Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
6/4/01
Contacts:
Brooke Edmunds, Extension Plant Pathology, (515) 294-1741,
bedmunds@iastate.edu
Elaine Edwards, Continuing Education and Communication
Services, (515) 294-5168, eedwards@iastate.edu
Yard and Garden Article for the Week Beginning June 8
Cedar-Apple Rust
By Brooke Edmunds
Plant Pathology Research Assistant
Iowa State University Extension
Is the cedar in your yard decorated with bright orange stars? Unless you were getting a jump on Christmas decorating, chances are those 'stars' are actually the fruiting body of a common Iowa fungus, Gymnosporangium juniperi-virgininae. Commonly called cedar-apple rust, this fungus causes considerable damage to some apple trees in the Midwest.
Wait a minute. How did apples get into the picture? How does a fungus on a cedar affect an apple tree?
This fungus is a crafty one. It is -- get ready for it &endash; heteroecious, meaning that it needs two different types of plants to complete its life cycle. In this case, the two plants are a cedar and an apple tree.
For most of its life, the cedar-apple rust fungus exists as a gall on twigs of cedars or junipers. Sometimes called cedar 'apples,' these galls are 1/4 to 2 inches across. They are round, brown, hard and can be dimpled like a golf ball.
After growing on the cedar twig for 18 months, the fungus wakes up as the spring rains begin. The gall soaks up water, expands and forms a fruiting body. This is where those bright orange "stars" or telial horns come in. The gelatinous horns produce innumerable microscopic spores that are picked up by the wind.
The next stage of the rust fungus' life story begins when the rust spores land on a susceptible apple cultivar.
Infection is most noticeable on the leaves of an apple tree. In late spring or early summer -- just about now -- bright yellow spots appear on the upper surface of the leaves. These spots enlarge and turn orange. Pycnidia, the small black fruiting structures of the fungus, form in the center of these spots.
In late summer, more strange and interesting things happen. If you look at the underside of an infected leaf, you'll notice tiny, white tubes growing from the bottom of each orange spot. These tubes, or aecia, are another type of fruiting body. The aecia produce spores that are blown on the wind back to a cedar and start the cycle over.
It's nice to we have all sorts of fungal fruiting bodies to admire, but how does this harm the apple tree? If there are many cedar-apple rust spots the leaves may give up and fall off. And as you know, premature defoliation causes the tree to suffer and there may be fewer apples to harvest in fall. The apples themselves also can be infected, leading to undersized, deformed fruit.
What can be done? Since this fungus has such specific steps to its life cycle, blocking any of these steps would break the cycle and thwart the disease. Some ways to accomplish this include:
1. Planting resistant apple cultivars, especially if cedars are nearby. Among the most resistant apple cultivars are Red Delicious, Redfree and Jonafree. Resistant cultivars of cedar or juniper can also be planted. It's a good idea is to avoid planting red cedar (Juniperus virginiana), which is the number one favorite cedar for the cedar-apple rust fungus. Most other cedars and junipers are far more resistant.
2. Since cedar-apple rust requires both cedar and apple to complete its life cycle, removing red cedar from the landscape can break the chain. This is not always feasible (your neighbors might notice that their trees have disappeared). Since the spores are very light, they can travel from other nearby cedars. For a cedar eradication plan to be fully effective, all cedars for 2 miles around your apples would have to be a removed! Nevertheless, the further apart you can keep the cedars and the apples, the better for rust control.
3. If you want to keep your cedars, check for and remove the brown galls in early spring (before about April 15). If you get rid of the galls before they form the orange telial horns, no spores will be released to infect your apples. This won't work too well if your neighbors don't remove the galls from their cedars as well.
4. If your apples are a susceptible cultivar and you can't remove all the cedars in your area, then fungicide sprays may be warranted. Funginex, a Benlate/Captan mix or other labeled fungicides can be used. Follow the label for application instructions. For backyard apple growers who would rather avoid the hassle and hazard of fungicide sprays, the other strategies make more sense.
If your apple trees were hit hard by cedar-apple rust last year, try following some of the steps above to prevent problems this year. Even though this fungus has a complicated life cycle, it doesn't have to complicate your life, too.
ml: isugarden
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