Colorful displays of three types of rust fungi are appearing on junipers in Ohio. The fungi belong to the genus Gymnosporangium and they complete part of their life-cycle on members of the plant genus Juniperus and the other part of their life-cycle on members of the family Rosaceae. In biological terms, this type of life-cycle is known as "heteroecious."
In terms of management, it's important to understand the two hosts are not "alternate hosts"; the fungi cannot complete their life-cycle confined to a Juniperus host OR a rosaceous host. They must alternate between the two types of hosts. Remove one of the hosts from the host-alternating life-cycle and these heteroecious fungi are doomed.
Eastern red cedar (J. virginiana) is a common Juniperus host for all three fungi. Of course, this is not a cedar. But, it's why "cedar" appears along with the name of a rose family host in the disease names of cedar-apple rust; cedar-quince rust; and cedar-hawthorn rust.
This disease is produced by the fungus, G. juniperi-virginianae. The "apple" in the disease name comes from apples and crabapples. The fungus produces large brain-like galls on the stems of juniper. Spectacular looking tentacle-like tangerine-orange growths, known as "telial horns", are extruded from mature galls in the spring during wet weather. Alas, the startling orange octopus effect disappears as the gelatinous horns dehydrate and collapse during dry spring weather.
The horns are composed of fungal teliospores, which give rise to basidiospores that are ejected into the air to drift onto apple hosts where they germinate, marking the beginning of the other half of the fungal life cycle. Occasionally, this fungus will infect hawthorns. Fungal infections cause little harm to both their juniper and apple hosts unless you’re an apple grower.
This disease is produced by the fungus, G. globosum. This fungus also produces irregularly shaped galls on junipers, but the growths may arise from the needles as well as the stems and measure only 1/4 - 1/2" in diameter. Short, blunt, gelatinous telial horns emanate from the galls in the spring. During periods of extended wet weather, the telial structures may ooze mucus-like onto the stems and foliage beneath the galls.
As its common name indicates, hawthorn is one of the required rosaceous hosts. However, the fungus will also infect apples and crabapples as well as occasionally infecting serviceberry, quince, and pear. As with cedar-apple rust, this fungus causes little harm to its juniper and apple hosts.
This disease is produced by the fungus, G. clavipes. Fungal infections on junipers appear as elongated canker-like stem swellings that produce longitudinal cracks in the bark. Bright orange spores ooze globule-like from the cracks during wet weather in the spring.
This rust has a much wider rosaceous host range compared to the other two rusts, this rust has a much wider rosaceous host range and is capable of producing more significant damage. Aside from infecting its namesake quince host, the fungus may also infect serviceberry, chokeberry, apple/crabapple, cotoneaster, mountainash, and hawthorn. Last year, we reported this rust on Callery pear.
Some of the most dramatic signs and symptoms of infection occur on the fruits (haws) of hawthorn as well as the fruits of serviceberry. Peculiar looking spore structures, known as aecia, sprout from the fruits. The aecia are filled with bright orange spores (aeciospores) making the fruits look like they're covered in orange hair.
Aside from reducing the aesthetic appeal of heavily infected trees, the bright orange aeciospores that are shed from the aecia may settle onto sidewalks, patios, outdoor furniture, slow-moving gardeners, etc., to bestow a noticeable burnt-orange patina.
The fungus may also infect hawthorn stems and thorns to produce more significant damage. Cankers associated with stem infections can produce significant twig dieback causing serious harm to heavily infested trees. The dieback may become widespread throughout the canopy with the overall tree symptoms mimicking a severe case of bacterial fire blight.
Of course, with multiple shared rosaceous hosts for cedar-quince rust and cedar-hawthorn rust, we occasionally see both rusts on the same host. A pathological twofer.