Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Onion (Allium cepa) -- Downy Mildew
 
Cause: Peronospora destructor, a funguslike microorganism that overwinters on infected plants or debris. No cultivar is resistant. Many Allium spp. are affected. Disease is favored by cool (less than 72°F) humid weather.
Symptoms: Slightly pale spots develop first, later taking on a light brown or purplish shade. As spots enlarge, often girdling the leaves, a grayish violet furry mold develops on the spot's surface. Plants may be dwarfed, distorted, and pale green. Plants often are not killed, but bulb quality is poor and often spongy. Lesion-weakened seed stems may break, causing seed to shrivel.

Note the gray growth on the upper leaf.
 
Cultural control:

  1. Practice a 3-year or longer crop rotation if possible.
  2. Avoid poorly drained soils.
  3. To minimize high humidity, plant in the direction of prevailing winds, avoid using wind breaks, manage plant density and spacing, and don't use overhead irrigation.
  4. Destroy onion plant refuse and cull piles.
  5. Eradicate volunteer or wild Allium spp.
Chemical control:
  1. Acrobat 50WP 6.4 oz/A on 5- to 7-day intervals in combination with another fungicide active against downy mildew. Do not apply more than twice sequentially before alternating with another fungicide with a different mode of action. Bulbs or leaves may be harvested on the day of the last application after sprays have dried.
  2. Agri-Fos at 2 quarts/A in 100 gal water as a regular preventative control or when disease first appears. Can be tank-mixed with Dithane. 4-hr reentry.
  3. Aliette WDG at 2 to 3 lb/A. Do not apply within 7 days of harvest. Do not combine within a copper spray program. For dry bulb onions. 12-hr reentry.
  4. Champ Formula 2 at 1.3 pints/A. 24-hr reentry.
  5. Cuprofix Ultra 40D at 1.25 to 3 lb/A on 7- to 10-day intervals. 12-hr reentry.
  6. Dithane DF Rainshield at 3 lb/A on 7-day intervals. Preharvest interval is 7 days. Do not apply to exposed bulbs. 24-hr reentry.
  7. Dithane DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on Allium seed crops. Washington and Oregon only (SLN WA-020028, SLN OR-020030). 24-hr reentry.
  8. Dithane F-45 Rainshield at 2.4 quarts/A on 7-day intervals. Preharvest interval is 7 days. Do not apply to exposed bulbs. 24-hr reentry.
  9. Forum at 6 fl oz/A on 5- to 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 another effective fungicide with a different mode of action. Bulbs or leaves may be harvested on the day of the last application after sprays have dried. 12-hour reentry.
  10. Kocide 2000 at 1.5 lb/A or Kocide 3000 at 0.75 to 1.5 lb/A on 7- to 10-day intervals. 24-hr reentry.
  11. Kop-R-Spray at 2 teaspoons/gal water.
  12. Legion 80 WDG at 2 to 3 lb/A on 7- to 14-day intervals. Do not apply within 7 days of harvest. 12-hr reentry.
  13. Maneb 75 DF at 2 to 3 lb/A. Do not apply within 7 days of harvest or more than 32 lb/A/season for bulb onions and 14.9 lb/A/season for green onions. 24-hr reentry.
  14. ManKocide at 2.5 lb/A on 3- to 7-day intervals. Do not apply within 7 days of harvest. 24-hr reentry.
  15. Manzate 75 DF at 3 lb/A. Dry bulb onions only. Do not apply to exposed bulbs or within 7 days of harvest. 24-hr reentry.
  16. Nu Cop 50 WP at 2 lb/A on 7- to 10-day intervals. Do not apply within 1 day of harvest. 24-hr reentry.
  17. Presidio at 3 to 4 fl oz/A on 10-day intervals as a tank-mix. Preharvest interval is 2 days. 12-hr reentry.
  18. Quadris at 9 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 days. 4-hr reentry.
  19. Quadris Opti at 1.6 to 3.2 pints/A on 7- to 14-day intervals. Do not apply more than one (1) foliar application of Quadris Opti or other Group 11 fungicide before alternating to a fungicide with a different mode of action. Do not apply within 7 days of harvest. 12-hr reentry.
  20. Reason 500 SC at 5.5 fl oz/A on 5- to 10-day intervals. Do not apply more than one (1) foliar application of Reason 500 SC or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Do not apply within 7 days of harvest. 12-hr reentry.
  21. Revus at 8 fl oz/A on 7- to 10-day intervals. Do not make more than two (2) foliar applications of Revus before alternating to a labeled fungicide with a different mode of action (non-group 40). Preharvest inter-val is 7 days. 12-hr reentry.
  22. Ridomil Gold Bravo SC at 2.5 pints/A before infection and continue on 7- to 14-day intervals. Do not apply to dry bulb onions within 7 days of harvest or to green onions within 14 days of harvest. 48-hr reentry.
  23. Ridomil Gold MZ at 2 to 2.5 lb/A. Do not apply within 7 days of harvest. For dry bulb onions only. 48-hr reentry.
  24. Topaz at 1 to 3 quarts/A in 100 gal water on 2- to 3-week intervals. Do not apply more than 6 times per crop per season. 4-hr reentry.
Biological control: Efficacy in Oregon unknown.
  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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