Issue 2, May 12, 2014

Gymnosporangium Rusts on Eastern Red Cedar

Three common Gymnosporangium rusts affect trees in Illinois landscapes: Cedar-apple rust, Hawthorn rust, and Quince rust. These pathogens require two hosts to complete their life cycles. The most damaging stage occurs on deciduous hosts within the Rosacea family. Infections to deciduous hosts occur during the spring and become evident later in the season. The Eastern red cedar is an evergreen species and serves as the alternate host for these pathogens. Infections to evergreen hosts are not as damaging, but serve an important stage in each pathogen's life cycle. These rust pathogens overwinter on Eastern red cedars as hard and pitted galls or swollen branches. Galls are light brown to reddish or chocolate brown and range from 1/8 inch to 2 inches in diameter.

Over-wintering Cedar-apple rust gall on Eastern red cedar. Photo taken Feb. 2014.

Internal tissue is solid. Insect galls may appear similar but contain chambers, exit holes, or insects. In recent weeks, the rust galls have started to form masses.

Early emergence of the telial stage of cedar quince rust on Eastern red cedar.

Rust gall on Eastern red cedar with telial growth starting to emerge. Photo taken April, 2014.

As spring temperatures increase and moisture fluctuates, the galls form distinct orange, gelatinous spore masses. These spores blow to and infect nearby susceptible deciduous hosts.

Gelatinized telia on Eastern red cedar. Photo credit: Nancy Pataky

The most common control strategies for Gymnosporangium rusts in the landscape focus on protecting the deciduous hosts. These recommendations start with utilizing resistant or immune species and varieties. Where feasible, infections to the broad-leaved host can be reduced by removing unwanted host trees within a ½ mile radius. Galls can also effectively be pruned out or hand-picked from small junipers during the fall and winter months. Fungicide sprays are effective at protecting susceptible trees from infection. Many of the fungicides used to protect deciduous hosts are also labeled for use on Juniperus spp. If fungicides are deemed necessary to protect the evergreen host, they should be applied at two-week intervals during July and August or as directed by specific product label recommendations. (Travis Cleveland)

Travis Cleveland

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