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Juniper (Juniperus spp.) - Rust
Cause Gymnosporangium sabinae (formerly G. fuscum), a fungus closely related to a number of other rusts that alternate between conifer and Rosaceous hosts. Most popular ornamental junipers are susceptible such as J. chinensis, J. sabina, J. scopulorum and J. virginianna. The species J. communis, J. horizontalis, and J. squamata are immune or highly resistant. This disease is well established in western Washington, coastal British Columbia and in Contra Costa County, CA. Found in western Oregon on many different pears since 2016.
Telia are extruded from galls during wet weather in early spring. Basidiospores are dispersed at night to rosaceous hosts causing an annual infection. Aecia form on the underside of leaves on rosaceous hosts such as pear, in the fall. Aeciospores are dispersed to nearby junipers in the fall causing a perennial infection. Symptoms on juniper do not develop for 18 months.
Symptoms There are no obvious symptoms on juniper. Careful examination of infected plants reveals elongated, swollen galls on older wood. Needles may be shed from this swollen area and small or thin twigs may die if infected. Red to orange, jelly-like telia are extruded from the galls during wet weather in early spring.
Cultural control
- Propagate from pathogen-free mother plants. Purchase only certified rust-free stock.
- Remove infected junipers.
- Remove infected pear leaves within 100 m before sporulation in the fall (late August in British Columbia).
Chemical control Spray susceptible juniper in the nursery at least three (3) times from mid-August to early October (earlier in California) when aecia are seen on rosaceous hosts.
- 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.
- 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 EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Ormrod, D.J., O'Reilly, H.J., Van Der Kamp, B.J., and Borno, C. 1984. Epidemiology, cultivar susceptibility, and chemical control of Gymnosporangium fuscum in British Columbia. Canadian Journal of Plant Pathology 6:63-70.