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
- Badge SC (Group M1) 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.
- 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-090020, SLN OR-090016). 24-hr reentry.
- 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 3.75 to 13 fl oz/A on 7- to 14-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- Phosphonates (Group P7) 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/100 gal water/A on 14- to 21-day intervals. 4-hr reentry.
- Fungi-phite at 1 to 2 quarts/A on 7- to 14-day intervals. 4-hr reentry.
- Presidio (Group 43) at 3 to 4 fl oz/A on 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.
- Regalia at 0.5 to 4 quarts/A plus another fungicide 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.
Biological control
- 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. 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 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