possible rust/ leaf spots on Mock Orange tree(Bush)

Asked September 26, 2019, 8:33 AM EDT

We have a county resident that has a mock orange bush/tree and she says it has burrs on the underside of the leaves. She sent me pictures, which I have attached below. It almost looks like a occurrence of a cedar rust infection that is found on apple trees. Can Mock orange get cedar rust? I asked her if they had any Cedars / yews close to this bush and she let me know that the neighbor has a cedar tree approximately 40 feet away. Do you have any suggestions?

Tuscola County Michigan trees and shrubs rust

1 Response

Hello, Rusts sometimes called juniper rusts caused by the genus Gymnosporangium infect mostly trees in the rose family which includes apples and hawthorn, with cedars (junipers), the alternate host. There is an exception though, and that is G. speciosum which infects mock orange and again, cedar (juniper) is the alternate host. I am attaching an article from the USDA about these rusts. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5347332.pdf You will note that one method of managing the problem is by removing one of the hosts. The hosts should be kept at a distance of two miles. Pruning out the affected parts in the juniper can reduce damage and preventive fungicides are available. If using fungicides ensure that it has been formulated to treat the specific rust. Also, read and follow the directions for application on the container. Usually the fungicide should be applied to the mock orange in the spring and to the juniper when the aecial stage is evident on the mock orange.