How to Manage Pests

UC Pest Management Guidelines


Downy mildew lesions.

Lettuce

Downy Mildew

Pathogen: Bremia lactucae

(Reviewed 8/07, updated 5/12)

In this Guideline:


SYMPTOMS

Downy mildew causes light green to yellow angular spots on the upper surfaces of leaves. 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. Bremia 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 isolates of B. lactucea 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 are overcome by virulent isolates of B. lactucea.

In the absence of resistant cultivars, the alternative is to apply fungicides before the development of the disease. Greenhouse grown transplants should especially be protected so that downy mildew is not distributed and brought to the field on transplants.

IRRIGATION SYSTEMS, such as drip irrigation, that reduce leaf wetness and humidity should reduce the severity of disease but will not prevent disease 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/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
 
When choosing a pesticide, consider the general properties of the fungicide as well as information relating to environmental quality. Not all registered pesticides are listed. Always read label of 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 material.
 
B. FOSETYL–AL
  (Aliette WDG) 2–5 lb 12 3
  MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33)
  COMMENTS: Spray at 7-21 day intervals if necessary. Do not apply more than 7 foliar sprays/season.
 
C. PHOSPHORUS ACID
  (Fosphite) See label 4 0
  MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33)
  COMMENTS: Spray at 7-21 day intervals if necessary. Do not apply more than 7 foliar sprays/season.
 
D. MANDIPROPAMID
  (Revus) 8 fl oz See label 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 1.2-1.6 qt 24 14
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Spray at 7-10 day intervals if necessary. Do not exceed 6.4 qt/acre/year.
 
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.
 
K. COPPER HYDROXIDE Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: Only marginally effective.
 
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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 (for more information, see http://www.frac.info/). 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Diseases
S. T. Koike, UC Cooperative Extension, Monterey County
R. M. Davis, Plant Pathology, UC Davis

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