On onion the disease becomes manifest in young seedlings and at any subsequent time in the growth period of the host. Abnormal yellowing of roots is commonly associated with pink root but is not necessarily a stage of the disease. Affected roots turn pink, shrivel, and die. As the plant sends out new roots, they in turn become diseased and functionless. That happens throughout the growing season. The affected plants are not usually killed, but the reduced food supply results in the formation of mere scallions or small bulbs.

The causal organism, Pyrenochaeta terrestris, is made up of many races, which vary in growth characters and in virulence upon onion. Black fruiting bodies smaller than a pinhead sometimes appear on the diseased onion roots. Within them myriads of microscopic spores are formed. They are not important in the spread and perpetuation of the fungus, because the fungus seems to depend upon fungus threads, which grow and persist more or less indefinitely in infected soil.

I know of no practical way to eradicate the fungus from infested soil. The development of resistant varieties is therefore important. Such a program is under way in the Department of Agriculture and several State experiment stations. At the Wisconsin Agricultural Experiment Station, R. H. Larson has worked out a method of subjecting breeding progenies to pure cultures of the fungus as they grow in white quartz sand at controlled, constant temperatures. After 2 weeks of growth in a liquid medium, the fungus threads are chopped into fine particles, which are mixed in the clean, sterile sand. The sand is placed in shallow metal trays supported in tanks of water regulated at a constant temperature of 80 F. Seeds are sown in the sand, and after 28 days the diseased seedlings are discarded and the resistant ones are transplanted to soil and grown on to bulbs. Thousands of seedlings can be tested in a month in this way. Resistant individuals are then used for further breeding.

Yellow Bermuda is one of the most tolerant of the common varieties. Plant breeders have discovered that they can increase that level of resistance by rigid selection. Resistance is an hereditary character that can be transferred to other types by breeding.

Chives, the Nebuka type of Welsh onion, and Giant Musselberg variety of leek also have considerable natural resistance. The Evergreen shallot, developed at the Louisiana Agricultural Experiment Station, and the Beltsville Bunching, a non-bulbing onion derived from a cross between onion and Nebuka Welsh onion and introduced by H. A. Jones of the Department of Agriculture, are resistant types developed by selection and breeding.

ONION SMUT was first reported in 1869 in the Connecticut River Valley. By 1888 it was of great economic importance there on old onion soils. It has become an important disease in most of the onion-growing areas throughout the Northern States as far west as Oregon and central California. In the United States the disease has remained strictly a northern one, although very likely the fungus has been transported frequently to southern regions. The disease also occurs on leek and Welsh onion.

Smut appears on the first leaf (cotyledon) soon after it emerges above ground as a dark, slightly thickened area. If most of the first leaf or later leaves is involved, they are swollen and tend to bend downward. On plants starting to bulb, raised black blisters appear near the bases of the scales. Lesions may break open and expose black, powdery masses of spores.

Most infected seedlings die within 3 or 4 weeks after they emerge from the ground. Some plants survive weakly until midseason or later, and occasional plants produce bulbs with lesions on the outer fleshy scale and in one or more underlying scales. The fungus does not produce a rot in storage, but affected bulbs may be more subject than healthy ones to invasion by storage-rotting fungi and bacteria.

The causal organism, Urocystis cepulae, as seen in the black smut pustules in the leaves, consists of microscopic spores, which can live for many years in the soil. When soil is infested it remains so for many years, although there is no evidence that the organism grows and multiplies in the soil. The spores are not ordinarily seed-borne but are transferred widely on diseased sets and plants and locally by wind and water-borne soil.

The onion seedling is susceptible to infection by fungus threads growing from the spores only during the early seedling stage. If the plant escapes infection until the first leaf has reached its full growth it will continue entirely free from disease. Healthy sets or transplants that are planted in infested soil grow with complete immunity from infection. The fungus is sensitive to high temperatures, and if the young seedlings are growing in soil with temperatures of 80 F. or above they escape infection, because the fungus is very inactive and the plant grows through its susceptible period more rapidly. This phase of the disease cycle was worked out by the writer, L. R. Jones, and F. L. Wellman, of the Department of Agriculture and the Wisconsin Agricultural Experiment Station. We interpreted the results as explaining why the disease is of no importance in southern areas where seed is sown in late summer in very warm soil.

A great deal of research has been carried out on resistance of onion and its relatives to smut. R. I. Evans, of the University of Wisconsin, found that as the first leaf grew it became gradually more and more resistant and even though the fungus continued to enter, it had less and less success in establishing itself.

Welsh onion, especially the Nebuka type, is more resistant than onion, because the tissue of its first leaf becomes incompatible to the fungus earlier and much more rapidly. Three of us at the Wisconsin Agricultural Experiment Station and the Department of Agriculture studied resistance in crosses between onion and Welsh onion. Resistance was highly dominant and the hybrid scallion onion, Beltsville Bunching, has nearly as high resistance as Welsh onion. In 1952 we had not been able to introduce the resistance of Welsh onion to the bulb-onion type, by backcrossing the hybrid to onion.

Control of onion smut centers around protection of the young seedlings from infection. About 50 years ago at the Ohio Agricultural Experiment Station. method was developed In which a stream of dilute formaldehyde was introduced in the furrow with the seed. The method, known as the formaldehyde drip, has been a successful control measure ever since. The standard treatment is 1 pint of 37 to 40 percent formaldehyde solution in 16 gallons of water applied at the rate of 1 gallon to 150 feet of row, or 1 pint in 8 gallons of water applied at the rate of 1 gallon to 300 feet of row. The treatment, however, is cumbersome, formaldehyde injury may occur in very dry soil, and heavy rains immediately after application may reduce its effectiveness.

A. G. Newhall, of the New York (Cornell) Agricultural Experiment Station, devised a method in which Arasan is pelleted on onion seed with the aid of Methocel sticker, when seed is sown at the rate of about 5 pounds an acre for bulb-crop onions, the pelleting of 1 pound of Arasan to 1 pound of seed is necessary. Dr. Newhall later found that a much simpler and equally effective procedure is to apply 5 to 6 pounds of Arasan an acre in the furrow with the seed by a special attachment on the seeder. When seed is sown to produce set onions, about 65 pounds of seed to the acre is used. Then 1 pound of Arasan applied to 10 of seed without a sticker controls the disease, and no special attachment for the fungicide is required.

J. C. WALKER is professor of pathology in the University of Wisconsin.