Cause Bremia lactucae, a fungus-like microorganism. The pathogen overwinters on wild hosts and weeds. It 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.
- Manage irrigation to reduce periods of high humidity.
- Spring-planted, summer-harvested crops have fewer problems than fall-harvested.
Chemical control
- Actigard 50WG (Group P1) at 0.75 to 1 oz/A every 7 days can suppress the development of downy mildew by inducing host-resistance pathways. Do not apply within 7 days of harvest. 12-hr reentry.
- CAA-fungicide formulations (Group 40) in combination with another fungicide that has a different mode of action. Do not apply more than once before alternating to a different mode of action
- Forum at 6 fl oz/A on 7-day intervals. Do not apply within 7 days of harvest. 12-hr reentry.
- Revus at 8 fl oz/A on 7- to 10-day intervals. 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.
- Bonide Liquid Copper is available for home gardens. H
- Champ Formula 2 at 0.66 to 1.3 pints/A on 7- to 10-day intervals. Preharvest interval is 0 days. 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
- Dithane F45 Rainshield (Group M3) 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- 240011). 24-hr reentry.
- MilStop SP (85% potassium bicarbonate) at 2 to 5 lb/A. 1-hr reentry. O
- Orondis Gold 200 (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 days. 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.
- OSO 5% SC (Group 19) at 6.5 to 13 fl oz/A on 7- to 14-day intervals for disease suppression. Can be applied the day of harvest. 4-hr reentry. O
- Phosphonates (Group P7; formerly Group 33) can be very effective.
- Aliette WDG at 2 to 5 lb/A on 7- to 21-day intervals. Do not apply within 3 days of harvest. 24-hr reentry.
- Fosphite at 1 to 3 quarts/A on 14- to 21-day intervals. Can be applied the day of harvest. 4-hr reentry.
- Fungi-phite at 1 to 2 quarts/A on 7- to 14-day intervals. 4-hr reentry.
- K-Phite 7LP at 1 to 4 quarts/A on 7- to 14-day intervals. Can be applied up to the day of harvest. 4-hr reentry.
- Rampart at 1 to 3 quarts/A on 2- to 3-week intervals. Can be applied up to the day of harvest. 4-hr reentry.
- Resist 57 at 1 to 3 quarts/A on 2- to 3-week intervals. 4-hr reentry.
- Presidio (Group 43) at 3 to 4 fl oz/A on 7- to 10-day intervals as a tank-mix. Preharvest interval is 2 days. 12-hr reentry.
- Quadris Flowable (Group 11) at 12 to 15.5 fl oz/A on 7- to 14-day intervals. Do not apply more than one (1) application before alternating to a non-Group 11 fungicide. Preharvest interval is 0 day. 4-hr reentry.
- Ranman 400SC (Group 21) at 2.75 fl oz/A on 7- to 10-day intervals. Preharvest interval is 0 days. 12-hr reentry.
- Reason 500 SC (Group 11) at 5.5 to 8.2 fl oz/A. Do not apply within 2 days of harvest. 12-hr reentry.
- Regalia (Group P5) at 0.5 to 1.5 quarts/A plus another fungicide on 7- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Vacres (potassium bicarbonate) at 2.5 to 5 lb/A on 7- to 14-day intervals. Preharvest interval is zero (0) days. 4-hr reentry.
- 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 0.5 to 1 teaspoon/gal water. H O
- Cease at 3 to 6 quarts in 100 gal water. For greenhouse plants only. Preharvest interval is 0 days. 4-hr reentry. O
- LifeGard WG 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. 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
- Stargus at at 0.5 to 4 quarts/A plus a nonionic surfactant on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O