Rust diseases require two hosts to complete life cycle

Originally published in the Kenosha News

By Horticulture Educator, Jeanne Hilinske-Christensen

Question: What is the difference between cedar-apple rust and cedar-hawthorn rust? I am familiar with cedar apple rust but was told by a landscaper my serviceberry tree may be affected by cedar-hawthorn rust. How can these diseases be managed? — J.S.

Answer: In addition to cedar-apple rust and cedar-hawthorn rust, there is a third on the list of common rusts that may affect plants growing in our region, and that is cedar-quince rust. All three of these rusts are closely related fungal diseases (caused by fungi in the genus Gymnosporangium) that require two hosts to complete their life cycle, junipers and a member of rose family, such as apple, hawthorn and quince. Other commonly known plants that may serve as alternate hosts to one of these rusts are serviceberry, crabapple and pear.

Each of these rusts are caused by different species of Gymnosporangium and may complete their life cycles on different members of the rose family, hence the different names for the diseases.

These diseases may also infect other parts of the plant, including the fruit and twigs. Cedar-quince rust causes damage mostly to the fruits and twigs of its rose family hosts rather than the foliage. Managing techniques for these three types of rust are similar.

To treat junipers affected by cedar-apple rust, prune the branch about four to six inches below the galls with pruners that are disinfected following each cut. Disinfect with a 10 percent bleach solution or with disinfectants containing 70 percent alcohol. Fungicides can be applied at seven- to 21-day intervals in July and August to prevent future infections.

Selecting rust-resistant varieties is probably the best way to avoid dealing with these diseases.

University of Wisconsin Garden Fact Sheets XHT1009 and XHT1009a offer additional information on cedar-apple rust. Both publications are available on the Wisconsin Horticulture website: https://hort.uwex.edu/articles/cedar-apple-rust/