Foundation seed plots a mile away from other potato fields can become infected with virus diseases transmitted by winged aphids. As the growing season progresses, insect vectors of virus diseases may increase, and infection, contracted by the tops during the current season, is more likely to have reached the tubers in late- than in early-harvested stock. Healthy plants harvested early show a lower percentage of virus diseases than do healthy plants harvested late in the season. Early harvesting of seed plots or foundation seed is now a common practice in some potato seed producing areas.

Hand pulling was one of the first methods employed to destroy potato vines in an effort to produce better disease-free seed. It soon became apparent that the method was impractical because of the great cost and the high labor requirement.

Flame burning destroyed the leaves but left stems standing, from which new growth was produced in late-maturing varieties. A similar situation occurred when the foliage was mascerated with a rotobeater machine.

The use of chemicals to kill the vines has become a common practice in many of the potato seed producing areas. It helps reduce the spread of virus diseases, prevent infection of tubers by the late blight fungus (Phytophthora infestans), complete harvest before freezing weather, regulate the size of seed tubers, and reduce skinning and bruising of tubers. Late-maturing varieties have been found to be more difficult to kill than early-maturing ones. Vascular discoloration of the tuber frequently occurs from the use of chemicals to kill vines.

More work needs to be done to determine the factors that cause the discoloration.

Because some weather conditions mask symptoms of some tuber-transmitted virus diseases, detection is sometimes difficult in the northern areas. The diseases can be detected when the plants are grown under field conditions in winter in some of the Southern States and California. Thus it is now mandatory to test all foundation seed (and much of the certified seed) in winter test plots. Through this means some northern growers of foundation seed have been able to maintain relatively disease-free stocks. The winter tests of samples from fields of potatoes grown for certification are completed in time so that the data may be used by certification officials of the seed-producing States. The information also helps growers of certified seed to keep from planting inferior stock.

It is generally agreed that very little progress would have been made in controlling mosaic and other tuber-transmitted virus diseases if winter tests had not been established for guidance in the program of production of good foundation and certified seed.

The production of the good, new, resistant varieties of potatoes is largely due to the accomplishments of the National Potato Breeding Program, which began in 1929. It is a Nationwide program in which State agricultural experiment stations and the Department of Agriculture cooperate.

The Katandin variety was the first distributed under this program. It has resistance to mild mosaic and some resistance to leaf roll. It is immune to net necrosis. Nearly 13 million bushels of certified Katandin seed were grown in the United States in 1952. In 1952 it represented more than 30 percent of all certified seed potatoes and led all other varieties. Since then, 42 additional varieties have been introduced, some of which have replaced older varieties to a significant extent. Not all are resistant to any of the major potato diseases but were released because of their reported superior horticultural characters.

POTATO seed certification dates from 1914 and the work of Professor J. G. Milward of the University of Wisconsin. It was the object of the work then, as now, to put upon the market the best seed that could be produced. Certification is recognized as a constructive preventive measure in tuber-borne diseases. The industry has expanded greatly. The average production of certified seed potatoes in the United States, for the years between 1949 and 1952, was 44.7 million bushels.

The enforcement of certification standards is the responsibility of the colleges of agriculture and State departments of agriculture in most States. In Nebraska, Utah, and South Dakota, the work has been conducted by growers' organizations.

Certified potato seed is grown under a system of inspection. The plants are inspected twice in the field during the growing season. The tubers are inspected in the bin after they are dug. The first field inspection is usually made early so as to identify and rogue the diseased plants. The second inspection is made in the period between the time of blossoming and just before the vines mature. For each inspection a maximum percentage of affected plants or tubers for each disease is allowed. This disease tolerance varies for disease and by certifying States but is usually between 1 and 5 percent.

Each State has jurisdiction over its own certification work and sets the tolerances permitted for various diseases. For example, the tolerances allowed for various diseases and varietal mixtures by Maine are (the figures are percentages allowed in first inspection and second inspection, respectively): Leaf roll, 2 and 1; mosaic, 3 and 2; spindle tuber, 2 and 2; yellow dwarf, 0-5 and 0.5; total virus diseases, 5 and 3; blackleg, 2 and 1; wilt, 2 and 1; bacterial ring rot, 0 and 0; total for all diseases, 6 and 4; giant hills, 1 (second inspection); varietal mixtures, 1 and 0.25.

The, production of certified seed potatoes depends upon the quality of foundation seed stocks from which they are grown. All plantings produced for certification must be grown from foundation stock of the best quality and should be thoroughly rogued. Improvement of foundation seed stock through field roguing permits the removal of diseased, weak, off-type, or varietal-mixture plants during the growing season. In order to insure freedom from virus diseases, the foundation seed is produced in areas isolated from other potatoes and free from disease-transmission insects. A comparatively small number of tubers indexed one year and planted in the field the next spring will give a sizable increase in seed stock for the following year. After one season's increase, a sufficient quantity would then be produced so that the grower could replace his old stock entirely with disease-free potatoes for the production of certified seed.

The most recent development is the operation of seed-source farms by the certifying agencies. The farms (in Maine and some other States) are operated to produce seed stocks for growers of foundation seed. Seed is carefully grown in isolated areas and is tested in winter field test plots. Seed thus produced is released to selected growers for the production of foundation seed. The varieties grown on the farms are planted by tuber-unit methods and carefully rogued by representatives of the certification agencies.

VEGETABLE SEED PRODUCTION areas have been shifted to areas where the weather is less favorable to the development of plant diseases. Such a shift is possible with crop-seed production because the total acreage of a particular crop is usually relatively small in proportion to the crop for which the seed is used.

Anthracnose (Colletotrichum lindemuthianum) and the bacterial blights (Xanthomonas phaseoli and Pseudomonas phaseolicola) are three major diseases of beans in the United States. They are wet-weather diseases, and their spread and development depend to a great degree on the presence of high moisture and suitable temperatures. Before 1925, the production of bean seed was largely limited to the New England-New York area and Michigan. In those regions the weather conditions are generally favorable for the development of the three diseases. Between 1916 and 1919, losses in some of these localities were as much as 25 percent. For many subsequent years the diseases were widespread and destructive, wherever beans were grown.

Since the causal organisms of the diseases are disseminated chiefly with the seed and it is imperative to have clean seed for planting, attention was centered on seed production in the West, where weather is unfavorable to the development of the diseases. Such areas were found in Idaho and California. Those centers of certified seed production have elevations of 2,000 to 3,000 feet, exceptionally low humidity, few showers, and little hail during the growing season. As a consequence of producing certified bean seed there, the three diseases have ceased to be the limiting factors in the Production of the crop.

At one time pea seed for gardens and canning was grown chiefly in the Northeastern States. Since 1925 or so, the center of production has shifted to the irrigated and more and parts of some of the Western States. Pea seed for these purposes is now produced in the Snake River Valley and the Twin Falls areas of Idaho, near Bozeman, Mont., the Palouse section of northern Idaho, and eastern Washington. This shift was made in order to produce seed free of bacterial blight (Pseudomonas pisi) and leaf and pod spot (Ascochyta species).

The growing of seed stocks of cabbage, turnip, rutabaga, and cauliflower used to be limited to the Midwest and East. More recently the ravages of blackleg (Phoma lingam) and black rot (Xanthomonas campestris) have caused the shifting of the seed-producing areas for these crops to localities along the Pacific coast. Because of the low rainfall in those areas during the time the seedling plants are growing in seedbeds, the two diseases do not become established. Hence a crop of disease-free seed can be produced from the seedlings, which are subsequently transplanted to clean fields.

Most of the cauliflower seed produced in the United States is grown in the coastal valleys of California.

In those areas, the seed has been relatively free from both blackleg and black rot.

THE USE OF RESISTANT VARIETIES, if available, is the most effective way of controlling the seed-borne organisms. That subject is discussed in the section that begins on page 165.

ERWIN L. LECLERG is a research coordinator in the office of the Administrator of the Agricultural Research Administration. He is charged with coordinating parts of the research program of the Department that have to do with sugar crops, dry beans and peas, seeds, weeds, forage crops, pastures and ranges, pesticides and insecticides, and related equipment. He holds degrees from Colorado Agricultural and Mechanical College, Iowa State College, and the University of Minnesota. He joined the Department in 1930.