I’ve been fascinated by galls lately.
Most of the galls I’ve looked at have been induced by insects. This one, though, is caused by a Rust fungus. I’m thinking that it’s one of the Gymnosporangium genus, probably Cedar-Apple Rust, Gymnosporangium juniperi-virginianae.
One of the interesting things about Juniper-apple Rust is that it requires two hosts. Part of its life cycle is on a Juniper, and the other part of its cycle is on Apples, Service-berries, Hawthorns, or other plants in the Rosacea.
The Juniper gets infected by spores that come from the Apple. The fungus spores germinate on a Juniper twig, and “tell” the tree to grow a woody swelling, as in these photos.
Then, after rainstorms in the Spring or early Summer, the gall grows what look like tentacles – bright orange, gelatinous, spore-producing sporangia. This phase is the ‘telial stage’ and is quite dramatic. Sadly, I do not have photos of those.
The Juniper gall’s “tentacles” produce spores for a couple of weeks, and these wind-borne spores infect Apples, Service Berries, Hawthorns, or other members of the Rosacea.
Here, the spores grow and produce yellow-orange lesions on the Apple’s leaves and fruit. During the dry weather in the late Summer, these lesions sprout small tubes or cups, which, in turn, release spore-powder into the wind.
The wind-blown spores from Apples, Hawthorns, etc, go on to infect Juniper trees.
And the cycle starts all over again.
The infected Juniper may lose a few twigs or branches, but the infection doesn’t seem to do much over-all harm to the Juniper.However, the Apple, Hawthorn or Service-berry infections can have serious effects on the infected plant, causing it to drop its leaves and fruit.
Sources:
Russo, Ronald A. 2021. Plant Galls of the Western United States (Princeton Field Guides, 142). Princeton University Press. ISBN-10: 0691205760.
Plant Disease Diagnostic Clinic. Cedar-Apple Rust: Gymnosporangium juniperi-virginianae. Cornell University, College of Agriculture and Life Sciences. Plant Disease Diagnostic Clinic, Plant Pathology and Plant-Microbe Biology Section.
