Cedar-hawthorn rust is caused by a fungal pathogen called Gymnosporangium globosum. This disease occurs on eastern red cedar, Rocky Mountain juniper, southern red cedar, common and prostrate junipers, apple and crabapple, most hawthorns (there are a few reported, resistant species), and sometimes on pear, quince and serviceberry. In order to survive, the fungus must "move" from one type of host to another (e.g., from juniper to hawthorn).
On evergreen hosts, very small (1/8 to 9/16 inches in diameter), roundish galls develop on needles throughout the tree. These galls slowly grow onto the twigs, often becoming flat on the twig side. Immature galls are reddish-brown, while mature galls are grayish-brown in color and scarred. When mature, these galls swell and produce reddish-brown, short, blunt, jelly-like telial horns during rainy spring weather. In contrast to cedar-apple rust galls, cedar-hawthorn rust galls seldom cause the death of infected evergreen twigs.
On deciduous hosts, small yellow spots first appear after infection in the spring. As the spots mature and enlarge, they take on an orange color and develop tiny black dots (spermagonia) in the center of the lesion. By mid-summer, tubes (aecia) are visible on the undersides of mature leaf lesions or within the lesions on fruit, petioles, or twigs infections. Hawthorn rust aecia (about 1/8 of an inch long) are longer than the aecia of cedar-apple rust.
With severe rust, hawthorn foliage may turn bright yellow and drop prematurely. In addition, fruit and young shoots may become infected. The symptoms vary when other hosts are infected. On quince, black flat lesions (without aecia) form on fruit. Dark brown to blackish spots with reddish halos form on the upper surface of pear leaves. In addition, aecia may develop on the petioles and lower surface of pear leaves.
From the telial horns on the evergreen host, basidiospores are released that infect deciduous hosts. Although most infections occur within several hundred feet of the source evergreen, infection has been reported even when there is more than fourteen miles between the host trees.
About 80 to 90 days after infection (about 10 longer than cedar-apple rust), aecia are produced. Most people only notice this stage after the aecia have split and take on a ragged appearance. Aeciospores, released from the aecia during rain or as morning humidity lowers, become airborne and infect susceptible evergreen hosts from midsummer into early fall.
The following spring, galls (consisting of both fungal and host plant tissues) begin to develop on the evergreen host. These galls continue to grow through the summer, and by fall they are full size (1/8 to 9/16 inches in diameter). Rainy weather during the following spring causes the telial horns to emerge and release basidiospores that infect the deciduous host. As spring rains subside, the galls become inactive until the following spring. In contrast to cedar-apple rust galls, cedar-hawthorn rust galls often are perennial (producing spores for more than one year).
In summary, the complete cycle of cedar-hawthorn rust takes 24 months to complete and requires infection of two different hosts.
Grow resistant varieties. Even though sanitation is not perfect follow 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-quince 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, University of Illinois at Urbana-Champaign.