Cause Gymnosporangium sabinae (sometimes referred to as G. fuscum), a fungus. The disease affects pear and juniper, which is the alternate host of the fungus. The disease was established only in southern British Columbia and Contra Costa County, CA. Then it was found on pear in the Bellingham, WA area just south of the Canadian border in 1997. Since then the disease has been commonly found on pear leaves in western Washington. Found in western Oregon on pear starting in 2016 (Benton, Clackamas and Marion counties only). As of 2018, the disease has not been a problem in commercial orchards in any PNW locations including BC.
Telia are extruded from Juniper galls during wet weather in early spring. Basidospores are dispersed at night to rosaceous hosts such as pear causing an annual infection. Aecia form on the underside of leaves late summer and into the fall. Aeciospores are dispersed to junipers causing a perennial infection.
Symptoms Yellow to bright orange leaf spots appear on leaves, fruit, twigs, and branches of pear. Occasionally, infected fruit mummifies and falls to the ground. Leaf lesions can grow together and cover much of the leaf surface when the disease is severe. These symptoms occur in spring and can be confused with other rust species. Unlike other rusts, acorn-shape aecia form on the lower leaf surface. They are up to 6 mm high and rupture along lateral slits. Aecia can develop by mid-June and into the fall and again another growing season if on perennial parts of tree.
- Eliminating either host is the only practical cultural control. If pear is the important plant, remove all juniper from a 1000-ft radius.
Chemical control The following do not specifically list pear rust but can be used to control the disease.
- Aprovia at 5.5 to 7 fl oz/A plus an adjuvant. Do not use within 30 days of harvest. Group 7 fungicide. 12-hr reentry.
- Inspire Super at 12 fl oz/A. Do not apply within 14 days of harvest. Group 3 + 9 fungicide. 12-hr reentry.
- Fontelis at 16 to 20 fl oz/A. Do not use within 28 days of harvest. Group 7 fungicide. 12-hr reentry.
- Mancozeb products such as Dithane, Manzate Pro-Stick, or Penncozeb at 3 or 6 lb/A. Do not combine 6 lb/A prebloom or 3 lb/A all-season schedule. See label for details. Do not apply within 77 days of harvest. Group M3 fungicides. 24-hr reentry.
- Procure 480 SC at 8 to 16 fl oz/A. Do not apply within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
- Tebucon 45 DF at 4 to 8 oz/A. It is unknown if there is a PGR effect on shape when using these products during bloom. Do not use within 75 days of harvest. Group 3 fungicide. 5-day reentry.
- Topguard at 8 to 12 fl oz/A. Do not use within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
- Trionic 4 SC at 8 to 16 fl oz/A. Do not apply within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
Note Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Merivon and Pristine.
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.