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 (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).
On evergreen hosts, infection occurs on needles and new shoots. In contrast to cedar-apple and cedar-hawthorn rust, 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.
Grow resistant varieties. Even though sanitation is not perfectfollow good cultural practices and remove as much of the infected twigs, fruit and leaves as possible. Follow recommended fungicide treatments (contact your local University of Illinois Extension office or a reputable garden center, landscaper, nursery or arborist).
See cedar-apple rust and cedar-hawthorn rust for additional information on rust diseases. More than one type of rust may be present on many of the plant hosts discussed. Although these rusts are quite similar, only cedar-hawthorn and cedar-quince rust galls produce spores for more than one year. Also see the rust differences chart.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud, Extension Specialist, Pesticide Applicator Training and Plant Pathology, Department of Crop Sciences, University of Illinois at Urbana-Champaign.