The Important Diseases of Lettuce

Guy Weston Bohn.

American gardeners grow several types of lettuce the loose leaf varieties, cos or romaine, butterhead, and iceberg. All are in the botanical species Lactuca sativa.

Most of the commercial acreage of lettuce is in the Southwestern States, where lettuce, most of it the iceberg type, is grown the year around and shipped in refrigerated cars to all parts of the country.

The lettuce plant has a compact mass of edible, tender leaves on a short stem. Its structure makes it very perishable. Harvesting and handling must be prompt. Lettuce is subject to a variety of diseases during its growth and its distribution to consumers.

SEED DECAY and seedling blight, diseases that reduce plant stands, are caused by several soil-inhabiting fungi, such as Rhizoctonia solani and Pythium ultimum.

The fungi attack the tender plants before and shortly after they emerge from the soil and cause the stems and roots to rot. Young plants may be attacked only at the soil surface. If that happens, a short section of the stem rots and the seedling falls over. The injury is called damping-off. Older plants are seldom attacked.

Conditions that reduce the rates of germination and emergence increase losses from seed and seedling rots in heavy, wet, poorly aerated soils, which tend to puddle. Losses in such soils are increased if seed is planted too deeply or if rain or irrigation water packs the soil about the seeds.

Lettuce seed germinates best at moderately cool temperatures in coarse, well-aerated soils. Losses from rots can be reduced, especially in heavy soils, by planting the seeds shallowly, and by irrigating before planting, or sub-irrigating. In districts where rain is likely to fall while the seedlings emerge, planting should be after, rather than before, a rain.

Seed decay can be reduced by dusting the seed with a fungicide, such as chloranil (Spergon), at the rate of 4 ounces to 100 pounds of seed; ferric dimethyl dithiocarbamate (Fermate), at 1 pound to 100 pounds of seed; or thiram (Arasan) at the rate of 1 pound to 100 pounds of seed. Yellow cuprous oxide (Cuprocide) is likely to injure the seed.

Upper left: Head lettuce affected with bacterial soft rot. Upper right: Cantaloup vine attacked by the powdery mildew fungus. The disease is important in the Imperial Valley of California. Lower left: Pink root, a disease of onion. One plant is healthy and three are diseased. Lower right: Alternaria leaf spot on tomato leaf.

SCLEROTINIA DROP often causes severe losses in localities where rains come during the growing season, as in the Eastern and Central States. It is especially severe if lettuce and other susceptible hosts are grown repeatedly in the same soil. The disease causes occasional losses in Arizona and California, but it is seldom severe there because most of the commercial crop is grown on raised beds and harvested during rain-free periods.

Sclerotinia drop is caused by the soil-inhabiting fungi Sclerotinia sclerotiorum and S. minor. The fungi attack the leaves of older plants where they touch moist soil. They attack the stem at the moist axils of large leaves near the base of the plant. They cause a soft, watery rot, which spreads rapidly over the stem and leaf bases. The entire plant suddenly collapses. It becomes a soft, watery mass and then turns brown and dry.

A cottony fungus growth can be observed on the stem and leaf bases of plants that show early symptoms. Small black sclerotia of various shapes can be found in the decayed tissues as they become brown and dry. The sclerotic are resting bodies that enable the fungi to pass through periods of unfavorable weather. When moist, cool conditions favor their development, the sclerotia produce mushroom-like apothecia. The apothecia produce innumerable spores which distribute the fungus to new host plants by wind, rain, irrigation water, and cultivating tools.

The fungi can persist for long periods in the soil. They can also attack other crop plants, such as bean, cabbage, celery, eggplant, potato, and tomato. In places where the disease occurs on those crops, large populations of the fungi are built up in the soil and increasingly severe losses occur when susceptible crops follow susceptible crops in soils that harbor the pathogens.

Sclerotinia drop can be controlled in seedbed soils with steam pasteurization or with chemicals: One part of commercial formalin in 50 parts of water applied at the dosage of 1 gallon to a square foot, or calcium cyanamide at the rate of 1,000 pounds an acre, 15 days before planting.

Losses in seedbeds and greenhouses can be reduced by providing good ventilation and by using cultural methods that maintain a dry atmosphere and dry plant and soil surfaces.

In commercial lettuce fields it is best to grow the plants during rain-free periods and maintain a dry mulch around the bases of the plants.

In the West the plants are grown on raised beds and irrigation water is applied in semi-permanent ditches. Irrigation water should be applied with care to prevent water-logging and to prevent wetting the soil surface in contact with the plants.

The building up of large populations of the fungi in the soil can be prevented by rotating lettuce and other susceptible crops with grains and other crops that are not attacked by Sclerotinia.

No variety of iceberg lettuce is known to be resistant to Sclerotinia.

The cos varieties are also susceptible but often are less severely injured than the iceberg varieties. The cos varieties have an upright habit of growth and present fewer moist infection courts to invading fungi.

BOTRYTIS ROT and gray mold of lettuce are caused by the soil-inhabiting fungus, Botrytis cinerea.

The plants usually develop brown, necrotic lesions on the stem near the soil line, on the bases of leaves near the soil line, or on leaves in contact with moist soil. Infection usually proceeds upward along the stem and inward through successive layers of leaves. Hence, the rot often occurs in one side of the plant. In moist weather a gray mold appears on the dead tissues and on spots elsewhere on the leaves.

Botrytis cinerea grows upon decaying vegetable matter in the soil and attacks numerous ornamental and vegetable crop plants during moist weather. It is often troublesome in greenhouses and occasionally causes losses in fields during wet, muggy weather.

Control measures for sclerotinia drop also apply to botrytis rot.

SLIME OR BACTERIAL ROT of lettuce occurs in the field during warm, muggy weather; in transit, in lettuce shipped without refrigeration; and in markets. It often is troublesome in Eastern and Central States. In the West it is seldom seen during cool weather but often occurs in spring.

Slime causes a wet, slimy decay of the large internal head leaves. The outer leaves and the small leaves at the center of the head are usually not affected at first, and the plants often appear normal in the field. They can be detected by twisting the top of the head. The outer and inner firm tissues separate readily at the rotted leaves.