Cedar, cypress, and juniper rusts—Gymnosporangium spp.
These fungi infect junipers in some urban areas and cedar, cypress, and juniper in cool, moist areas of their native growing range such as the Sierra Nevada. Rosaceous species alternate hosts of the fungi (e.g., pear and serviceberry) can also be infected.
The most conspicuous symptom on conifers is the presence of dark green, tight bunches of shoot growth (witches' brooms) on perennially infected branches. The brooms are often scattered throughout the canopy of the tree or shrub. The largest and oldest brooms usually occur in the lower canopy. Some brooms can grow more than 3 feet in diameter.
Small, round, reddish cushions that form on conifer needles in spring also indicate infection by Gymnosporangium spp. Later in the year, when these cushion-like, spore-forming bodies mature and become wet, they appear as orange, gelatinous masses.
On the fruit, green stems, and leaves of the rosaceous species alternate hosts, groups of small, cup-shaped fruiting bodies containing orange fungal spores appear as pustules and are symptomatic of infection.
Certain mistletoes could be confused with the witches' brooms caused by Gymnosporangium rusts. For example, Phoradendron juniperinum =P. libocedri, infects incense cedar and junipers; when viewed from a distance it resembles brooms caused by Gymnosporangium rusts. The mistletoe is distinguished by the presence of leafless, green shoots growing in rounded, clusters. The mistletoe's green shoots appear to be leafless because its leaves are only 1/10 inch long and lie scalelike along stems.
Gymnosporangium spp. require two hosts in different plant families to complete their life cycle. Most have four or five spore stages. The fungus on its conifer host produces spores in spring and summer. These spores disperse in air to infect the young tissues of the alternate host, a Rosaceae. The fungus grows in the alternate host and eventually produces airborne spores that infect the conifer host, usually in late summer or fall.
Incense cedar rust, Gymnosporangium libocedri, also called cedar broom rust, alternates between incense cedar and mainly serviceberry, Amelanchier spp. Apple, hawthorn, mountain ash, pear, and quince are also alternate hosts.
Gymnosporangium rusts cause galls, stunted and bushy branches (e.g., witches' brooms), stem dieback, and orange gelatinous spore masses on conifers. The damage usually has minimal effect on plant health. Young infected conifers are often deformed and sometimes die, but larger trees are seldom killed, even though some growth reduction and brooming occur.
Only when trees are heavily infected or exhibit extensive brooms do they suffer growth reduction and mortality. The lumber quality is degraded in trees grown for harvest.
On their rosaceous species alternate hosts (e.g., apple, hawthorn, and pear), Gymnosporangium rusts cause nonlethal swellings and colorful spots on fruit, leaves, and twigs. The rosaceous hosts usually suffer little damage. An exception is when pear fruit become infected and deformed by G. libocedri, a disease named pear rust, or Pacific coast pear rust.
Prune and dispose of infected conifer twigs and limbs in the fall, before spores are produced in the spring. Remove galls by making cuts in healthy wood below swollen, infected tissue. Avoid overhead watering. Removing nearby alternate hosts of the fungus may help to reduce new infections. Consult Diseases of Pacific Coast Conifers or Pests of the Native California Conifers for more information.
Juniper rust witches' brooms
Orange, spore-forming bodies
Gymnosporangium rust spots
Cup-shaped fruiting bodies