In most fields, airborne contamination largely nullifies the benefits derived from planting disease-free seeds. The only practical solution to the problem is to sow adapted, disease-resistant varieties where available.

Blackstem diseases of alfalfa, clovers, and vetches induced by Ascochyta spp. are among the most prevalent and destructive seedborne troubles of these crops. Heavily infected alfalfa fields have yielded 30 to 50 percent of diseased seed. Inspection of random alfalfa seed samples has revealed that 1 to 40 percent of the seeds are infected by the blackstem fungus. Blackstem severely restricts seed production if infection occurs during the flowering and seedset period. Seed infection can be reduced by treatment with fungicides.

Some diseases occur infrequently or are locally important. They usually reduce seed production and may be spread to uncontaminated fields with the seed. An example is blackpatch, a fungus disease of red clover and several other legumes. During wet seasons, heavy infection by this fungus reduced seed yield up to 50 percent in fields of red clover in West Virginia. Treating seed with fungicides was only partly successful in controlling the disease.

The bacterial wilt disease of lespedeza is seedborne and is widely distributed. Reductions of 30 to 50 percent in forage yield have been recorded in Missouri. The disease occurs chiefly in annual lespedeza. Some experimental strains of plants are more tolerant than others, but no resistant varieties are available. Only seeds from disease-free fields should be planted.

Several diseases are seedborne in lupines. During wet seasons, the fungi that cause anthracnose (Glomerella cingulata) and brown spot (Pleiochaeta setosa) attack plants and often infect the seeds.

In northern Florida and southern Georgia, the incidence and destructiveness of seedborne virus diseases is one of the major factors limiting seed production of sweet yellow lupine. Blue and white lupines are damaged less severely. Incidence of the diseases can be reduced by planting seed from disease-free fields. No resistant varieties were available in 1961.

Stem nematodes perhaps are introduced into new areas on seed or in plant debris carried with the seed. The stem nematode disease of alfalfa and red clover is most serious in the Western States, but infested fields have been reported in New York, Virginia, and North Carolina. Only seed from noninfested fields should be planted. One should avoid moving soil or irrigation water from infested to noninfested fields. Volunteer plants should be destroyed when a field is plowed. Nonsusceptible crops should be grown for at least 3 years. Lahontan and Nemastan, which are resistant varieties of alfalfa, should be grown in the West.

MANY DISEASES of ornamentals are seedborne and cause serious losses, but much of the seed is grown in Western States, where the dry weather during seed formation and harvest favors the production of disease-free seed. Some bacteria, fungi, and viruses attack ornamental plants grown for seed. They exact a toll in loss of seed yield and poor stands.

The heterosporium disease of nasturtium, incited by Heterosporium tropaeoli, is internally and externally seed-borne in up to 93 percent of the seed. Infection occurs in maturing fruits when humidity is high. The fungus survives in seeds for at least 3 years. Stem lesions develop in seedlings from infected seeds, and the fungus multiplies and spreads to adjacent plants. The disease is controlled by treating seeds in hot water.

The alternaria disease of zinnias, caused by Alternaria zinniae, may be seedborne. The disease occurs oftenest in the humid Eastern States, where it causes spotting of blossoms, leaves, and stems. Treating seeds with a fungicide to reduce the hazard from seedborne inoculum is suggested. Field and garden sanitation should be practiced, because the fungus may overwinter in the soil.

Fusarium wilt, caused by Fusarium oxysporum f. callistephi, is the most serious disease of China aster. It causes damping-off of seedlings, a wilt of mature plants, and decay of flowers in storage. Seeds presumably become contaminated with spores during threshing. Spores can also be carried on debris mixed with the seed.

The seedborne fungus is even more destructive in steamed soil and is thus more severe in greenhouses and seedbeds. Soil once infested with the fungus is ruined for growing susceptible varieties of asters. Seeds therefore must be treated with mercury-containing fungicides before planting them in noninfested soil, but seed treatment does not protect seedlings growing in infested soil. Varieties possessing some resistance to fusarium wilt are available and should be grown in infested soil.

Erwinia phytophthora, the organism that causes bacterial crown, stem, and bud rot of delphinium, is carried in seeds. Development of the disease is favored by excessive soil moisture. The crop should be irrigated with the least amount of water required and water should be applied in furrows somewhat distant from the rows. Treating seeds in hot water is helpful.

A bacterial blight of garden stock, Mathiola incana, caused by Phytomonas incanae, has occurred since 1933 in plantings for seed production in the coastal areas of California. The principal damage has been a serious reduction in seed production in some years. In home gardens and commercial cut-flower enterprises, many plants may be killed or severely stunted. Since the organism is seedborne, seedlings are often infected and killed. Under humid conditions the disease may spread rapidly from the infected seedlings to neighboring plants. The disease can be controlled by treating seeds in hot water.

K. W. KREITLOW is research leader for forage crop disease investigations in n the Forage and Range Research Branch, Agricultural Research Service, Beltsville, Md. He received degrees from the University of Minnesota and Louisiana State University. He has investigated forage disease problems since 1941.

C. L. LEFEBVRE is Assistant Director for Plant Science Programs in the State Experiment Stations Division, Agricultural Research Service, Washington, D.C. He is a graduate of the University of Minnesota and Harvard University. He joined the Department in 1937.

J. T. PRESLEY is research leader for cotton pathology investigations in. the Cotton and Cordage Fibers Research Branch, Agricultural Research Service, Beltsville, Md. He received degrees from the University of Maryland and the University of Minnesota. Since 1935 he has devoted the major part of his time to cotton diseases.

W. J. ZAUMEYER is Leader, Bean and Pea Investigations, Vegetables and Ornamentals Research Branch, Agricultural Research Service, Beltsville, Md. He is a graduate of the University of Wisconsin.