Issue 10, June 26, 2009

Cercospora Blight of Juniper

Cercospora blight of juniper is not usually a big problem in Illinois. It is more common in areas where Rocky Mountain juniper (Juniperus scopulorum) is grown. This year, however, we have seen this disease. Eastern red cedar (Juniperus virginiana) can also become infected, although the literature says that most landscape cultivars of eastern red cedar have resistance. Maybe this is just another example of increased disease following our wet, wet spring.

Cercospora blight actually begins in late summer and fall, so problems seen now began last year. Illinois arborists should be familiar with Phomopsis tip blight of juniper (see issue 7 of this newsletter). That fungal disease infects the newest growth. Cercospora blight, on the other hand, starts on the oldest foliage on the lower branches and spreads upward. Look for brown lower and inner growth with green branch tips at the top of the plant. Another big difference is the fungal fruiting body and spores. Fruiting bodies of Cercospora are fuzzy looking cushions with long spores.

The first image shows Cercospora blight on Eastern red cedars (junipers) in a windbreak. The image is credited to the USDA Forest Service Archive, USDA Forest Service, Compare this to the image of Phomopsis tip blight on the right credited to Oregon State University.

The Cercospora blight fungus, Cercospora sequoiae var. juniperi, lives from year to year on dead needles, whether on the plant or on the ground. Most infection occurs in early summer (now) but as long as wet weather continues, more disease will occur. Fungicides can be used to stop this disease in susceptible junipers. Sprays are made in late June and early July. Registered products include Armicarb, Camelot, Dithane, Fore, Formec, Junction, Kocide, Manhandle, and Protect. Repeat 1-3 times according to label directions from June through July if wet weather continues. Be sure to coat the tree well, especially the bottom of the tree. Home growers can use products containing copper, copper sulfate, mancozeb, or potassium bicarbonate.

Resistant junipers are available in the nursery industry. A table of resistance ratings is available in Diseases of Woody Ornamentals and Trees in Nurseries, edited by Jones and Benson.

There are other noninfectious problems that can also cause decline of junipers. Site stress, weather stress, planting problems, girdling roots, and many other factors could cause trees to die from the bottom upward. Such problems could also predispose plants to infection by Cercospora. Make sure you are getting the true and entire picture of the problem before you spray fungicides.--Nancy Pataky

Nancy Pataky

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