Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Cabbage and Cauliflower (Brassica) -- Downy Mildew (Staghead)
 
Cause: Peronospora parasitica, a funguslike microorganism that overwinters in roots or infected plant debris, affects nearly all cultivated and weed plants in the Crucifer family. Several strains of the microorganisms, however, are specific to certain groupings of crops. For example, the radish strain causes little damage to cabbage. Spores may be spread as contaminants on seeds. There is no proof that the fungus enters the seed. High humidity, fog, drizzling rains, and heavy dew favor disease development and spread. Optimum conditions for disease development are night temperatures of 46 to 61°F for 4 or more successive nights, and day temperature about 75°F or lower.

Resistance is being incorporated into commercial cultivars, but at present no resistant commercial cultivars are available.

Symptoms: The first observable symptom is small, light green-yellow lesions on the upper leaf surface, later showing on the undersurface. The spots turn yellow as they enlarge. During periods of high humidity, a grayish white moldy growth is on the leaf's underside. Later the leaf may become papery and die. Cabbage heads develop sunken black spots which may be minute or larger than 1 inch in diameter. Frequently, some plants become systemically infected in the seedling stage, but the black streaking and discoloration in leaves and heads does not become apparent until near harvest.

Heavily infected leaf.

Close-up of sporulation on the underside of the leaf.

 
Cultural control:
  1. Eradicate cruciferous weeds (wild mustards, etc.) that may harbor the fungus.
  2. Manage irrigation to reduce periods of high humidity.
  3. Spring-planted, summer-harvested crops have fewer problems than fall-harvested.
Chemical control: Apply first spray as soon as seedlings appear; repeat three times a week until plants are set out in field. Spraying for mildew requires completely covering the plant. In general,high-gallonage ground application has been more effective than air application.
  1. Agri-Fos at 1.25 quarts/A in 25 gal of water to 2.5 quarts/A in 250 gal water/A on 1- to 3-week intervals. Do not use with acidifying compatibility agents. 4-hour reentry.
  2. Aliette WDG at 2 to 5 lb/A. Do not apply within 3 days of harvest. Do not use within 7 days of any copper sprays. 12-hr reentry.
  3. Alude at 1.25 to 2.5 quart/A on 7- to 21-day intervals. 4-hr reentry.
  4. Bravo Ultrex at 1.4 lb/A with water to cover. 12-hr reentry.
  5. Cabrio EG at 12 to 16 oz/A on 7- to 14-day intervals. Do not make more than one application of Cabrio EG or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 0 days. 12-hr reentry.
  6. Copper products which at high rates can cause phytotoxicity on some cabbage cultivars in the form of flecking on wrapper leaves.
    1. Champ Formula 2 at 0.33 to 0.66 pt/A. 24-hr reentry.
    2. C-O-C-S WDG at 3 to 4 lb/A. 24-hr reentry.
    3. Cuprofix Ultra 40D at 0.75 to 1.25 lb/A on 7- to 10-day intervals. 12-hr reentry.
    4. Kocide 2000 at 0.75 to 1.5 lb/A or Kocide 3000 at 0.5 to 0.75 lb/A on 7- to 10-day intervals. 24-hr reentry.
    5. Nu Cop 50 WP at 0.5 to 1 lb/A on 7-day intervals. Do not apply within 1 day of harvest. 24-hr reentry.
  7. Dithane DF Rainshield at 2 lb/A for seed production only. Washington (SLN WA-020028) and Oregon (SLN OR-020030) only.
  8. Echo 720 at 1.5 pt/A on 7- to 10-day intervals when disease threatens. Preharvest interval is 7 days. 12-hr reentry.
  9. Forum at 6 fl oz/A on 7-day intervals. Must be applied as a tank-mix with another fungicide that has a different mode of action (non-group 15). Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. Do not apply within 7 days of harvest. 12-hour reentry.
  10. Legion 80 WDG at 2 to 5 lb/A on 7- to 21-day intervals. Do not apply within 3 days of harvest. 12-hr reentry.
  11. Maneb 75 DF at 1.5 to 2 lb/A. Do not apply within 7 days of harvest. 24-hr reentry.
  12. MilStop (85% potassium bicarbonate) at 2 to 5 lb/A. 1-hr reentry.
  13. Presidio at 3 to 4 fl oz/A on 10-day intervals as a tank-mix . Preharvest interval is 2 days. 12-hr reentry.
  14. Quadris at 6 to 15.5 fl oz/A on 7- to 14-day intervals. Do not apply more than one (1) foliar application of Quadris or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 0 day. 4-hr reentry.
  15. Revus at 8 fl oz/A on 7- to 10-day interval. Do not make more than two (2) foliar applications of Revus be-fore alternating to a labeled fungicide with a different mode of action (non-group 40). Preharvest interval is 1 day. 12-hr reentry.
  16. Ridomil Gold Bravo SC at 1.5 pints/A or Ridomil Gold SL at 0.125 to 0.25 pint/A tank-mixed with other fungicides registered for control of downy mildew. Warning Resistance to Ridomil has been confirmed in the Pacific Northwest. To minimize further development of resistance, do not make foliar applications if Ridomil was used at planting, do not use as a curative spray once disease has developed, and use only as a preventive spray and only in combination with another registered fungicide. Do not apply within 7 days of harvest. 48-hr reentry.
  17. Topaz at 1 to 3 quarts/A on 2- to 3-week intervals. 4-hr reentry.
Biological control:
  1. Sonata at 2 to 4 qt/A on 7- to 14-day intervals. Can be applied up to and on the day of harvest. 4-hr reentry.
Content edited by: Cynthia M. Ocamb on January 1, 2009
 
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