P&PDL Picture of the Week for
April 23, 2007

Rust Galls on Juniperus virginiana—the three amigos

By Gail Ruhl, Senior Plant Disease Diagnostician, Department of Botany & Plant Pathology, Purdue University

Three different rust fungi are currently causing gall-like protrusions on eastern red cedar (Juniperus virginiana) trees.

The bright orange gelatinous galls with tendril-like protrusions (Fig 1) are caused by Gymnosporangium juniperi-virginianae. This fungal disease is known as Cedar Apple Rust. The galls develop on the cedar tree (Fig 2) and ‘sprout’ fungal spore-bearing tendrils during rainy periods in the spring (Fig 3 and 4). Spores from these galls infect mostly apples and crabapples.

The smaller, less conspicuous branch galls, with horn-like protrusions (Figs 5,and 6), are known as Hawthorn Rust and are caused by the fungus Gymnosporangium globosum. The fungus alternates between Juniperus hosts and hawthorn, crabapple, and apple in addition to several other rosaceous hosts.

Gelatinous swellings on the cedar branches (Figs 7, 8, 9 and 10) are yet signs of another type of rust disease known as Quince Rust, caused by Gymnosporangium clavipes. This disease alternates between Juniperus and various rosaceae plants. Hawthorn is one of the most notable alternate hosts of this Quince Rust on Cedar.

As noted above, these rust fungi require two separate hosts to complete their life cycle and thus the spores produced from the cedar galls are dispersed by wind-blown rain to infect succulent parts of various angiosperm hosts. During midsummer, spores are produced from infected angiosperm hosts and these spores are then blown back to their Juniperus hosts, completing the life cycle of this rust fungus. Sinclair and Lyon’s book “Diseases of Trees and Shrubs” has an excellent section on Gymnosporangium Rusts, beginning on page 260.

The cedar-apple rust and hawthorn galls that form on eastern red cedar are unsightly, but usually cause little harm to the tree. During dry weather, galls can be removed and destroyed. The spindle shaped stem swellings of quince rust may eventually girdle branches and cause some minor branch dieback. For more information on this disease please refer to BP-35, Cedar Apple and Related Rusts.

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Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service