Symptoms and Signs
Downy mildew causes light green to yellow angular spots on the upper surfaces of leaves. The white fluffy growth of the pathogen develops on the lower sides of these spots. With time these lesions turn brown and dry up. Older leaves are attacked first. Severely infected leaves may die. On rare occasions the pathogen can become systemic, causing dark discoloration of stem tissue.
If downy mildew infects the cotyledons of young seedlings, the plants can die. Greenhouse-grown lettuce transplants can also be infected.
Comments on the Disease
Damp, cool conditions and moisture on leaves are required for the pathogen to infect lettuce and cause symptoms to develop. The short-lived spores are dispersed by winds during moist periods. Cultivated lettuce is the main host. Bremia lactucae apparently does not survive in the soil.
Bremia lactucae is a complex organism, consisting of multiple races (pathotypes) in California. New races continue to occur as the pathogen changes. B. lactucae has been reported to infect other plants such as artichoke, cornflower (Centaurea cyanus), and strawflower (Helichrysum bracteatum). Because B. lactucae isolates from these other hosts generally only infect the original host, B. lactucae probably consists of a number of host-specific forms. Within each pathotype, some isolates have developed a lack of sensitivity to some fungicides.
Management
The most effective means of controlling downy mildew is to grow resistant cultivars. Some cultivars are resistant to most of the isolates of B. lactucea that are currently in California. However, such resistant cultivars are not available for all areas and seasons. Also, because the pathogen is highly variable and dynamic, resistant cultivars do not remain resistant indefinitely and can be overcome by new virulent isolates of B. lactucea.
In the absence of resistant cultivars, apply fungicides before the development of the disease. Protect greenhouse-grown transplants so downy mildew is not distributed and brought to the field on transplants.
Irrigation Controls
Irrigation systems, such as drip irrigation, that reduce leaf wetness and humidity may reduce the severity of the disease but will not prevent it when weather conditions are conducive to epidemics.
Treatment Decisions
Available fungicides are mostly protectants, and must be applied before infection occurs for best results.
| Common name | Amount per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | MEFENOXAM | |||
| (Ridomil Gold SL) | 1–2 pt | 48 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) | ||||
| COMMENTS: Some isolates are insensitive to this pesticide. | ||||
| B. | FOSETYL–AL | |||
| (Aliette WDG) | 2–5 lb | 12 | 3 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33) | ||||
| COMMENTS: Spray at 7 to 21 day intervals if necessary. Do not apply more than seven foliar sprays per season. | ||||
| C. | PHOSPHORUS ACID | |||
| (Fosphite) | See label | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33) | ||||
| COMMENTS: Spray at 7 to 21 day intervals if necessary. Do not apply more than seven foliar sprays per season. | ||||
| D. | MANDIPROPAMID | |||
| (Revus) | 8 fl oz | 4 | 1 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Carboxylic acid amide (40) | ||||
| E. | FLUOPICOLIDE | |||
| (Presidio) | 3–4 fl oz | 12 | 2 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Benzamide (43) | ||||
| F. | ACIBENZOLAR-S-METHYL | |||
| (Actigard 50WG) | 0.75–1 oz | 12 | 7 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Benzothiadiazole (P1) | ||||
| G. | MANCOZEB | 2 lb | 24 | 14 |
| (Mancozeb 75WDG) | ||||
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3) | ||||
| COMMENTS: Spray at 7 to 10 day intervals if necessary. Do not exceed 8.5 lb (6.4 lb a.i.)/acre per crop. | ||||
| H. | FAMOXADONE/CYMOXANIL | |||
| (Tanos) | 8 oz | 12 | 1 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11), Cyanoacetamide (27) | ||||
| COMMENTS: For use on head lettuce only. | ||||
| I. | FENAMIDONE | |||
| (Reason 500 SC) | 5.5–8.2 fl oz | 12 | 2 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: Use allowed under a Supplemental Label. | ||||
| J. | DIMETHOMORPH | |||
| (Forum) | 6 oz | 12 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Carboxylic acid amide (40) | ||||
| COMMENTS: Use allowed under a Supplemental Label which expires on December 18, 2018. | ||||
| K. | COPPER HYDROXIDE | Label rates | see label | 0 |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| COMMENTS: Only marginally effective. | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number. |