The orange balls sometimes spotted on evergreen branches occur as the result of infection by a Gymnosporangium fungus. Different forms of the fungus cause one of two similar types of galls.
Cedar-hawthorn rust galls are caused by the fungus Gymnosporangium globosum. Affected trees include eastern red cedar, Rocky Mountain juniper, southern red cedar, common and prostrate junipers, apple and crabapple, most hawthorns, pear, quince and serviceberry, according to the University of Illinois Extension.
Galls have a 24-month life cycle that requires two hosts---an evergreen and a deciduous tree. Galls occur in the spring, following infection of the tree during the previous fall. Spring rains trigger mature galls to begin to develop gelatinous telial horns. These finger-like projections release spores that subsequently infect other trees. The spores have been recorded as infecting trees up to 14 miles away, according to the University of Illinois Extension. Galls grow during the summer months and reach their full size in fall.
Galls rarely kill the twig that holds them, but severe infections may cause the foliage to drop from affected hawthorn trees. The aecia (tube formations) form between day 80 and 90 of infection, approximately, according to the University of Illinois Extension. The galls are perennial, producing spores over multiple years. The immature form of the gall appears reddish brown or orange, and the mature gall turns brownish-gray.
Cedar-apple rust galls affect the juniper family of evergreen trees and shrubs, much like the cedar-hawthorn rust. According to the North Dakota State University Extension Service, the infection alternates between these plants and the pome fruit group of the rose family. This includes apple, pear, quince, hawthorn, mountain ash and juneberry.
Spread of Infection
Cedar-apple rust gets its name from the reddish-brown galls that mark the infection. Just as with cedar-hawthorn rust, this infection produces gelatinous tendril horns in spring that release spores. These spores travel to infect apple leaves and fruits.
Homeowners may have difficulty judging which infection is present on a tree. Observers can tell cedar-hawthorn rust galls from cedar-apple rust galls by comparing the aecia. The aecia of hawthorn rust galls are 1/8 inch longer than those of cedar-apple rust, and the cedar-hawthorn infection lasts approximately 10 days longer than cedar-apple rust, notes the University of Illinois Extension.
Treatment and Prevention
Trees can host multiple rust infections simultaneously. Extension services suggest growing resistant varieties, removing infected plant material and using fungicide to treat and prevent infection. Homeowners should avoid planting juniper and fruit trees, which are potential targets of infection near each other.