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, and then necrotic. Soft rot pathogens may invade areas with powdery mildew.
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.
- Practice a 3-year rotation with nonsusceptible crops and manage crucifer weeds.
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. HO
- 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
- Demethylation-inhibiting (DMI) Fungicides (Group 3) are labeled for use.
- Procure 480SC at 6 to 8 fl oz/A on 14-day intervals. Do not apply within 1 day of harvest. 12-hr reentry.
- Rhyme (Group 3) at 5 to 7 fl oz/A. Alternate or tank-mix with a non-Group 3 fungicide for resistance management. Preharvest interval is 14 days. 12-hr reentry.
- Tebuconazole formulations (Onset 3.6L, Tebustar, Monsoon, Tebu-Crop 3.6F, Toledo 3.6F, etc.) at 3 to 4 fl oz/A on 10-day intervals for Chinese cabbage only. Do not apply within 7 days of harvest. 12-hr reentry.
- Miravis Prime (Group 7 + 12) at 11.4 fl oz/A on 7-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.
- JMS Stylet-Oil at 3 to 6 quarts/100 gal water. 4-hr reentry. O.
- 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 5- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Rhyme (Group 3) at 5 to 7 fl oz/A. Alternate or tank-mix with a non-Group 3 fungicide for resistance management. Preharvest interval is 14 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.
- Reason 500 SC at 5.5 to 8.2 fl oz/A. Do not apply within 2 days of harvest. 12-hr reentry.
- Sulfur formulations are registered for cabbage and cauliflower (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.
- Premixes of Group 3 + 11 fungicides are available for use. Make no more than two (2) sequential applications.
- Quadris Top (Group 11 + 3) at 12 to 14 fl oz/A. Do not apply within one day of harvest.12-hr reentry.
- Premixes of Group 3 + 9 fungicides are available for use. Make no more than two (2) sequential applications.
- Inspire Super at 16 to 20 fl oz/A on 7- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
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 0.23 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.