Breeding work is under way to incorporate into the hybrids resistance to two major diseases, downy mildew and pink root, and to one insect pest, thrips. Though sporadic in appearance, the downy mildew is probably our most destructive disease of onions. In the North, damage is done chiefly to the bulb crop. On the west coast, it is particularly serious on the crop grown for seed; much of the seed acreage, therefore, has been shifted to other areas where the disease is less prevalent. No entirely satisfactory control method by the use of sprays or dusts has been developed. Apparently the only satisfactory means of control will be by the use of resistant varieties.

In breeding for resistance to downy mildew, Italian Red 13-53 has again come to our aid. Its high resistance to this disease was first observed in 1934 in the breeding plots at Davis. Under California conditions, infection is usually confined to the tips of the leaves, and the spread toward the base is slow. The seed stalks of 13-53, however, are immune. Lesions have never been found even during the most severe epidemics. Now you can begin to see why we consider that 13-53 may be the most important onion selection ever made. Besides its contribution to hybrid onions, it has also given us the best source of resistance to downy mildew. To produce hybrids resistant to downy mildew, it will be necessary to incorporate resistance into the female lines, as well as into the pollen parents.

Pink root is also a major disease in most areas, but it is especially destructive in the South. As the organism lives and multiplies in the soil, chemical control is not practical. Again, we think the use of resistant varieties is the only permanent solution. Resistant lines of Yellow Bermuda are being developed by the Department in cooperation with the experiment stations of Wisconsin and Texas. At the Wisconsin station, the young seedlings are given a severe test. Those surviving are grown to maturity and the bulbs are sent to Beltsville. The resistant plants are used for crossing with female plants, and the various progenies are tested in the South for yield and other desirable characters.

Breeding is also being done to develop lines that resist smut, purple blotch, and yellow dwarf.

The onion thrips—not a disease, but an insect—is without question the most destructive pest on the onion crop the world over. They puncture the surface cells of the leaves and suck out the contents, causing local injury. In severe cases the plants are killed prematurely, and theyield is greatly reduced. Satisfactory chemical control has only recently been developed. DDT is the first insecticide that has given commercial control. Resistant varieties would require less spraying or dusting and thus reduce cost of growing.

Breeding for resistance to thrips has been under way for some time. Characters have been determined that either reduce the number of thrips per plant or cause the plant to show less injury, but the character that can be used most advantageously in breeding for resistance is glossy foliage. Most onion plants secret a waxy layer, or "bloom," on the surface of the leaf, a deposit that gives a grayish cast to the leaf. It is easily rubbed off. When this waxy covering is absent, the foliage has a glossy appearance. In all field tests, glossy plants show considerable resistance to thrips. This glossy character, derived from a single plant selection from a field of Australian Brown onions and from the variety White Persian, which was obtained in Persia by W. E. Whitehouse of the Department, is being bred into our commonly grown varieties and into the lines that arc being developed for the production of hybrid seed. Here, too, it is necessary to have two resistant glossy male parents for the production of resistant hybrids: One to perpetuate the female line; the other to combine with the female line for the production of hybrid seed.

This hybrid work makes possible the combining of those lines that are resistant to the attack of thrips and various diseases, and it provides a method for attaining great uniformity of shape, color, time-of-maturity, and edible quality in the onion and at the same time gives a means of obtaining the greatest amount of vigor. The production of hybrid seed opens up an almost unlimited field for the production of improved onion varieties. We now have unusual opportunities for accomplishment. Excellent tools are available, but improvements will be made only in proportion to the efforts put forth.

THE AUTHORS

H. A. Jones, olericulturist, is in charge of potato and onion investigations at the Bureau of Plant Industry, Soils, and Agricultural Engineering. He was head of the Division of Truck Crops at the University of California for 14 years, previous to 1936, when he joined the Department. In 1944 Dr. Jones was awarded the William Herbert medal by the American Plant Life Society in recognition of his important contribution to onion breeding. He is a graduate of the University of Nebraska and the University of Chicago.

A. E. Clarke is a cytologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering and has carried on cytogenetic and breeding investigations with potatoes and onions since 1936. After graduating from the University of Alberta, and the University of Wisconsin, Dr. Clarke was awarded a National Research Fellowship in the Biological Sciences and continued his studies at the University of California and the California Institute of Technology. In 1943, for their study of male sterility in onions, Jones and Clarke received the Vaughan Research award offered by the Ameri­can Society for Horticultural Science.