Cause Several fungi can cause leaf spots on primrose. Ramularia primulae has been reported in Washington and isolated at the OSU Plant Clinic. Can be serious on cultivars grown in rock gardens. The fungus survives in soil and can be spread by soil movement as well as through airborne conidia. Conidia infect older, mature leaves.
Symptoms Yellow, angular blotches with ash-color centers on older leaves. A chlorotic halo may surround spots.
Cultural control Remove and destroy infected and fallen leaves.
- Exotherm Termil at 1 can/1500 sq ft of greenhouse. It ignites to form a vapor that condenses back on the plants. See label for details. Group M5 fungicide. 24-hr reentry with no ventilation or 12-hr reentry with ventilation.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Pageant at 12 to 18 oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Adebayo, A.A. 1973. Field and laboratory studies on Ramularia primulae and Cercosporella primulae. Transactions of the British Mycological Society 61:159-165.