Darrell Blackwelder: Cedar apple rust

Published 12:00 am Sunday, April 19, 2020

I got an email last week with a couple of photos about cedar trees. The trees had some kind of orange slimy growth and the person was afraid it was going to kill the trees. The cedars are 25 years old and never had this problem. The problem in the email is cedar apple rust Gymnosporangium juniperi-virginianae. It’s a fungal disease of both cedars (junipers) and apple trees. It is an uncommon fungal disease that requires two alternate hosts to complete its lifecycle. Ironically, the fungi must thrive on two entirely different types of plants to complete its lifecycle.

Brown galls about the size of a golf ball are within the cedars during the winter months. After warm spring rains, the galls swell producing gelatinous orange-colored horns with infectious spores. These spores are transmitted to nearby apple tree leaves up to three miles away. These spores then emerge as orange splotches later on the undersides of the apple leaves completing the cedar apple rust fungus lifecycle. The orange gel may also infect limbs and trunks. This fungus requires two years to complete this rather complex lifecycle.

Cedar apple rust does little harm to cedars or junipers. Apple leaves will become pale and fall off the tree. Many apple cultivars are resistant to cedar apple rust. Fungicidal sprays may help, but the disease usually does not kill either tree. For more detailed information on cedar apple rust go to https://content.ces.ncsu.edu/cedar-apple-rusts .

Darrell Blackwelder deblackw@ncsu.edu is the retired horticulture agent and director with the North Carolina Cooperative Extension Service in Rowan County.