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. 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 Small, discrete white patches develop on both leaf surfaces. Later, patches coalesce until a powdery mass of white mycelium and spores cover the entire leaf. Leaves later become yellow, brown and prematurely defoliate.
- 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.
- Practice a 3-year rotation with nonsusceptible crops and manage crucifer weeds.
Chemical control Foliar fungicide applications can help when under severe disease pressure.
- Bi-Carb at 4 teaspoons/2 gal water on 7- to 10-day intervals. Thorough coverage is critical. May be applied up to the day before harvest. H 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.
- 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.
- 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.
- Quadris Flowable at 6 to 15.5 fl oz/A applied prior to disease development. Preharvest interval is 0 days. 4-hr reentry.
- Sulfur formulations are registered for radish (Microthiol Disperss O, 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.
- 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
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.