Vegetable Production Guide

Lettuce
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Disease Control

This section was updated - 12 September 2018

Anthracnose

Small, tan to straw-coloured spots on the lower leaves and mid-ribs. The leaf spots eventually fall out leaving a shot-hole appearance. This disease frequently occurs during unusually wet springs.

Control

  1. Clean up all lettuce residue in and around greenhouses or cold frames where early lettuce transplants are started.
  2. Avoid unnecessary overhead watering.
  3. Turn under crop residue in the field after harvest.
  4. Fields which produced a seriously diseased crop should be rotated out of lettuce for at least one year.

Bacterial Slime Rot

Occurs as a wet, slimy decay on lettuce in the field, in transit or in the markets. Usually the large internal leaves are affected first.

Control

  1. Do not crowd plants.
  2. Avoid overwatering.
  3. Harvest as soon as mature.
  4. Pre-cool harvested heads to 1°C and keep cool.

Bottom Rot

The common soil fungus, Rhizoctonia, infects the basal leaves of lettuce heads as they approach maturity causing rust-coloured, sunken lesions which first appear on the midrib. Under warm, damp conditions the lesions can enlarge and rot the entire midrib. There are no obvious spores or mycelium associated with bottom rot. The fungus does, however, overwinter by producing sclerotia.

Control

  1. Avoid growing head lettuce in fields with a history of bottom rot.
  2. Lettuce types with an upright growth habit are more likely to escape infection.
  3. Rotate with grasses, cereals or legumes.

Damping-off/Stunt

(Pythium)

Pythium species in soil cause damping off of seedlings under wet conditions. If Pythium invades the crown, plants become stunted. Ridomil Gold 1G is registered on head lettuce as a granular application at seeding for control of damping-off and stunt. Growers wishing to evaluate this treatment should follow the same procedure as suggested for the cavity spot of carrot treatment (see Carrot section). The recommended rate is 115 g/100 m of row or 25 kg/treated ha (10.1 kg/treated acre). Make only one application per year, at seeding.

Torrent 400 SC (cyazofamid) is also registered on greenhouse lettuce transplants for control of Pythium Damping-Off and Root Rot.  See Table 3, Lettuce Disease Control for details.

Drop

(White Mould)

Drop of lettuce, caused by Sclerotinia, is so-called because the outer leaves of infected plants tend to collapse and lie flat on the ground. Uprooting such plants reveals white mycelium and in later stages, dark sclerotia. The large (5 – 15 mm) sclerotial form of the fungus tends to predominate in the spring and spores can blow into a crop from adjacent fields and waste areas. The small (0.5 – 2 mm) sclerotial form of the disease infects directly from contact with the soil under moist conditions at any time during the growing season.

Control

  1. Rotate with corn, cereals, forages, onions or potatoes.
  2. Roguing is an effective means of reducing inoculum for future crops only with the small sclerotial form.
  3. Incorporate diseased crop refuse promptly to reduce sclerotial production.
  4. Do not dump infected produce near production fields as the large sclerotial form produces spores which can be blown back into the field.
  5. Prolonged flooding will reduce survival of sclerotia.
  6. See Table 3, Lettuce Disease Control. Because of its high cost, it is only recommended in fields with a history of drop or in situations of severe disease pressure.
    Or Rovral applied for grey mould control may also help to control drop.

Downy Mildew

(Bremia)

This fungus infects lettuce leaves during cool, damp weather in the spring and again in the fall causing irregularly-shaped yellow to brown lesions. The undersides of the lesions may be covered with glistening white spores. Early infections at the seedling stage can cause stunting and death of plants.

Control

  1. Avoid locating early and late plantings in fields with a history of downy mildew.
  2. Turn under diseased crop residues promptly to hasten breakdown of the fungus.
  3. Plant resistant varieties such as Patriot, Calgary and El Dorado for the harvest period from August to the end of the season.
  4. If a fungicide program is to be used it must begin before the disease becomes well established as it is only preventive, not curative. See Table 3, Lettuce Disease Control.

Grey Mould Rot

(Botrytis)

Grey mould is a very common fungus which sporulates profusely on dead and dying plant material. It is favoured by cool, wet conditions and on lettuce is most damaging in greenhouses and in the spring and fall crops outdoors.

Control

  1. Improve air and soil drainage.
  2. Plow down crop refuse promptly.
  3. Do not over-fertilize with nitrogen, but provide adequate calcium nutrition.
  4. See Table 3, Lettuce Disease Control for the recommended fungicide.

Tipburn

Tipburn is a physiological disorder that is related to calcium deficiency and is made worse by high soil fertility and high temperatures. The initial symptoms of tipburn are small, dark-brown spots along the margins of the interior or exterior head leaves. These spots later merge and the entire margin becomes brown. Tipburn is more serious when it occurs on the internal leaves of the head because it cannot be detected and makes the heads unmarketable. The internal dead areas are ideal sites for the development of secondary rots that cause a watery breakdown of the tissue, sometimes known as slime.

Control

There are no procedures that the grower can use that will guarantee freedom from tipburn in field-grown lettuce. The following will minimize the problem:

  1. The main control for tipburn has been the use of tolerant varieties, with Alpha, Summertime and Target showing promise locally.
  2. Hold nitrogen levels as low as possible for adequate yields. Apply nitrogen in the nitrate form rather than the ammonia form, and do not apply excessive potash (K), magnesium (Mg), or other fertilizers that might compete with calcium uptake. Take soil samples if you are not sure of your pH and calcium levels. Foliar sprays of calcium will not prevent the disorder.
  3. Follow good irrigation practices and avoid water stress of the crop.
  4. During periods when the lettuce crop is highly susceptible to tipburn, the problem can be lessened by harvesting before full maturity and before tipburn develops sufficiently to reduce the marketability of the lettuce.

Root Rot - Suppression

RootShield WP (Trichoderma harzianum Rifai strain KRL-AG2) is a biological fungicide for the suppression of Root Rot in greenhouse vegetable transplants.  When applied to transplants, greenhouse planting mix or soil, the product grows into plant roots as they develop to protect roots  against Pythium, Rhizoctonia and Fusarium.

  1. Suspend RootShield WP using a spray volume of 30 to 45 g/100 L and apply at the rate of 55 to 110 g per cubic metre (loose) of greenhouse potting mix, soil or planting beds. 
  2. Apply using hand-held backpack or ground spray equipment. 
  3. Becomes active when soil temperatures are above 10 degrees C and is not effective while soils remain cold.
  4. More effective at suppression of root rot in neutral or acidic soils.  
  5. Should be used within 12 months of the date of manufacture. 
  6. Contains the allergens wheat and sulfites.

smileyRootShield WP is used in organics.  It is OMRI-USA listed.  OMRI-USA = Organic Materials Review Institute of USA.  Check with your certification body before using in an organic operation.

 

When applied after sowing seeds or transplanting, Trianum WP (Trichoderma harzianum Rifai strain T22) protects plants against infection by soil-borne pathogens via the root system.  It is a biological fungicide that out-competes plant pathogenic fungi for space and nutrients, colonizing plant roots ahead of pathogens.

  1. Suspend 1 volume of Trianum WP in 5 volumes of water and mix thoroughly. 
  2. Add suspension to required amount of water to make up final drench volume.
  3. Apply immediately after sowing seeds or transplanting.
  4. After sowing: use 1.5 kg/m2 of cultivated area in 1.33 L of water.
  5. After transplanting:  use 6 g in 1 L of water and apply in 500 L/ha of suspension.
  6. Becomes active when soil temperatures are above 10 degrees C and is not effective while soils remain cold.
  7. More effective in neutral or acidic soils.  
  8. Use within 6 months of the date of manufacture.
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