The dimpled, red galls on this eastern red cedar (Juniperus
virginiana) are signs of cedar-apple rust, a fungus that divides its time
between two completely different hosts. One part of its life cycle is completed
on junipers, where these golf ball-like masses can be spotted in winter. The other
part is completed on plants in the rose family, such as apple trees. On each host, the appearance of
the fungus is so different that it can be hard to connect the two as belonging to the same organism.
In spring, cedar-apple
galls on junipers sprout gelatinous, orange “horns.” These gummy tentacles produce and release spores that can infect the leaves of apples, crabapples and
sometimes hawthorns. As the fungus grows on apple trees, the leaves
develop yellow or orange spots on their upper and lower surfaces. In summer, spores released from the spots on the lower surfaces of the leaves are spread by wind back to junipers, where they form overwintering galls. And so the cycle is continues.
Cedar-apple rust usually doesn’t have severe effects, although infection can cause susceptible apples and crabapples to lose their leaves early. Fruits may also develop unattractive spots. Many varieties of apples and crabapples are resistant to cedar-apple rust. For a list, see the University of Minnesota Extension Service link below.
Spots on apple leaves can also be caused by
other fungi. Apple scab is one example.
rust and related rust diseases. R. Koetter and M. Grabowski, University of
Minnesota Extension Service. Accessed
of the week: Cedar apple rust (Gymnosporangium juniperi-virginianae Schwein.).
D. Taylor, U.S. Forest Service. USDA. Accessed online 2/27/21.
Agrios, G. N. 1988. Cedar-Apple Rust. Pages 462-466 in Plant
Pathology, third edition. Academic Press, Inc. New York.