GardenLine | Fruit to Grow and Pick | Cedar Apple Rust

Kabaluk was a graduate student with the Department of Horticulture Science. This column is provided as an extension service by the Division of Extension and Community Relations and the Department of Horticulture Science, University of Saskatchewan.

If you are the owner of a hawthorn or saskatoon berry plant, you've probably noticed bright orange spots on the leaves and perhaps to a lesser extent on the fruit. This is a fungal disease whose common name is cedar apple rust. Despite the common name, this disease does not occur on cedars. The western red cedar on which this disease does occur is actually a juniper.

Life Cycle

Cedar apple rust, which is caused by the fungus Gymnosporangium, needs juniper plants and certain Rose family plants (such as saskatoons, hawthorns, and in some cases, apples) to complete its life cycle. On junipers, the disease appears as woody, spherical galls. In the spring (early May), brown, horn-like projections called "telia" grow out of the woody galls. During wet weather, the telia absorb water, swell up immensely, and become orange and gelatinous. At this stage the disease emits spores that infect the Rose family plants to cause the bright orange spots. The orange spots will eventually produce their own horn-like structures called "aecia" on the fruit and underside of the leaves; from the aecia, spores are produced that re-infect the junipers in the late summer. The disease must pass from junipers to Rose family plants to junipers again; it cannot spread between Rose family plants.

Control

So, what do you do about cedar apple rust? First decide whether or not it is a problem. If you have a few saskatoon plants in the backyard, it is hardly worth spraying them because you probably don't get much fruit off them anyway. The disease is serious only when there has been a lot of spring rain. If you are growing your saskatoons and hawthorns for ornamental purposes, you'll notice that cedar apple rust is itself quite ornamental; if you appreciate aesthetics, you may actually want the disease!

To control the disease, you can remove the telial galls every year from your juniper plants, remove your juniper plants entirely, or grow resistant junipers (Scandia and Arcadia junipers are resistant, as are some Rocky Mountain Juniper varieties; inquire at your local nursery.) Ferbam and Thiram have been suggested for saskatoons, applied at the recommended rates every 7 days for 4-5 weeks after the telia on the juniper galls have become orange and gelatinous. Although no fungicides have been registered for cedar apple rust on hawthorns, the same spraying schedule would likely be effective. Adding dish soap at the rate of 1/4 teaspoon per quart will help the solution stick to the leaves of the plant.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.