Pacific Northwest 1998 An Online Guide to Plant Disease Control

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Lettuce (Lactuca sativa) -- Anthracnose
 
Hosts: All lettuce cultivars as well as several Lactuca weed species are affected.
Cause: Microdochium panattonianum (formerly Marssonina panattoniana), a fungus that overwinters on crop refuse or in soil as microsclerotia, surviving in soil as long as 4 years. It also can be harbored in weed hosts such as prickly lettuce (Lactuca serriola). The disease is important only in prolonged periods of wet weather when it may cause heavy losses. Optimum temperatures for infection are 68 to 72°F, but infection can occur between 50 and 86°F. Spores are produced on foliar lesions and can spread by splashing water.
Symptoms: Small, circular watersoaked lesions appear first on lower leaves, enlarge to become more elliptical, and turn brown. The centers often fall out, leaving black-margined holes in the leaf (shothole). Spots progress from older leaves to inner, younger leaves. Outer leaves often break off.

Small, circular watersoaked lesions appear first on lower leaves, enlarge to become more elliptical, and turn brown. The centers often fall out, leaving black-margined holes in the leaf (shothole).

 
Cultivar Resistance: Romaine and head lettuce types are more susceptible to the disease than green leaf, bibb, or red leaf types.
Cultural control:
  1. Sanitary measures that reduce crop refuse assist in control. Plow and cultivate lettuce parts deeply into the soil.
  2. Sanitation measures for greenhouses.
    1. Treat benches and seedling trays with a dilute bleach solution.
    2. Remove and destroy infected seedlings promptly.
    3. Sterilize soil with steam, if available.
  3. Practice crop rotation, ensuring that fields are out of lettuce production for 4 to 5 years.
  4. Avoid cultivating during wet weather.
  5. Prevent moving infested soil to lettuce fields by washing equipment between fields.
  6. If only a few infections occur, remove affected plants and any old dead leaves as soon as spots are visible.
  7. Control prickly lettuce and other weed hosts.
  8. Plant more-resistant lettuce types.
Chemical control:
  1. Cabrio EG 12 to 16 oz/A on 7- to 14-day intervals. Do not make more than one application of Cabrio (Group 11 fungicide) before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 0 days. 12-hr reentry.
  2. Dithane DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on lettuce seed crops. Washington and Oregon only (SLN WA-020028, SLN OR-020030). 24-hr reentry.
  3. Dithane F45 Rainshield at 1.5 quarts/A on 7- to 10-day intervals can be used on seed crops only. Oregon only (SLN OR-090016). No portion of treated seed crop may be used or distributed for food or feed for 1 year (365 days) after the last application. 24-hr reentry.
  4. Heritage at 3.2 to 8 oz/A on 7- to 14-day intervals. May be applied the day of harvest. 4-hr reentry.
  5. Quadris Flowable at 6.2 to 15.4 fl oz/A on 7- to 14-day intervals. Do not apply more than one (1) foliar application of Quadris or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. May be applied the day of harvest. See label restrictions for tank mixes with insecticides. 4-hr reentry.
Content edited by: Cynthia M. Ocamb on January 1, 2010
 
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