Stem and Cone Rusts of Pine
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS

Gymnosporangium Rusts
Gymnosporangium spp

Hosts: Junipers and Arizona cypress

Figure 230. Figure 230a. Telia stage of G. speciosum on unidentified juniper.

Symptoms/Signs:

Figure 230b. Telia stage of G. clavariiforme on common juniper. Figure 230b. Telia stage of G. calvariiforme on common juniper.
 There are eight species of Gymnosporangium rusts of juniper in the Southwest and one on cypress. Most Southwestern Gymnosporangium rusts alternate on members of the Rosaceae family (especially serviceberry and hawthorn). The very showy G. speciosum alternates on littleleaf and false mock orange, members of the Hydrangeaceae. On the evergreen host tree, these rusts cause witches’ brooms, galls, other branch distortions, and dieback of twigs and branches. Brown to orange hornlike or cushionlike projections (telia) are produced in the spring; these swell and gelatinize during wet periods and are quite spectacular. On alternate hosts, the rust develops colorful spots and localized swellings on leaves, fruits, and green twigs, followed by casting or distortion and death.

Biology:  Some Gymnosporangium species complete their life cycles in 1 year, but many require 2 years. Two spore stages form in spring through late summer on the alternate host. The second spore type is wind disseminated and infects juniper or cypress (depending on the rust species). Gymnosporangium species overwinter in their evergreen hosts, where they produce telia in the spring. The telia produce the final spore stage that infects the alternate host. Many of these rust species become perennial in juniper or cypress and produce telia annually on galls, swellings, or witches’ brooms, until the infected area is killed by the infection.

Effects:  In the Southwest, these fungi generally cause minimal damage to junipers, other than some deformities. Impact on the foliage and fruits of the alternate hosts may be more significant in years with adequate rainfall, causing early defoliation.

Similar Diseases:  Witches’ brooms caused by some of the juniper rusts could be confused with mistletoe infection.

References:  79, 93, 117