Cedar Apple Rust Fungus on Juniper

We saw this Cedar Apple Rust on Juniper when we did a three-hour walk on the Upper Plateau at Edworthy Park on May 23rd. In a few places, it had formed beautiful "rosettes" like this one.

"Cedar apple rust, which is caused by the fungus Gymnosporangium, needs juniper plants and certain Rose family plants (such as saskatoons, hawthorns, and in some cases, apples) to complete its life cycle. On junipers, the disease appears as woody, spherical galls. In the spring (early May), brown, horn-like projections called "telia" grow out of the woody galls. During wet weather, the telia absorb water, swell up immensely, and become orange and gelatinous. At this stage the disease emits spores that infect the Rose family plants to cause the bright orange spots. The orange spots will eventually produce their own horn-like structures called "aecia" on the fruit and underside of the leaves; from the aecia, spores are produced that re-infect the junipers in the late summer. The disease must pass from junipers to Rose family plants to junipers again; it cannot spread between Rose family plants."

gardenline.usask.ca/fruit/rust.html