Downy Mildew (fungus – Bremia lactucae): First symptoms are yellowish or light green areas on the upper surface of the leaves. As the lesions enlarge, the fungus produces downy growth on the underside of the leaf (See Photo). The affected tissue turns brown, sometimes comprising extensive areas of the leaf. Infected leaves are usually invaded by other microorganisms, causing considerable damage during shipping. The fungus is rarely seed-borne and does not persist long in soil. The source of infection is infested crop residue and, occasionally, weeds. Spores of the downy mildew fungus are wind-borne and are carried considerable distances. Symptoms are produced within 5-10 days after infection. Cool, humid weather with rain, heavy dew or fog favors disease development. There are several races of the fungus and varieties differ in their reaction. Resistant varieties may become susceptible when new races of the fungus develop. Varieties resistant to known races should be planted. Preventative fungicide applications at frequent intervals will give satisfactory control of the disease.
Sclerotinia Drop (fungus – Sclerotinia sclerotiorum: The cause is a soilborne fungus that infects the stem of lettuce at or near ground level. Lower leaves wilt and collapse, the entire plant wilts and dies (See Photo). Later, a soft, watery, brown rot occurs. A white, cottony growth of the fungus may be found on the surface of leaves and stems (See Photo). Hard, black resting bodies called sclerotia, can be found in the cottony growth. The fungus is soilborne and can remain in the field for several years. Avoid planting in infested fields.
Mosaic (virus): Young plants first show a slight leaf rolling, followed by light green to yellow mottling. Foliage has a yellowish color and plants are small or dwarfed. Symptoms vary according to type or variety of lettuce. The virus is seed-borne and overwinters in wild hosts and weeds. Infected seed is the most common source of infection. Once infected plants are present in the field, aphids will continue to spread the virus. There is no cure for mosaic once a plant is infected. The control is to use virus-free seed.
Big Vein (virus): Infection may appear at any stage of growth. Very young seedlings are killed. Surviving plants are smaller in size, the quality of heads is affected and maturity is delayed. Infected leaves have an upright habit of growth. Areas around the veins clear, causing the veins to appear enlarged (See Photo). The virus is transmitted by Olpidium brassicae, a soilborne fungus found in the fine roots of many plants. The virus may persist in the fungus in soil for up to eight years. Avoiding excessive soil moisture and rotation are the means of reducing disease losses. Varieties may differ in tolerance.
Tip Burn (physiological): Margins of leaves die and turn brown. The first symptoms produced are small, dark brown spots about a quarter of an inch from the edge of the leaf. Tissue around these spots dies and turns brown along the margins. Tip burn usually becomes infected with secondary bacteria, causing soft rot. The disease is believed to be related to fluctuations in soil moisture, drastic changes in temperature, and available calcium. No satisfactory control for this disease has been developed.
Damping-Off (fungi – Rhizoctonia sp., Pythium sp.): Small plants wilt and die soon after emergence. Excessively wet and prolonged cool periods of weather are conducive to damping-off. Use only high quality seed which have been treated with a fungicide. Plantings should be on raised beds.
Soft Rot (bacterium – Erwinia carotovora): The bacterium enters through mechanical wounds or following damage caused by other diseases, insects or frost. Once the disease is established, it may spread rapidly in the field under warm, wet conditions. There is no practical control of the disease once it becomes established and losses will increase during transit and marketing. Infected or damaged lettuce should not be packed; lettuce waiting to be shipped should be kept refrigerated.
Nematodes: (See section on Nematodes)