Cause Bremia lactucae, a fungus-like microorganism of which there are at least three strains. The pathogen overwinters on wild hosts and weeds. It is serious in field-grown winter crops as well as in greenhouses and is worse in damp, foggy weather and between 43°F and 53°F.
Symptoms Light green or yellowish areas on the upper leaf surface are matched by downy patches on the undersurface. Affected parts turn brown, and leaves die. Older leaves are attacked first.
Cultural control
- Cultivate thoroughly. Particularly control weeds in the vicinity, because many weeds are hosts.
- Use resistant strains of lettuce, such as those developed in the Imperial Valley. These lettuce varieties are resistant to the downy mildew strains noted: Bullseye (II, III), Target (I, III), Dynasty (I), Bounty (I), Alpine (I), and Salinas (I). The cos variety 'Valmarie' is tolerant.
Chemical control
- Actigard at 0.75 to 1 oz/A every 7- to 10-days for up to four (4) applications per crop. Do not apply within 7 days of harvest. 12-hr reentry.
- CAA-fungicide (Carboxylic Acid Amides) formulations (Group 40) as a tank-mix with a fungicide with a different mode of action.
- Forum at 6 fl oz/A on 7-day intervals. May be applied the day of harvest. 12-hr reentry.
- 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 different mode of action. Preharvest interval is 1 day. 4-hr reentry.
- Copper formulations (Group M1) are not effective as stand-alone products
- Badge SC at 0.75 to 1.5 pints/A on 5- to 10-day intervals. Preharvest interval is 0 days. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
- Champ Formula 2 at 0.66 to 1.3 pints/A. 48-hr reentry.
- Cueva at 0.5 to 2 gal/100 gal water on 7- to 10-day intervals. May be applied on the day of harvest. 4-hr reentry. O
- Nu-Cop 50 DF at 1 to 2 lb/A on 5- to 10-day intervals. 48-hr reentry. O
- Previsto at 0.75 to 2 quarts/A on 7- to 10-day intervals. 48-hr reentry. O
- Curzate 60 DF at 3.2 to 5.0 oz/A on 5- to 7-day intervals. Do not apply within 3 days of harvest. 12-hr reentry.
- Mancozeb formulations (Group M3) are labeled.
- Dithane F45 Rainshield at 1.5 quart/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-220008, SLN OR-090016). 24-hr reentry.
- Roper DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-1300003, SLN OR-130003). 24-hr reentry.
- MilStop SP (85% potassium bicarbonate) at 2 to 5 lb/A. 1-hr reentry. O
- Orondis Gold (Group 49) at 4.8 to 9.6 fl oz/A at planting, in-furrow or by drip. Do not apply more than two (2) sequential applications. Preharvest interval is 0 day. 4-hr reentry.
- Orondis Ultra (Group 49 + 40) at 5.5 to 8 fl oz/A on 7- to 10-day intervals. Do not apply more than two (2) sequential applications. Preharvest interval is 1 day. 4-hr reentry.
- Phosphonates formulations (Group P7) should be very effective.
- Aliette WDG at 2 to 5 lb/A. Apply in at least 10 gal/A water. Do not apply within 3 days of harvest. Do not use in conjunction with copper-based fungicides. To reduce leaf speckling, adjust pH to 6 or higher when long periods of wet weather are expected. Do not mix with surfactants or foliar fertilizers. 24-hr reentry.
- Presidio (Group 43) at 3 to 4 fl oz/A on 10-day intervals in combination with another fungicide that has a different mode of action. Preharvest interval is 2 days. 12-hr reentry.
- Regalia (Group P5) at 0.5 to 1.5 quarts/A on 5- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Revus (Group 40) at 8 fl oz/A on a 7- to 10-day interval with no more than two (2) consecutive applications. Do not apply within 1 day of harvest. 4-hr reentry.
- Ridomil Gold SL at 0.125 to 0.25 pint/A in a tank-mix with another fungicide registered for downy mildew. 48-hr reentry.
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
- Cabrio EG 12 to 16 oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
- Dexter Max (Group M3 + 11) at 1.7 to 2.25 lb/A on 7- to 10-day intervals. Remove residues from head lettuce by stripping and trimming. Preharvest interval is 10 days. 24-hr reentry.
- Quadris Flowable at 12 to 15.5 fl oz/A on 7- to 14-day intervals. May be applied the day of harvest. 4-hr reentry.
- Reason 500 SC at 5.5 to 8.2 fl oz/A on 5- to 10-day intervals. Do not apply within 2 days of harvest. 12-hr reentry.
- Trilogy at 0.5% to 1%. Not labeled for use in Oregon. Do not use above 90°F or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Poor control as a stand-alone product. 4-hr reentry. O
- Zampro (Group 40 + 45) at 14 fl oz/A for no more than three (3) application per season. Do not apply more than two (2) applications before alternating to a fungicide with a different mode of action. Preharvest interval is 0 days. 12-hr reentry.
Biological control
- Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 4-hr reentry. O
- Actinovate Lawn & Garden at 1/2 to 1 teaspoon/gal water. H O
- Double Nickel LC at 0.5 to 4.5 pints/A on 3- to 10-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- LifeGard WG (Group P6) at 1 to 4.5 oz/A on 7- to 14-day intervals for activating plant resistance. Refer to label for appropriate rate per application volume. Studies by the University of Florida using higher rates showed a reduction in disease severity. Preharvest interval is 0 days. 4-hr reentry. O
- Romeo at 0.45 to 0.68 lb/A on 7- to 10-day intervals starting prior to infection. Preharvest interval is 0 days. 4-hr reentry. O
- Sonata at 2 to 4 quarts/A on a 7- to 14-day interval for disease suppression. Can be applied up to and on the day of harvest. 4-hr reentry. O
- Stargus at 6 to 8 fl oz per 1,000 ft row as an in-furrow treatment, 3 to 4 quarts/A as a soil drench (drip or chemigation) on 10- to 21-day intervals, or 2 to 4 quarts/A as a foliar spray on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
Reference Raid, R.N., Hartman, D., and Vital, J. 2017. Evaluation of SAR compounds alone and in combination with a protectant for control of lettuce downy mildew, Spring 2017. Plant Disease Management Report: Report No. 11:V154.