Cause Several fungi in the genus Gymnosporangium cause rusts on juniper and members of the rose family such as serviceberry and pear. Seven different species are reported from Oregon, nine from Washington, and six from Idaho. Gymnosporangium bethelii, G. harknessianum, G. kernianum, and G. nelsonii have been reported on J. occidentalis (California juniper) in Oregon. Spore stages produced on junipers infect only members of the rose family in early spring over a 2- to 3-year period. The spore stages produced on serviceberry, pear, or others in the rose family only infect junipers during the course of one summer.
Cedar apple rust is common east of the Rockies on J. virginiana. In the West, it is rare on J. virginiana and J. scopulorum. Galls on juniper in the West are more likely due to other species of rust.
Symptoms G. nelsonii (and G. bethelii) may produce woody, round, to elongate, galls on juniper branches and small stems. Dark brown, tongue-shaped structures (telia) emerge from these galls or just the juniper branches after a rain in the spring. G. kernianum will produce large witches' brooms in junipers. G. harknessianum produces reddish-brown, gelatinous masses on juniper leaves.
Cultural control To prevent juniper infection, remove Rosaceous hosts from the vicinity, or remove infected leaves from them before they begin to sporulate in mid- to late summer.
Chemical control Apply when spores are being shed from the Rosaceous host.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Bayleton 50 T&O at 5.5 oz/275 to 550 gal water. Landscape only, not for use on plants for sale. Group 3 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Mural at 4 to 7 oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Terraguard SC at 2 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Tourney 50 WDG at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trigo at 3 to 9 oz/100 gal water. Group 3 + 11 fungicide. 12-hr reentry.
Reference Scharpf, R.F., technical coordinator. 1993. Diseases of Pacific Coast Conifers. USDA Forest Service Ag Handbook 521.