Downy mildew is caused by the fungus-like organism Bremia lactucae, and is a serious disease of lettuce worldwide. The disease can be devastating in temperate climates if not controlled. Infection occurs when wind, seed, or soil-borne spores (oospores or sporangia) germinate on leaves in the presence of free moisture or relative humidity near saturation when temperatures are cool to moderate (40 to 86ºF). High and low temperatures extend the latent period and slow disease developed, but extended periods of morning leaf wetness favor infection. The pathogen survives between lettuce crops in and on lettuce seeds, pathogenically on wild Lactuca spp., and soil-borne oospores, but wind-blown spores can also be important in disease development.
Downy mildew symptoms first appear as angular, variably sized light green or light yellow lesions, but later become yellow or necrotic. Lesions are often bounded by large veins. When temperatures are moderate and humidity is high, sporulation is evident on leaves, especially on the lower sides of leaves. Older lesions become brown and necrotic. Severe infection can kill seedlings, but adult plants are rarely killed. Early infections can also become systemic and cause a dark brown discoloration of vascular tissues. Low levels of infection can downgrade the crop, causing significant trimming losses at harvest, and promoting decay during post-harvest storage. High levels of downy mildew can cause an entire crop to be unmarketable.
Biological control strategies have not been developed for downy mildew.
Plant lettuce varieties resistant to downy mildew. Resistant varieties effectively control the disease, but new races of the downy mildew pathogen have developed that can overcome single-gene resistance. Other varieties are available and can provide high levels of resistance to all races of the pathogen and have been more stable sources of resistance. Use subsurface drip irrigation; avoid sprinkler or furrow irrigation. If sprinkler irrigation is used, time irrigations to allow foliage to dry before dusk to prevent extended periods of leaf wetness.
Fungicides can provide effective downy mildew control, but strains of the pathogen insensitive to metalaxyl are widespread. Fungicides with different modes of action should be used if metalaxyl-insensitive strains are present.
|
Trade/Common Name |
Product per Acre |
Application Frequency |
Remarks |
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Azoxystrobin and Strobilurin Mixtures |
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Amistar |
4-5 oz |
5-7 days |
Maximum of 1.88 pounds or 4 applications per season; Rotate with fungicides with a different mode of action such as maneb or Aliette; 0 day PHI |
|
Quadris Flowable |
12.3-15.4 |
5-14 days |
Rotate with fungicides with a different mode of action such as maneb or Aliette; Maximum of 2.88 quarts or six applications per season; 0 day PHI |
|
Tanos |
8 oz |
5-7 days |
Rotate and tank-mix with fungicides with a different mode of action such as mancozeb or Aliette; Maximum of 36 ounces per season; 3 day PHI |
|
Copper |
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Copper fungicides |
Varies |
7-10 days |
Various formulations, such as Kocide 101, Top Cop + S, Tri Basic Copper, and Champ among others; 0-1 day PHI |
|
Dimethomorph |
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Acrobat 50WP |
6.4 oz |
5-10 days |
Tank-mix with another fungicide activity against downy mildew; do not make more than 2 sequential applications; 0 day PHI |
|
Forum |
6 oz |
5-10 days |
Tank-mix with another fungicide activity against downy mildew; maximum of 5 applications per season, maximum of 30 oz per acre per season, do not make more than 2 sequential applications; 0 day PHI |
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EBDC |
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Maneb |
Varies |
7-10 days |
Various formulations such as Maneb 75DF and Manex; 10 day PHI |
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Fosetyl-Al |
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Aliette 80 |
2-5 lb |
7-21 days |
Maximum of 7 sprays; must be applied in at least 15 gpa water; |
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Mefenoxam |
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Ridomil Gold EC |
1-2 pts |
Pre-plant incorporated soil drench or soil band (a 7” band is recommended) |
Suppression of early season downy mildew |
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Phosphorous Acid |
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Agri-Fos |
0.5 gallons |
7-21 days |
Apply in at least 200 gpa |
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: Lettuce, Disease, Downy Mildew
Date: 04/01/2007