There are more than 1,200 species in the genera Cercospora. And these fungi attack a wide range of plants. Cercospora sequoiae juniperi attacks junipers. Most selections of Eastern Red Cedar and Chinese junipers have good resistance to this disease. However, Rocky Mountain juniper is known to be one of the most susceptible juniper species. This disease can also attack the Oriental Arborvitae.
The disease spreads best during warm, wet weather. The spores penetrate new to one-year-old foliage during this time. At about 75 degrees, complete infection takes about sixteen hours. Symptoms may be visible between two to three weeks after infection. The pathogen is able to over winter in the infected foliage on the trees. The disease usually starts near the base of the plant and works upward. It also starts usually on the inner foliage and works outward. The affected foliage turns bronze to light brown color then grayish. Small, fuzzy or hairy spore-bearing structures, easily seen with a hand lens, form on the dead foliage. Eventually the dead foliage and twigs falls off the branch. With time all the inner foliage and twigs are gone, leaving only foliage at the very tips of the branches. Occasionally adventitious buds may develop new growth on the "naked " lower branches. Left uncontrolled, the tips of the lower branches die too. Thus leaving only green growth at the very top of the plant. On highly susceptible trees, the fungus can also kill the entire plant, sometimes in as little as three years. Initially the green tips of the affected branches help to separate this disease from Phomopsis and Kabatina Blights. Phomopsis and Kabatina kill from infection (canker or wound) out to the tip in the same growing season.
Avoid over-crowding the plants to allow sufficient sunlight and air circulation. During wet early summers, fungicides applications may be necessary on susceptible junipers. Make two to three applications at labeled intervals between early June and early July.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud, Extension Specialist, Pesticide Applicator Training and Plant Pathology, University of Illinois at Urbana-Champaign.