Ways To Combat Disorders of Tomatoes

S. P. Doolittle.

Every tomato grower is faced with the hazard of loss from disease. Infections by fungi, bacteria, or viruses are responsible for much of the loss, but extremes of temperature and moisture or excess or deficiency of mineral elements in the soil at times may damage the crop.

The symptoms thus caused can be classified as wilts, leaf spots, fruit spots or rots, and abnormalities of growth of the foliage or fruits.

Fusarium wilt, caused by the fungus Fusarium oxysporum f. lycopersici, is serious on tomatoes. Its first symptom is a yellowing of the oldest leaves, usually on a single stem. Beginning at the base, the yellowed leaves wilt and die until all the leaves are killed. Often a single shoot dies before others are severely injured, but eventually the entire plant is affected. There is no soft decay of the stem, but if one cuts it lengthwise near the ground, he can see in the woody portion next to the outer "bark," or cortex, a brown discoloration of the water-conducting tissues; fibrovascular bundles which is characteristic of the disease.

The fungus that causes fusarium wilt can persist in the soil for many years. Infection occurs through the roots. From them the fungus passes into the water-conducting vessels of the stem. Apparently it produces a toxic substance that causes the wilting of the foliage and eventual death of the plant. The fungus is most active at temperatures between 80 and 90 F. Losses from the wilt are most common and severe in the warmer sections of the country. Clean fields can be infested by planting diseased seedlings, or by drainage water, farm implements, or any agency that carries infested soil even in small amounts to the clean field.

Other wilt diseases of tomato are caused by soil-borne bacteria and fungi. Unlike fusarium wilt, they are characterized by a decay of the stem near the ground and a wilting of the entire plant with little or no yellowing of the leaves.

Upper left: Pea streak, caused by a virus, on pea pods. Upper right: Anthracnose of cucumber Plants; the disease also affects watermelon and muskmelon growing in humid regions. Lower left: Bacterial spot of tomato on two fruits. Lower right: A plant of Great Lakes lettuce affected with rib blight, a physiologic disease.

Bacterial wilt, caused by Pseudomonas solanacearum, is an example. That organism causes a watery decay of the pith, which soon causes the wilting and death of the plant. These wilt diseases are most common in the southern United States but are of much less economic importance than fusarium wilt.

LEAF SPOT DISEASES of tomato cannot be described solely on a basis of leaf symptoms, since certain of their causal agencies also are responsible for serious spotting of the stems and spotting or rot of the fruits.

The most common and destructive leaf spots are caused by bacteria and fungi that produce spots in which the cells of the leaf have been killed. In some of the diseases an area of yellowed tissue surrounds the dead spots. With the exception of one fungus that destroys areas of tissue almost too large to be called spots, the organisms invade only a limited area around the point of infection and produce spots of characteristic size and appearance. They may be irregular or circular and vary from one thirty-second inch to one-half inch in diameter. Both the petioles and leaflets may be spotted, and those injuries commonly cause the death of many leaves. Some viruses also cause dead spots or patches on the leaves but the spots are of a type that usually is not confused with those caused by bacteria or fungi.

The leaf spot caused by the fungus Septoria lycopersici is an example of a disease that severely injures the leaves but does no appreciable harm to the fruit.

On the other hand the fungus (Alternaria solani) that causes alternaria leaf spot and the fungus (Phytophthora infestans) that causes late blight produce severe injury to the foliage and are also important because of their effect on the fruits.

Septoria leaf spot is characterized by numerous water-soaked spots that soon become roughly circular and have gray centers with dark margins. Later the centers of the spots show tiny dark specks (pycnidia), the bodies in which the fungus spores are produced. The spots are from one-sixteenth to one-eighth of an inch in diameter and may occur in such numbers as to cover the leaf. The fruits rarely are affected, but stems and blossoms may have spots.

The fungus also attacks wild solanaceous plants, such as horsenettle, jimsonweed, and nightshade. It over-winters in the soil on the remains of tomato plants, its weed hosts, and other decaying vegetation. Spores produced on such material cause the infection of tomato leaves near the soil. Spores then are produced in the spots on those infected leaves and, in wet weather, are exuded on the surface of the spot. From there they are splashed to other leaves by rain and may be carried on the hands or clothing of farm workers or on farm equipment.

The disease may occur on plants of any age, but in the field it seems to develop more rapidly after some fruit is set. If temperatures are moderately high and rains frequent, there is a progressive loss of foliage until only a few leaves may remain at the tips of the stems. When that occurs, yields are reduced and many of the fruits are injured by exposure to the sun. In hot, dry seasons the disease is much less damaging.

Septoria leaf spot causes serious loss in some sections of the Central and Atlantic States as far south as Arkansas, Tennessee, and South Carolina, but is of minor importance in the Mountain and Pacific States.

The first symptoms of alternaria leaf spot (early blight) consist of small, dark-brown spots on the older leaves. The spots may enlarge until they are one-half inch in diameter and have an irregular or circular outline. The large spots often are marked with rings in a target pattern. The tissues surrounding the spots often turn yellow and this discoloration may cover the leaflet. The disease is common on seedlings, where the spots usually are small and dark. In the field, the spotting and yellowing of the leaflets, together with spotting of the petioles, often kills much of the foliage.

Plants in the field also show many spots on the stems. They are light brown, with darker margins, and from one-eighth to one-half inch in size. On seedlings the fungus causes large, dark cankers (collar rot) that may girdle the stems. The varieties Southland and Manahill have high resistance to collar rot and some resistance to alternaria leaf spot.

Alternaria solani also causes a disease of potatoes known as early blight. Occasionally it affects peppers. It can live on decaying plant tissue in the soil. Most of the primary infection in tomatoes comes from that source, although contaminated seed may possibly cause occasional infection.