Tipburn is characterized by the death and dark-brown discoloration of marginal bands of the larger head leaves. In non-heading varieties, the symptoms appear in the rapidly expanding large leaves as they approach maturity. The mature leaves and the very young leaves are less often injured.

The first symptom of tipburn is usually the breakdown and brown discoloration of small spots of tissue near the edge of the leaf. The spots usually occur first between the larger veins. The spots increase in numbers and coalesce as the disease progresses until the entire marginal band of tissue is killed. The symptoms may appear on only one or two leaves or on most of the leaves in the head. Under very favorable conditions most of the leaves may be involved. The dead tissues remain dry and are confined to the marginal portions of the leaves unless they are invaded by microorganisms. Various bacteria and other fungi may grow in the dead tissues, producing slime. Rotting then proceeds until the entire head is involved.

Tipburn in lettuce appears to be caused by the accumulation of excessive respiratory products in the sensitive tissues during warm nights. Injury seldom occurs at temperatures below 65 F. or during daylight.

Environmental conditions that favor rapid, succulent growth predispose the plants to tipburn injury excessive soil fertility, excessive soil moisture, and warm temperatures.

Environmental conditions that favor rapid respiration and the accumulation of respiratory products in the large head leaves favor the development of tipburn warm night temperatures and high relative humidity.

Tipburn can be controlled by growing lettuce during seasons when night temperatures do not exceed 65 F. as the heads mature; growing lettuce in soils that do not favor excessively rapid succulent growth; limiting the amount of fertilizers, especially nitrogenous fertilizers; using irrigation water sparingly when the heads approach maturity; and using resistant varieties.

The Department of Agriculture, in cooperation with several State agricultural experiment stations, has released several varieties of lettuce that are tolerant of conditions that cause tip-burn. The most resistant varieties, Imperial 456, Progress, Great Lakes and some of its derivatives, and Alaska, are adapted to culture during the summer. The moderately resistant varieties, Imperial 410, Imperial 615, Imperial 847, Imperial 850, and Jade, are adapted to culture during the spring. Resistance to tipburn is not complete, and all varieties suffer tipburn when conditions are very favorable. The resistant varieties often show little or no tipburn, however, under conditions that render susceptible varieties worthless.

REDHEART OF LETTUCE, a physiological disease, is characterized by the chestnut brown discoloration and breakdown of the small, inner head leaves. The outer leaves may appear normal or they may develop numerous, elongate, brown pits on the midribs and veins and sometimes on the tissue between the veins. It often occurs in transcontinental shipments of lettuce, especially in spring.

The cause appears to be lack of sufficient oxygen, which results from poor aeration or prolonged exposure to low temperatures during shipment and storage. It also results from bacterial rot of the outer leaves, which occurs in shipments without adequate refrigeration.

Redheart can be controlled by providing adequate aeration and prompt and continuous cooling of lettuce to 39 to 41 F. in shipping containers, refrigerator cars, and terminal storage, and by prompt movement of the produce from the grower to the consumer.

PREMATURE YELLOWING, rib blight, and several other obscure diseases occur in head lettuce in the field and during shipping and marketing. The diseases occur most commonly during the spring and appear to be caused by various physiological disturbances.

Premature yellowing is associated with poor development of the root system and the production of small, infirm heads. Losses result from reduction in yield and from poor quality. Yellowing in early spring lettuce in Arizona was found to be associated with poor aeration, excessive soil moisture, and salt accumulation in the root zone. This complex of adverse conditions results from soil compaction by farm machinery and untimely applications of irrigation water. Losses can be reduced by avoiding the use of heavy machinery, especially on wet soils, and by avoiding excessive applications of irrigation water.

Rib blight is characterized by the occurrence of a dark-brown, necrotic strip of tissue along the midvein near the center of the leaf. This symptom occurs in only one or two or in several of the large head leaves. The plants often appear otherwise normal and the disease can be detected only by removing the outer leaves.

Rib blight appears to be associated with rapid, succulent growth; the plants are often vigorous and dark green in color. The disease predisposes heads to attack by bacteria and losses are often attributed to slime. The cause of rib blight is unknown. Excessive applications of fertilizers and irrigation water should be avoided they favor rapid, succulent growth and predispose lettuce to injury by rib blight, tipburn, and other physiological disorders.

GUY WESTON BOHN is a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering at the United States Horticultural Field Station, La Jolla, Calif. Before joining the Department, he was an instructor in genetics in Texas Agricultural and Mechanical College.