Transplants Grown in the South

Huey I. Borders.

More than 90 percent of the seedlings used to produce tomatoes for canning in the Central and Atlantic States are grown in the South. Field-grown tomato transplants are produced in Florida, Mississippi, Texas, North Carolina, Arkansas, Tennessee, and Virginia, but most are grown in southern Georgia, where the industry was established in 1908.

Field-grown seedlings are sturdier than plants produced in greenhouses or cold frames and can be produced as early in the spring as the northern grower needs them. Their low cost gives them a competitive advantage over plants grown under glass in the North. Consequently the industry has grown to comprise more than 5,000 acres in Georgia alone. Each year hundreds of millions of plants are shipped to northern growers by truck, train, and airplane.

The production of disease-free transplants is vital to the grower and to the producers of tomato crops for canning in the North. A few leaf spots on a large tomato plant in the field may not greatly affect the yield of fruit, and the grower can obtain a good crop despite the presence of some disease. But a few leaf spots on a tomato Seedling, up to transplanting size, may profoundly affect the ultimate vigor and yield of the plant. Infected seedlings are likely to become unthrifty Plants that may die shortly after they are transplanted.

Furthermore, the presence of one or two diseased seedlings in a bundle of plants may lead to the infection of others under the moist conditions that occur in a closely packed package of plants during shipment and before they are transplanted after arrival. Therefore it is vitally important to keep even a small amount of disease out of a field of tomato seedlings; growers of tomato transplants try to have perfect not merely good, control of disease in their fields.

To that end, the Georgia Department of Entomology maintains a system of plant certification. Fields registered for certification must meet requirements as to crop rotation, seed treatment, and spraying for control of insects and diseases. The fields are inspected regularly during the season. Those that meet the requirements are certified as free from disease.

The diseases most common on field-grown tomato seedlings are caused by fungi, bacteria, and nematodes. Most important are late blight, caused by the fungus Phytophthora infestans, and alternaria or early blight, caused by Alternaria solani.

They cause stem and leaf infections that severely damage seedling plants and also are the cause of serious losses in field-grown tomatoes.

Another group of diseases is caused by organisms that attack the plants through the roots and underground parts of the stem or at the ground line. In plant fields the most common diseases of this type are southern blight, caused by the fungus Sclerotium rolfsii; bacterial wilt, caused by Pseudomonas solanacearum; and injury from the root knot nematode, Meloidogyne species, formerly known as Heterodera marioni. The organisms live in the soil and attack the seedlings during the warmer parts of the season.

Fortunately for the southern grower, the control of virus diseases is not a difficult problem, as the mosaic diseases that are so common in commercial tomato fields and greenhouses are rarely found on field-grown seedlings probably because the aphids that transmit the viruses are not particularly active during the early spring, or because there is little handling or other contact with the plants until they are pulled for shipment, or because the viruses are not often seed-borne.

The control of diseases starts with the soil.

Root knot nematodes are microscopic eelworms that attack and invade the roots of the seedlings and cause swellings or galls. The seedlings may become sickly and develop into unproductive plants.

Of the diseases caused by the soil-borne fungi and bacteria, the most serious are southern blight and bacterial wilt. The organisms that cause them grow best at higher temperatures and consequently are generally confined to the South.

So far, from the standpoint of the grower of tomato seedlings, the best means of control is to plant only in soil free of soil-borne pathogens. A field can be used for the production of certified tomato seedlings only three or four seasons. By that time either southern blight or bacterial wilt or both will usually appear and make the field unfit for the production of certified seedlings.

The Georgia State authorities require that only land that has not been planted during the previous 3 years to crops susceptible to root knot nematodes be used for certified tomato seedlings. An exception: A grower may replant a field to tomato seedlings following a tomato seedling crop that was approved for certification or that was not disapproved because of infections by bacterial wilt or southern blight or infestations of the root knot nematode.

Clean seed is important. The seed is usually treated with 5 percent ethyl mercury phosphate (New Improved Ceresan), a fungicide that destroys surface-borne fungi and bacteria and protects the germinating seedlings from the attacks of damping-off fungi.

Dry seed can be treated by applying the material as a dust at the rate of 0.05 percent by weight of seed. Seed also can be treated by a liquid soak method soaking the seed for 5 minutes in a 1-1,200 solution of the commercial compound. The treatment can be used on either dry or freshly extracted seed.

Georgia regulations require that the plants shall be grown from certified seed obtained from a source approved by the Georgia Department of Entomology. To determine the efficiency of the seed-disinfectant treatment, a composite sample from each day's treatment is tested by certification authorities to determine the efficiency of the surface disinfection. Only seed that meets these requirements is acceptable for the production of certified plants.

The seeds are sown in open fields in Georgia in the early spring. Planting usually begins around the last week in February and continues up to the middle of April or so. Such plantings give a succession of seedlings of the right size and obviate the need to hold plants after they have reached commercial pulling size. Plants held in the field after reaching commercial pulling size become hard and fibrous and deplete the available nitrogen in the soil as they grow older. Such old, nitrogen-deficient plants are more susceptible to alternaria infection than young, vigorously growing seedlings. Successive plantings therefore help in reducing the amount of disease. Another factor that encourages disease is wilting or mechanical injury to seedlings during harvesting and packing.

The control of alternaria blight and late blight, which affect the leaves and stems of the plants, depends on the use of the proper fungicides but also on the selection of disease-free soil and the maintenance of a sufficiently high level of nutrition in the seedlings to assist them in resisting disease.

Tomato seedlings that are allowed to develop hunger signs were found to be susceptible to alternaria stem canker and leaf spot. In regulating fertilizer applications, however, the grower must remember that periods of warm, wet weather will cause increased growth and nitrogen intake by the plants with the result that they may become too soft or succulent. Such plants will suffer injury during shipment and when exposed to hot, dry weather after transplanting.

In spraying tomato seedlings for disease control many factors must be considered: Row width; number, angle, and height of the spray nozzle; size of spray disk orifice; pressure of the spray pump; and speed of spray rig, as well as the choice of spray materials.

The rows must not be too close together or it will be impossible to achieve proper coverage of the plants with the spray. Although tomato seed is sown in rows of various widths, rows spaced 16 inches part-18 inches is better will permit a much better spray dispersal and coverage than can be had in the narrower rows.

Fungi of late blight and alternaria blight produce their spores or conidia in greatest abundance on the under side of the leaves. Therefore spray nozzles should be placed low enough so that the cone of spray will be directed upward to cover that area. If the side nozzles are too high the leaves may be forced downward around the stem of the plant. Then spray material would not only be prevented from reaching the under side of the leaves but also would be kept from portions of the stem where canker spots or lesions can develop.

To protect the seedlings from late blight, alternaria leaf spots, and stem cankers, the grower should start spray applications as soon as the seedlings have produced their first pair of true leaves and continue them at intervals of 5 to 7 days until the plants have reached commercial pulling size (more than 5/32 inch in diameter of stem). A final fungicidal spray immediately before pulling may protect plants against disease during shipment and until the first sprays can be applied after transplanting in the North.

A spray should be applied with sufficient force to break it up into a fog of fine droplets that will cover both surfaces of the leaves and the stem but without sufficient excess spray to cause coalescence of the drops and runoff. A power sprayer is needed. Pump pressures of 200 to 400 pounds the square inch will give adequate coverage without injury to the plants. The best results are obtained by using No. 3 disks in the spray nozzles and by maintaining a tractor speed of not more than 4 miles an hour. Approximately 150 gallons of spray material an acre is applied.

Our experiments have shown that some sprays give almost perfect control of alternaria blight but that the same materials used in the form of dusts give practically no control. Dusts can be properly applied only in early morning or late evening when the dew has formed and the wind has died; sprays can be applied at any time during the day after the dew has dried on the plants unless the wind velocity is excessive.

Of many fungicidal materials tested at the Vegetable Seedbed Investigations Laboratory of the Department of Agriculture in Tifton, Ga., dithiocarbamates as sprays gave the most effective control. The four best spray materials were nabam (Dithane D 14) plus zinc sulfate and lime; ziram (Zerlate); zineb (Dithane Z-78); and tribasic copper sulfate. Besides good control of alternaria blight under severe disease conditions encountered in these tests, zineb and nabam plus zinc sulfate and lime also gave a high degree of control of late blight and stemphylium or gray leaf spot of tomatoes (Stemphylium solani).

HUEY I. BORDERS is a plant pathologist who conducts research on diseases of vegetable seedlings grown in the South. He joined the Department of Agriculture in 1947. Previously he was extension plant pathologist in Georgia and was engaged in research at the Subtropical Experiment Station, Homestead, Fla.