Test Your Knowledge - February

Cedar-apple rust galls
Cedar-apple rust galls
Division of Plant Industry Archive, Florida Dept. of Agriculture

Yes, these are cedar-apple rust galls.

These brown stem galls may be overlooked on cedars (Juniperus species) during winter and early spring, but when spring rains come and the galls swell to several times their original size and develop bright orange, jellylike horns, they are not so easily missed. These “horns” produce spores and are a very visible sign that the cedar is infected with the rust fungus, Gymnosporangium juniperi-virginianae.

Bright orange, jelly-like horns that have developed from a cedar-apple rust gall
Bright orange, spore-producing "horns"
Joseph O'Brien, US Forest Service

Interestingly, this fungus requires two plant hosts to complete its life cycle. The jelly-like horns eventually produce spores which are carried by the wind (up to 3 miles) to apple and crabapple trees, where they infect leaves and fruit. However, these particular spores cannot infect cedars.

Cedar apple rust lesions on apple leaves
Cedar apple rust lesions on apple leaves
Clemson University - USDA Cooperative Extension Slide Series

After infection, orange-colored lesions develop on apple leaves and fruit, and the fungus produces more spores, which are released in late summer and are also carried by wind. The spores produced in the apple leaf and fruit lesions are able to infect cedars, but not apple trees.

The need for alternate plant hosts to complete its life cycle means that if one of the hosts is removed, the fungus cannot survive. Elimination of junipers in commercial apple growing areas has been attempted in the past in an effort to control this disease. Unfortunately, elimination of one host is not often very practical since spores can be blown several miles by wind. A more effective control option is to plant resistant cultivars of apple, crabapple, and junipers. If only a few plants are involved, galls can be removed by hand from the junipers before they start releasing spores in the spring. When cedar-apple rust is an annual problem, fungicides can be applied as protection against infection. Once lesions appear on apple leaves and fruit, it is too late for fungicides that season.

For more information on this disease as well as other diseases of apples and crabapples, see HGIC 2000, Apple & Crabapple Diseases.

Janet McLeod Scott
HGIC Extension Agent

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