Cedar-apple Rust Galls Noticeable on Junipers

With the recent rains, the cedar-apple rust and cedar-hawthorn rust galls are very evident on cedar trees. These bright orange balls with gelatinous protrusions are quite striking and seemingly appear out of nowhere after a spring rain. The gelatinous protrusions produce spores which will infect apple, hawthorn and other rosaceous hosts. In Missouri, we commonly see these rust infections on apple, crabapple and hawthorn. Chances are that if you have these deciduous hosts in your landscape, you have seen these infections as well.

Symptoms of cedar-apple rust and hawthorn rust on apples and hawthorns are circular, yellow spots (lesions) on the upper surfaces of the leaves that appear shortly after bloom. Although the lesions from both of these diseases initially appear similar, those caused by hawthorn rust are generally a yellowish red and do not usually reach over 1/6 inch in diameter. As the disease progresses, the undersides of these leaves below the yellow spots will develop raised orange structures that will ooze from the center, then later turn black and appear as black dots. Eventually, this area will produce the orange and brown rust colored spores that infect the Juniperus hosts in late summer. Severely infected leaves may drop prematurely, especially during a dry summer.

In situations where chemical control is desired, there are several preventative fungicides labeled for use on rosaceous hosts. Fungicide sprays are aimed at protecting developing foliage from infection during the time the galls on the junipers are orange and gelatinous. This usually lasts for several weeks, and fungicide applications are not necessary once the galls become dry and inactive. Research conducted by Kansas State University indicates that two applications of a triazole fungicide are effective in managing these rusts when applied at three-week intervals beginning shortly after bloom. Triazole fungicides include propiconazole, myclobutanil and tridiamefon. Always read and follow label directions.

Laura Kabrick, Extension Plant Diagnostic Clinic, UMC (573) 882-3019

Above: Cedar-apple Rust Galls on Juniper

Below: Cedar-apple Rust Infection on Crabapples