Rust Diseases Affecting Junipers and Trees and Shrubs in the Rose Family

By Jeff Osborne - Forest Stewardship Program Associate 

Last week, as I was enjoying a snack of juneberries, I noticed a sight I had not seen on the shrub before. Some berries were double the size of others and had short columns protruding from them. I suspected a fungus and was able to identify the culprit as a rust fungus. There are several closely related fungal rust diseases in Pennsylvania which need to infect plants in the Cupressaceae family, junipers (like the eastern red cedar, Juniperus virginiana, with its incorrect common name), and plants in the Rosaceae family, including apples, hawthorns, and juneberries, to complete their life cycles. Three common rusts are cedar-quince (Gymnosporangium clavipes), cedar-apple (Gymnosporangium juniperi-virginianae), and cedar-hawthorn (Gymnosporangium globosum). Because of confusion with common names, like the eastern red cedar, some references refer to other names for the fungal infections such as juniper-apple rust or American hawthorn rust. Northern white cedar, even though a member of the Cupressaceae family, is not a host for these rusts. Let’s examine rust diseases that you may be seeing the effects of now on your apples, hawthorns, and juneberries.

The rusts have a basic life cycle which is similar, but their signs, hosts, and hosts’ symptoms may differ. In the spring, after rains during temperature ranges of 45 to 75 degrees Fahrenheit, the fungus, which has been waiting in galls or cankers on the juniper, begins to develop telia, which are orange gelatinous masses. These telia are made of teliospores. The teliospores form basidiospores which are then dispersed by wind to the other host. The basidiospores infect leaves, fruits, or new twigs on the deciduous host and form pycnia. The fungus then produces pycniapores and forms structures, aecia, which develop aeciospores. To complete the cycle, aeciospores then infect the juniper and form galls or cankers and may take up to two years for the galls to develop.

The graphic above shows activity of the rusts in relation to the seasons as well as possible management points and methods.

Cedar-quince rust:

Eastern red cedar and common landscaping junipers are susceptible, as well as many species in the rose family including apples, quince, mountain-ash, hawthorn, chokeberry, and juneberry. The rust forms cankers on junipers, from younger twigs to the main stem. Fruiting bodies may emerge from these cankers each spring for four or more years. This can lead to girdling of a significant portion of the plant. On the deciduous plants, this rust disfigures fruits and can form cankers on twigs. Leaves may show no damage or few yellow spots.

 The picture above shows the telia of cedar-quince rust on a canker on a juniper twig

Picture by Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Bugwood.org

 The photo above shows a juneberry shrub with about a quarter of its fruit infected by cedar-quince rust. Photo by author.

 The photo above shows columns protruding from the fruit. These columns are aecia, or reproductive structures which produce aeciospores. Photo by author.

Cedar-apple rust:

Eastern red cedar and common landscaping junipers are susceptible to this rust, as well as apples. This rust produces very conspicuous telia on newer growth of junipers in the spring. The fungal infection on apple species mainly targets leaves and can cause significant defoliation and blemished fruit, and can cause fruit to be under-developed. 

The picture above shows telia on an eastern red cedar.

Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org

The picture above shows pycnia on the upper side of an apple leaf.

Brian Olson, Oklahoma State University, Bugwood.org     

Cedar-hawthorn rust:

This rust has a host list similar to cedar-quince rust, and a sign and symptom list similar to cedar-apple rust. Hawthorns can be severely defoliated.

The picture above shows aecia on the lower side of a hawthorn.
Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org

Prevention and treatment:

Infection severity generally decreases as distance between the two host species increases, so removing one host from an area will benefit the other host, although spores can travel miles in the wind. If you are planning on planting any of these species, there are many listings of resistant varieties. If rust is a perennial problem, there are also several resources to spraying the rose family host, such as  Spray Guide for Growing Apples at Home, from Kansas State University.

The Pennsylvania Forest Stewardship Program provides publications on a variety of topics related to woodland management. For a list of publications, call 814-863-0401, send an email to PrivateForests@psu.edu, or write to Forest Stewardship Program, The Pennsylvania State University, 416 Forest Resources Building, University Park, PA 16802. The Pennsylvania DCNR Bureau of Forestry, USDA Forest Service, Penn State Extension, and the James C. Finley Center for Private Forests at Penn State, in Partnership through Penn State’s Department of Ecosystem Science and Management, sponsor the Forest Stewardship Program in Pennsylvania.