Cause Peronospora sp., a fungus-like organism. Has not been reported in the Pacific Northwest but has been found in Washington. Favored by high moisture and crowded plant conditions.
Symptoms Yellow spots with either diffuse to angular borders and white to grayish-purple, fuzzy fungal growth under leaves.
- Scout for the disease and discard infected plants.
- Heat and ventilation will lower humidity and reduce condensation on plants.
- Avoid watering plants where they remain wet for extended periods of time.
- Ideal growing conditions include a pH of about 5.8 and day temperatures of 71°F to 79°F and 62°F to 67°F at night.
- Maintain a steady, relatively dry environment by keeping greenhouse humidity below 90%, increasing spacing between plants for good air circulation, and taking care not to splash water on foliage during watering. Heating the greenhouse at night or venting around sunset may be necessary.
Chemical control Focus on cultural control tactics first. Tank-mix and/or alternate products with different modes of action to prevent the build-up of resistant fungi. Limit the use of any one group during crop production.
- Alude at 1 quart/100 gal water applied as a foliar spray at 14- to 21-day intervals. Group P7 fungicide. 4-hr reentry.
- Compass 50 WDG at 1 to 2 oz/100 gal water. Do not use organosilicate additives. Group 11 fungicide. 12-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Heritage at 2 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Mefenoxam 2 AQ at 0.49 to 0.98 fl oz/100 gal water plus another fungicide with a different mode of action. Group 4 fungicide. 48-hr reentry.
- Micora at 4 to 8 fl oz/100 gal water. Group 40 fungicide. 4-hr reentry.
- Phyton 27 at 1.5 to 3 oz/10 gal water. Group M1 fungicide. 48-hr reentry.
- Segovis at 0.6 to 3.2 fl oz/100 gal water plus another fungicide. Group 49 fungicide. 4-hr reentry.
- Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
Reference Thines, M., Telle, S., Ploch, S., and Runge, F. 2009. Identity of the downy mildew pathogens of basil, coleus, and sage with implications for quarantine measures. Mycological Research 113:532-540.