Issue 2, April 25, 2016

Gymnosporangium Rusts on Eastern Red Cedar

Three Gymnosporangium rusts commonly affect trees in Illinois landscapes: Cedar-apple rust, Cedar-hawthorn rust, and Cedar-quince rust.  As their name suggests, these pathogens require two hosts to complete their life cycles. A portion of each disease's life cycle occurs on Juniper (Juniperus spp.) hosts, while the remainder occurs on one of several deciduous host within the Rosaceae family. These rust pathogens overwinter Eastern red cedar and other Juniperus spp. as hard and pitted galls or spindle shaped swellings on young branches. Galls are light brown to reddish or chocolate brown and range from 1/8 inch to 2 inches in diameter (Image 1). Galls formed on evergreen hosts are not usually damaging, but serve as an important stage in each pathogen's life cycle. Currently, telial sporehorns are emerging from overwintering galls. Sporehorns have a distinct, orange, gelatinous appearance (Image 2). Sporehorns swell when moisture is present, then discharge spores as the dry. Discharged spores can be carried several miles by wind, but mostly infect susceptible trees within several hundred feet. Sporehorns exhaust all their spores approximately 30 days after apples and crabapples have bloomed.

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

Gelatinized telia on Eastern red cedar. April 2016

Apples, crabapples, hawthorns and quince are some of the more commonly affected deciduous species. They are also the hosts we are most concerned about when we apply controls. Infections to deciduous hosts occur in the spring, beginning as apples and crabapples are in their pink-bud to early bloom stage. This timeframe coincides with the development of sporehorns on Juniperus spp. Cedar-apple rust and Cedar-hawthorn rust causes mostly aesthetic injury in the form of pale yellow to orange leaf spots. often with a reddish border. Severe infection may result in moderate defoliation, especially during dry summers. Cedar-quince rust can be quite damaging to hawthorn, causing deformed swellings, galls, infected fruit, and stem tip dieback.

Early emergence of the telial stage of cedar quince rust on Eastern red cedar. April 2015.

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. Though not always practical or feasible, infections to broad-leaved hosts  can be reduced by removing unwanted host trees within a ½ mile radius. Overwintering galls can also be hand-picked or pruned from small junipers during the fall and winter months. Fungicide sprays are effective at protecting susceptible trees from infection, but are rarely utilized on Juniper hosts. They are commonly applied to deciduous hosts beginning at the pink-bud stage, with re-applications at labeled intervals until 1-2 weeks after petal fall. Fungicide options are listed in the Commercial Landscape & Turfgrass Pest Management Handbook and Pest Management for the Home Landscape.

The Report on Plant Disease (RPD): Rust Diseases of Apples, was updated this past December and can be viewed via the following link: (Travis Cleveland)

Travis Cleveland

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