Witches' Broom Rust
Gymnosporangium spp. [VERIFY: hosts inconsistent with G
13 host plants · Fungal
Last updated
This profile contains verified disease data from extension databases. Regional field notes and expert review are in progress.
Dense, bushy branch proliferation forms like a broom in the canopy, caused by rust fungi or pathogens infecting branches. Prune out brooms well below the affected area, or remove heavily infested trees.
Rust fungi often need two different host plants to complete their life cycle, so identifying and managing the alternate host can break the cycle. Remove heavily infected leaves and dispose of them away from the garden. Improve air circulation to speed leaf drying after rain. Fungicide applications are most effective as preventive treatments before symptoms appear; once pustules are visible, the current infection cycle is already underway.
Quick Reference
Management
Juniperus (telial host): telia present March-May during spring rains. Rosaceae hosts: susceptible during bloom period (April-May in western Washington) through early summer when new foliage emerges and buds are swelling. (Source: PNW Plant Disease Management Handbook)
Wet spring weather at temperatures 50-65°F triggers telia extrusion and basidiospore release from Juniperus galls. Infection of Rosaceae hosts occurs when spores land on susceptible tissue (leaves, buds, shoots) during bloom. (Source: Gymnosporangium lifecycle phenology)