Apple Cedar Rust Infection Periods During April
Posted: May 4, 2011
On leaves, cedar-apple rust, caused by the fungus Gymnosporangium juniperi-virginianae, first appears as small, pale yellow spots on the upper surfaces. The spots enlarge to about 1∕8 inch in diameter. Eventually, tiny, black, fruiting bodies (pycnia) become visible. Often a number of orange-yellow protuberances, called aecia, are produced in each spot on the underside of the leaf. Infected leaves may remain on the tree or may become yellowed and drop. Fruit lesions appear on the blossom (calyx) end. They are somewhat like leaf lesions but much larger and often cause fruit to become disfigured or to develop unevenly. Light brown to reddish brown galls form on the branches of red cedar. When they are dry and hard they may be 1/2 to 2 inches in diameter and are known as “cedar apples.” The galls’
surfaces are covered with depressions much like those on a golf ball. In the spring, when the “cedar apples” become wet, a yellow, gelatinous horn (telial horn) up to 2 inches long protrudes from each depression.
Spores discharged from these gelatinous telial horns on red cedar are easily windborne, infecting apple leaves and fruit. Spore discharge begins about the pink stage of apple bloom and is usually completed in a few weeks. Following a few wet
periods, the cedar galls die. Spots on apple leaves can be seen about 10 days after infection. Visible fruit infections require a somewhat longer time. Aecia on the undersides of apple leaves or on fruit lesions themselves produce spores. Borne by winds, the spores may be carried back to the red cedar.
Details on control can be found at http://agsci.psu.edu/tfpg.