Gymnosporangium clavipes is a widespread rust (fungus) that infects over 480 species in 11 genera. Also called cedar quince/hawthorn rust, G. clavipes is heteroecious, meaning that it requires two alternate hosts to complete it life cycle.
One host, various members of Juniperus, is infected in late summer or early fall. The spores germinate, infect the host and then remain asymptomatic until the following spring. Infections result in mild swelling of twigs. The bark becomes flaky and many twigs die the first year of infection. Those twigs that survive become perennially infected and can produce spores for up to 20 years. The infection first appears as orange masses in bark cracks. After spring rainfalls, the infection gelatinizes into gooey masses (telia). The telia mature and release basidiospores which are carried by the wind. Any basidiospores that land on members of the rose family (second host), to which serviceberry (Amelanchier alnifolia) belongs, can germinate. Cedar quince/hawthorn rust may infect leaves, petioles, thorns, young branches or fruit of this second host. When the fruit is infected, long tubes (aecia) up to 1/2 inch form on the fruit. The fruit dies. In late summer or fall, the aecia release aeciospores, which are borne by the wind back to the primary host (Juniperus species) to begin the cycle again.
Generally, G. clavipes infection itself is not considered life-threatening to the hosts. However, infection can weaken the host and predispose it to injury from insects, weather or other pathogens.
Gymnosporangium clavipes galls on black hawthorn (Crataegus douglasii) are pictured in my post “Cedar Quince/Hawthorn Rust” from 08-03-16.
These serviceberry galls were on infected shrubs near the Lower Ash Creek Campground bridge (Lassen County CA).