Cause The fungus Erysiphe aquilegiae (Erysiphe polygoni) has been reported from Washington, and the disease has been found by the OSU Plant Clinic. The fungus thrives under humid (greater than 90%) conditions when leaves remain dry. Spores spread by wind and human movement of plant material.
Symptoms White powdery patches develop on leaves. Patches may look netted or grainy. Colonies may cover entire leaflets. Sometimes the leaves may turn purplish.
- Space plants for good air circulation.
Chemical control Fungicides will do best when used 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.
- Bicarbonate-based products. Might be used to supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Thorough coverage is essential. O
- 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. Oregon and Washington only. 1-hr reentry.
- Monterey Bi-Carb Old Fashioned Fungicide at 4 teaspoons/2 gal water. H
- Cosavet-DF (80% sulfur) at 3 to 10 lb/100 gal water. Group M2 fungicide. 24-hr reentry. O
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Labeled for rust control but also gives good control of powdery mildew. Group 3 fungicide. 24-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.
- Kumulus DF (80% sulfur) at 3 to 10 lb/A. Group M2 fungicide. 24-hr reentry. O
- 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.
- Regime at 20.5 to 45.7 fl oz/A. Reapply if rain occurs within 12 hours of original application. Group BM01 fungicide. 4-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide H
Reference Bradshaw, M., 2018. First report of powdery mildew caused by Erysiphe aquilegiae on columbine in the United States. Plant Health Progress 19:69-70.