Cedar-Quince Rust is caused by a fungal pathogen called Gymnosporangium clavipes. This fungus occurs on a wide range of rose family plants, including mountain ash, hawthorn, quince, flowering quince, serviceberry, crabapple, and apple (though apples are somewhat resistant). In addition, eastern red cedars, common, prostrate, Rocky Mountain and savin junipers are possible evergreen hosts. In order to survive, the fungus must “move” from one type of host to another (e.g., from juniper to hawthorn). this rust causes flaky, perennial branch swellings rather than distinct, roundish galls. Most of these swellings girdle and kill small twigs, but some survive and remain infectious for many years. Most people do not notice the branch swellings until the telia become wet, swell and gelatinize to a bright orange color. On deciduous hosts, leaves, petioles, young branches and fruit are usually infected and symptoms vary widely among the various hosts. On hawthorn, the pinkish aecia (tubes) occur mainly on branches, thorns, and fruit. Hawthorn and serviceberry fruit often becomes heavily covered with aecia. Branch and thorn infections result in spindle-shaped, perennial cankers that expand each growing season. However, most infected branches are girdled by the canker during the second season, causing die-back to a bud or side-shoot. From the telial swellings on the evergreen host, basidiospores are released that infect deciduous hosts such as hawthorn. Seven to ten days after infection, spots or swellings develop, followed a few days later by the formation of tiny black dots (spermagonia) within the spots.
Four to seven weeks later, aecia are formed. Aeciospores, released from the aecia during rain or as morning humidity lowers, become airborne and infect susceptible evergreen hosts during late summer and fall. The following spring (or one year later), swellings (consisting of both fungal and host plant tissues) develop on the evergreen host. When the swellings are mature, a few hours of wet, cool (74 and 78°F is optimal) spring weather is sufficient for repeated telial swelling and release basidiospores that infect the deciduous host. In contrast to cedar-apple rust galls, cedar-quince rust swellings may remain infectious for 4-6 years or more. Proper use of fungicides, coupled with a regular fertilization program, will help to reduce symptoms and maintain the overall health of the plant.