Cause Erysiphe aquilegiae (formerly Erysiphe polygoni), a fungus that is frequently on plants growing in shaded areas. Has been reported from Washington and observed in Oregon. The disease can reduce both plant quality and vigor. The time between infection and symptom development ranges from 4 to 11 days at temperatures from 50°F to 85°F, respectively.
Symptoms White powdery patches on leaves, petioles and stems, which can involve the entire leaf surface. Small dark specks that look like pepper (cliestothecia) will develop in the powdery areas late in the season.
Chemical control Fungicides will do best when used on immature tissues before symptoms develop. Few materials have good eradicant activity. Use at 7- to 14-day intervals; using shorter intervals when environmental conditions favor disease development. Alternate or tank-mix products from different groups that have different modes of action.
- Although not specifically registered, according to IR-4 data, products such as Heritage are safe on this crop. These products provide good control of powdery mildew on many other crops. Group 11 fungicide.
- MilStop (85% potassium bicarbonate) at 2.5 to 5 lb/A in the field or 1.25 to 5 lb/100 gal water in the greenhouse. Might be used to supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Thorough coverage is essential. Oregon and Washington only. 1-hr reentry. O
- Ortho Rose & Flower Disease Control at 2 fl oz/gal water. Group 3 fungicide. H
- Safer Garden Fungicide (Ready To Use 0.4% sulfur) thoroughly sprayed over the entire plant. Do not use when the temperature is over 85°F or within a few weeks of an oil spray. H
- Seido at 4 to 5 fl oz/100 gal water plus an adjuvant. Group 50 fungicide. 4-hr reentry.
- Thiolux (80% sulfur) at 3 to 10 lb/A. Group M2 fungicide. 24-hr reentry. O
Reference Xu, X. and Robinson, J. 2001. The effects of temperature on the incubation and latent periods of powdery mildew (Erysiphe polygoni) on clematis. Journal of Phytopathology 149:565-568.