Corn stunt occurs in the San Joaquin Valley of California, the Rio Grande Valley of Texas, and Mexico. Symptoms include bud proliferation, successive tillering, chlorotic spots and streaks on leaves, and general stunting of the plants. The disease is caused by a virus transmitted by the leafhopper (Dalbulus maidis).

It has not been transmitted mechanically from plant to plant, but it has been transmitted by means of fine needles to the leafhopper vector. This technique was used to prove that corn stunt virus multiplies in its insect vector as it does in the plant. This discovery proved that the virus persisted in the vector.

Two distinct strains of corn stunt have been studied. A strain prevalent in Texas can immunize the leafhopper vector against a strain prevalent in Mexico. The immunized insect cannot transmit the other strain of the virus nor support its multiplication. The strain from Mexico will neither prevent multiplication of the strain from Texas nor prevent its transmission by the vector. Although one other case of cross-protection between virus strains had been demonstrated in leafhoppers, this was the first known case of unilateral protection.

The significance of this unilateral protection in the vector was brought out by the discovery that a parallel unilateral protection occurred in the corn plant. These findings provide a tool to identify and classify viruses that are difficult to study by the usual methods.

Barley yellow dwarf virus is a persistent, aphid-borne virus that sometimes causes considerable reduction in seed production of several small grains. Oats and barley are most severely affected, but wheat and many other grasses also are susceptible.

Infected plants develop diffuse yellow blotches on leaves and have heads in which little or no grain develops. Barley yellow dwarf virus was not discovered until 1951 in California, but the disease may have been present for many years.

Yellow dwarf of oats (also called red leaf) was the worst oat disease in the United States in 1959. Yield reductions were particularly heavy in parts of the Midwest and Northwest.

Many different strains of the barley yellow dwarf virus exist in nature. Some cause a more severe disease than others and infect certain grasses that are not infected by others, although all strains of this virus have a very wide host range among grasses. Another kind of variation among strains of barley yellow dwarf virus involves the aphids that transmit it.

Transmission by aphids is the only known means of spread of this virus, but about six species may be vectors.

Research on the relations between the virus and its aphid vectors has revealed an unusual type of virus specialization, called vector specificity. Some strains of the virus are transmitted by one aphid species. Other strains are transmitted by other species. This specificity between the virus strain and its vector is the only way we know to differentiate the virus strain involved.

Breeding disease-resistant varieties appears to offer the most hope at present for control of barley yellow dwarf, but the presence of so many strains of the virus may make it difficult to develop varieties that are resistant in all areas and under all conditions of fluctuation in aphid populations.

Hoja blanca, or white leaf, disease of rice was first found in the United States near Belle Glade, Fla., in 1957. Rice plants with symptoms of the disease also have been observed in Mississippi. A similar disease in other rice-growing areas of the world is known to be caused by a virus transmitted by leafhoppers.

The leafhopper (Sogata orizicola) has been reported as an important vector in Cuba. Since this leafhopper has been collected in Florida, it is thought to be the vector that was involved in the minor outbreaks of the disease that have occurred in the United States.

Hoja blanca was considered a potential threat to the rice crop in the United States before the disease was found in this country. It was found in Panama as early as 1952, but was of minor importance. It was first reported in Cuba in 1954 and increased rapidly in importance in that country. The disease also occurs in Venezuela, Colombia, Surinam, Costa Rica, El Salvador, Guatemala, and Dominican Republic.

Symptoms of the disease include narrow, longitudinal white leaf stripes, nearly white leaf blades, or mottled leaves. Diseased tillers are dwarfed. Floral parts are absent or sterile if present. Normal tillers may be found on diseased plants.

The Department of Agriculture has tested many rice selections for resistance to the disease since 1957 by planting cooperative nurseries in Cuba and Venezuela. A fairly large number of the selections tested were resistant to hoja blanca. Many of the resistant selections came from Japan, China, or Korea. If the disease should become serious in the United States, use of resistant varieties may be a means of control.

These examples are only a few of the known plant virus diseases transmitted by insects or mites to seed crops. There are many other maladies of unknown causes which some day may be attributed to insect-borne viruses.

ORIN A. HILLS became Leader of Western Vegetable Insect Investigations of the Department of Agriculture, at Mesa, Ariz., in 1960. He first entered the service in 1929 as an entomologist in Oregon, where he conducted studies on the beet leafhopper in desert breeding areas. These studies were continued in Colorado during 1934-1938. Since 1938 he has been in Arizona conducting research on insects affecting cantaloups, lettuce, and sugarbeets grown for seed.

KENNETH E. GIBSON became Head of the Twin Falls, Idaho, laboratory of the Entomology Research Division in 1957. He first entered the service of the Department of Agriculture in 1927. Mr. Gibson has done research on aphids as vectors of potato leaf-roll and on the beet leafhopper as the vector of curly top, as well as the chemical control of wireworms and the western bean cutworm.

W. F. ROCHOW joined the Department of Agriculture in 1955 as a plant pathologist. He is engaged in research on viruses infecting cereal crops. Previously Dr. Rochow spent 1 year in the Virus Laboratory of the University of California. He holds degrees from Franklin and Marshall College and Cornell University.