Cause The fungus, Erysiphe cruciferarum (syn. E. polygoni), can infect most crucifer crops and cruciferous weeds, but strains exist that have specificity and will infected only a subset of the crucifers. Older leaves are more susceptible to powdery mildew than younger leaves. Stressed plants will be more susceptible to disease. The fungus survives on infected hosts or as chasmothecia (formerly cleistothecia, sexually-produced survival structures) on infected crop debris. Disease is promoted under cool conditions of low rainfall and reduced relative humidity.
Symptoms A powdery, whitish-gray coloring consisting of the fungus and its spores develops on infected leaves. In advanced stages, leaves are distorted, twisted, and retarded in growth; mature rutabaga leaves may turn purple with severe infections. Leaves ultimately turn yellow and then die prematurely. On resistant rutabaga varieties, a purplish coloration may develop around infections sites.
Cultural control
- Plant resistant varieties if available.
- Avoid over-application of nitrogen fertilizers; use an optimum, balance fertility regime.
- Avoid drought stress.
- Avoid continuous cropping of susceptible crucifers.
- Crop rotation. If only powdery mildew is a concern, it is enough to rotate out of crucifers just one growing season so long as all volunteers and cruciferous weeds are controlled.
Chemical control Foliar fungicide applications can help when under severe disease pressure.
- Bicarbonates are registered to control powdery mildew. Thorough coverage is essential.
- Bi-Carb Old Fashioned Fungicide at 4 teaspoons/2 gal water on 7- to 10-day intervals. May be applied up to the day before harvest. H O
- Carb-O-Nator at 2.5 to 5 lb/100 gal of water on 5- to 14-day intervals, depending on disease pressure. Preharvest interval is zero (0) days for all listed crops. 4-hr reentry. O
- Kaligreen at 2.5 to 3 lb/A on 7- to 10-day intervals. May be applied up to the day before harvest. 4-hr reentry. O
- MilStop SP at 2 to 5 lb/A on 7- to 14-day intervals. Can be applied up to and including the day of harvest. 1-hr reentry. O
- Cevya (Group 3) at 3 to 5 fl oz/A on minimum interval of 7 days. Preharvest interval is 7 days. 12-hr reentry.
- Fontelis (Group 7) at 16 to 30 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 0 days. 12-hr reentry.
- Merivon Xemium (Group 7 + 11) at 4 to 5.5 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) applications before alternating to a fungicide in different FRAC groups (non-Group 7 and non-Group 11). Preharvest interval is 7 days. 12-hr reentry.
- Miravis Prime (Group 7 + 12) at 6.8 fl oz/A on 7- to 10-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 7 days. 12-hr reentry.
- Oils are labeled to control powdery mildew. Thorough coverage is essential. Cannot be used with or close to sulfur applications. Do not spray if temperature is below 50°F or above 90°F or when plants are wet or under heat or moisture stress. See label for details.
- Trilogy at 0.5% to 1%. Not labeled for use in Oregon. Poor control as a stand-alone product. 4-hr reentry. O
- Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide on 7- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than two (2) applications of any Group 11 fungicide per season.
- Cabrio EG at 12 to 16 oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
- Sulfur formulations are registered for kale (Microthiol Disperss O, Sulfur 6L, etc.). Sulfur is fungitoxic in its vapor phase and, therefore, is effective only when air temperatures promote volatilization. Sulfur volatilizes above 65°F but becomes phytotoxic above 95°F. Using it above 85°F is not recommended. Although sulfur reduces sporulation of established infections, it is primarily a protectant and must be applied before infection. Begin applying when weather conditions are appropriate and continue at weekly intervals. See label for details on rates and reentry intervals. Note: Frequent sulfur applications can negatively affect beneficial insects.
Biological control Efficacy unknown in Oregon.
- Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 4-hr reentry. O
- Actinovate Lawn & Garden at 1/2 to 1 teaspoon/gal water. H O
- Double Nickel LC at 0.5 to 4.5 pints/A on 3- to 10-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- Ecoswing at 1.5 to 2 pints/A. Preharvest interval is 0 days. 4-hr reentry. O
- Romeo at 0.45 to 0.68 lb/A on 7- to 10-day intervals starting prior to infection. Preharvest interval is 0 days. 4-hr reentry. O
- Sonata at 2 to 4 quarts/A on 7- to 14-day intervals. Can be applied up to and on the day of harvest. 4-hr reentry. O
Reference Shattuck, V.I. 2007. Powdery Mildew. p. 37-39 in Compendium of Brassica Diseases. Rimmer, S.R., Shattuck, V.I., and Buchwaldt, L. (eds.). St. Paul, MN: APS Press.