completing the disease cycle.
Type of Sample Needed for Diagnosis and Confirmation
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents. If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us
Management of cedar-apple rust
If you have followed along with this cycle, you might immediately think that a good way to break it would be to avoid planting susceptible hosts next to each other. Unfortunately, this is often impractical, because the fungal spores can travel as far as two miles.
If cedar-apple rust is a problem on your existing apple or crabapple trees, fungicide sprays can be used to protect trees from infection. For adequate control, make sure to read the label and follow its instructions. As indicated on the label, sprays are applied in the spring at the pink and petal-fall stages of flowering.
An easy way to avoid this disease is to plant disease resistant apple or crabapple varieties. Fortunately, there are many varieties available now that show good disease resistance. Redfree, Liberty, William's Pride, and Freedom are examples of new apple varieties that are immune to cedar-apple rust. These varieties are also immune to apple scab and show good resistance to powdery mildew and fire blight. Examples of apples that are susceptible to cedar-apple rust include Jonathan, Rome, Wealthy, York Imperial, and Golden Delicious. Many mail-order nurseries offer disease-resistant apples
Rust resistant crabapples are also readily available. A sampler of disease-resistant varieties includes Adams, David, Donald Wyman, Indian Summer, Prairie Fire, Professor Sprenger and White Angel. These varieties vary in flower color, fruit color, height, and width, so consider these traits before making a selection. Avoid for crabapple varieties that have serious disease problems, such as Hopa, Radiant, and Vanguard.
Cedar-apple rust generally does little damage to cedars or junipers. In many cases, the disease is not noticeable except in the spring when the galls are producing the bright orange spore horns. If the disease is very severe, however, twig dieback can occur. Rust galls on small cedars and junipers can be pruned out in late winter or early spring (by April 1). Avoid planting susceptible varieties. Susceptible cedars and junipers are eastern red cedar (Juniperus virginiana) and its varieties, and Rocky Mountain juniper (J. scopulorum) and its varieties.
See this article for more information about cedar rusts.