This foliar disease is caused by the fungus-like organism Peronospora destructor. Infection is favored by temperatures less than 72°F, and free moisture from rain, dew, or overhead irrigation in early spring or late summer. The fungus overwinters in volunteer onion, culls, and wild Allium species.
Downy mildew symptoms appear on older leaves as elongated patches which vary in size and are slightly paler than the rest of the foliage. With moisture, these areas become covered with a violet gray mycelium, which contain spores that may be spread to surrounding healthy tissue. Leaves generally fold over at affected areas, and leaf tips die. Another fungus (Stemphylium botryosum) may develop on primary lesions and produce a brown to black fungal growth. Affected plants may be dwarfed and leaves become pale green, deformed and often are covered by fungal mycelium. Bulb size and quality are reduced by infection occurring before maturity.
Biological control strategies have not been developed for downy mildew.
Practice a three-year or longer to non-hosts such as small grains and corn. Reduce over wintering and survival of the pathogen by eliminating culls and volunteers. Avoid dense plantings of late maturing varieties and overhead irrigation. Apply adequate but not excessive nitrogen fertilizer before bulb initiation. Orientate rows parallel to the prevailing wind direction and avoid wind breaks to minimize periods of leaf wetness. Onion varieties with high levels of downy mildew resistance are being developed.
Chemical Control
Chemical controls are most effective when combined with sound cultural practices. Copper fungicides and chlorothalonil are registered for control of downy mildew, but are not highly effective.
Product List for Downy Mildew:
Pesticide |
Product per Acre |
Application Frequency
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Remarks |
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Chlorothalonil and Chlorothalonil Mixtures—not all formulations listed |
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Bravo S |
3-4 pt |
7-10 days |
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Bravo Ultrex |
0.9 –2.7 lb |
7 days |
Maximum of 18 pounds per season; 7 day PHI |
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Bravo Weather Stik |
1-3 pt |
7 |
Maximum of 20 pints per season; 7 day PHI |
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Chlorothalonil 4L |
1 -2 1/8 pt |
7-10 days |
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Equus 720 |
1-2 pt |
5-10 days |
Maximum of 20 pints per acre; May be applied through sprinkler irrigation at no more than 10-day interval; other formulations of Equus are available |
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Quadris Opti |
1.6-3.2 pt |
5-14 days |
Maximum of three applications per season; Rotate with fungicides with a different mode of action; 14 day PHI |
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Ridomil Gold Bravo |
2 lb |
14 days |
Maximum of 4 applications, 7 day PHI |
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Ridomil Gold Bravo Liquid |
1 pack per 10 acres |
14 days |
Maximum of 4 applications, 7 day PHI |
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Terranil 6L |
1-2 pts |
7-10 days |
7 day PHI |
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Copper-based Fungicides—not all formulation listed |
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Champ DP |
1.33 lb |
7-10 days |
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Copper-Count-N |
2 qt |
7-10 days |
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Cuprofix |
2.5-6 lb |
7-10 days |
Can be phytotoxic |
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Cuprofix MZ Disperss |
5-7.25 lb |
3-7 days |
Maximum of 78 pounds per season; 7 day PHI |
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Kocide 2000 |
1.5 lb |
7-10 days |
Can be phytotoxic to leaves |
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Kocide 3000 |
0.75 lb |
7-10 days |
Can be phytotoxic to leaves |
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Nordox |
2-4 lb |
7-10 days |
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NuCop 50WP |
2.0 lb |
7-10 days |
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Top Cop with S |
2-3 qt |
7-10 days |
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Dimethomorph |
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Acrobat 50WP |
6.4 oz |
5-7 days |
Maximum of 5 applications per season; Must be tank-mixed and rotated with fungicides with a different mode of action; 0 day PHI |
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Forum |
6 oz |
5-7 days |
Maximum of 5 applications and 30 oz/acre per season; Must be tank-mixed and rotated with fungicides with a different mode of action; 0 day PHI |
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EBDC and EBDC Mixtures—not all formulations listed |
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Cuprofix MZ |
5-7.25 lb |
3-7 days |
Maximum of 78 pounds per season; 7 day PHI |
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Dithane DF |
3 lbs |
7 days |
Maximum of 32 pounds per season; Amended with a non-ionic surfactant to improve performance; 7 day PHI |
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ManKocide |
2.5 |
3-7 days |
Maximum of 160 pounds per season; 7 day PHI |
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Manex |
1.2-2.4 qt |
7 days |
Maximum of 24 quarts per acre; 7 day PHI |
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Penncozeb 75DF |
2-3 lb |
7 days |
Maximum of 24 pounds per season; 7 day PHI |
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Ridomil Gold MZ |
2.5 lb |
14 days |
Maximum of 4 applications; 7 day PHI |
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Fosetyl-Al |
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Aliette WDG |
2-3 lb |
7-14 days |
Maximum of 7 applications per season; 7 day PHI |
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Phosphorous Acid |
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Phostrol |
2.5-3.75 pts |
7-14 days |
Maximum of 7 applications per season |
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Strobilurin and Strobilurin Mixtures |
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Cabrio |
8-12 fl. oz |
14 days
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Maximum of 6 applications and 72 oz per season; Rotate with fungicides with a different mode of action; 7 day PHI |
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Pristine |
10.5-18.5 oz |
14 days |
Maximum of six applications per season; Do not make more than 2 consecutive applications before rotating to a fungicide with a different mode of action; 7 day PHI |
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Quadris FL |
6.2 - 15.4 fl oz |
5- 14 days |
Maximum of four applications per season; Rotate with fungicides with a different mode of action; 0 day PHI |
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Quadris Opti |
1.6-3.2 pt |
5-14 days |
Maximum of three applications per season; Rotate with fungicides with a different mode of action; 14 day PHI |
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The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality. |
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Categories: Onion, Disease, Downy Mildew
Date: 04/01/2007