THE PLAINS WHEAT MOSAIC viruses were collected by L. E. Melchers from a few scattered wheat plants growing in nurseries and fields near Manhattan, Kans., in 1931 and by me in 1941. Tests with the soil in which the mosaic plants were growing failed to indicate that the viruses are soil-borne. The viruses transmit by manual inoculation with some difficulty even at favorable temperatures of from 60 to 65 . They cannot be maintained in plants growing at summer temperatures at Beltsville.

We isolated green mosaic and yellow mosaic strains of virus. The green mosaic strain does not induce rosette in mosaic-rosette-susceptible Harvest Queen wheat. Unfortunately the viruses were lost in the summer of 1942, when no cool-temperature culture chambers were available.

Soil-borne viruses isolated recently from collections made in Kansas and Oklahoma are so similar to the plains wheat mosaic viruses that it is suspected that the viruses collected in 1931 and 1941 may have been soil-borne. The negative results obtained with the earlier collections of soil may have been due to a low level of infestation, and the plains wheat mosaic viruses may be strains of Marmor tritici.

SOIL-BORNE VIRUSES of oat mosaic occur in Alabama, Georgia, South Carolina, and North Carolina. The mosaics were observed first in the experiment station plots at Auburn, Ala. When they are abundant, they reduce the yields of grain and forage of susceptible oat varieties such as Bond, Camellia, Victoria, and Letoria. Growing resistant and tolerant varieties Anderson, Atlantic, Appler, Arlington, Custis, Fulgrain, Fulwin, Lee, Lemont, Victorgrain, Winter Resistant should prevent widespread losses from the mosaics. The mosaics have been observed in fields only in winter-grown oats and in oat relatives.

The viruses can be transmitted by manual methods of inoculation when the oat plants are grown at temperatures near 60 to 65 F. Two viruses have been isolated. One, Marmor terrestre var. typicum, causes apical mosaic, a light-green mosaic mottling that tends to be prominent toward the tip portion of the leaves of Letoria oats. The other, M. terrestre var. oculatum, causes eyespot mosaic, which in Letoria oats causes light-green or yellow, nearly elliptical spots, which have green centers. Some plants in certain oat strains that are grown in infested soil develop a typical rosette. A specific virus for oat rosette has not been isolated. The viruses of these mosaics are not carried in seed from diseased plants. In the soil they have shown much the same characteristics as the soil-borne wheat mosaic viruses.

WHEAT STREAK MOSAIC viruses were first collected at Salina, Kans., in 1932, but records of L. E. Melchers make it seem likely that similar mosaics may have been present in Kansas before 1930. In 1922 G. L. Peltier found a wheat mosaic in Nebraska that had characteristics of streak mosaic.

The disease has been worst in western Kansas, but it has caused damage in Nebraska, Colorado, and South Dakota. Some damage has been reported in Wyoming and western Iowa. The disease is known to occur in Oklahoma, the Salt River Valley in Arizona, California, and Canada. In Nebraska and South Dakota spring wheats as well as winter wheats are affected.

Streak mosaics gradually increased over the years. In 1949 they caused an estimated loss of 7 percent of the wheat crop in Kansas. That amounted to a loss of 30 million dollars. The loss in 1951 also was heavy. It is not uncommon to find entire fields and sometimes entire communities in which streak mosaic has wiped out the wheat crop. The destruction of the chlorophyll in the leaves frequently is so great that the plants are yellow. Growth is halted and the plants fail to set seed.

The viruses do not overseason in the soil. They are easily transmitted by manual inoculation at summer temperatures characteristic of Washington, D. C. Infection is erratic at high summer temperatures, such as those that prevail at Stillwater, Okla. They are not seed-borne. Maybe some insect carries them.

The strains of streak mosaic virus range from those that induce severe yellow spotting, streaking, and mottling (Marmor virgatum var. opi. cum), to those that cause mild, light-green or weak-yellow streaking and mottling (M. virgatum var. viride). With the strains of virus studied thus far, chlorophyll destruction or yellowing is favored by cool temperatures with the daily averages ranging from 60 to 70 . Wheat varieties differ in the degree of yellowing caused by a given strain of virus. Under certain conditions streak mosaics can be confused with the soil-borne virus mosaics.

The thermal death point of these viruses in plant juice is near 129.2 , with an exposure of 10 minutes. The survival time at room temperature in air-dried leaf tissue has not been more than 40 days. The dilution end point in water is near 5,000 times. The virus passed a Berkefeld "W" filter, pore size 3 to 4 microns.

Streak mosaic viruses occur in several wild grasses. They can infect corn, oats, barley, several species of winter annual bromegrasses, sand lovegrass, Canada bluegrass, some of the crabgrasses, and other range and pasture grasses.

In some oats, the virus favors reddening in association with the chlorotic patterns. In Golden Giant sweet corn, streak mosaic viruses induce a rather typical mosaic mottling, rings, dots, dashes, and short streaks, some of which are elliptic and have green centers. The symptoms generally resemble those induced in field corn by the sugarcane mosaic virus. The long, yellow, or bleached stripes associated with so many other viruses in corn are not characteristic of streak mosaic. The plants are never killed, and infection has never been obtained in all inoculated sweet corn plants. Some lines of sugarcane develop local lesions when the leaves of the true seedlings are inoculated with streak mosaic virus. The virus does not become systemic, though. Sorghum and the perennial smooth brome, Bromus inermis, are highly resistant or immune. Several of the susceptible forage and wild grass species may contain resistant varieties. Tests have failed to reveal any plant outside of the grass family that is susceptible to the streak mosaic viruses.

Because methods for inducing natural infection have not been worked out, the wheat variety test nurseries have to be inoculated by spraying. Varieties of wheat show some differences in the reaction to streak mosaic, but none has shown a safe degree of resistance to the severe yellow strains of the virus.

High resistance and immunity have been found in rye, several wheat-grasses (species of Agropyron), and some of the hybrids between wheat and the Agropyron species.

BARLEY STRIPE MOSAIC virus was identified with the barley false stripe disease in 1950, but the disease has been known since about 1910. It used to be thought to be of nonparasitic origin. False stripe was first noted in spring barley growing at the Wisconsin Agricultural Experiment Station at Madison. Pressed specimens of the diseased plants, prepared by A. G. Johnson on June 27, 1913, and still in existence, leave little doubt that the symptoms are the same as those induced by the virus. Perhaps it was the first virus disease on a grass species to be collected in the United States.

The disease occurs throughout the spring barley region in the United States, and it occurs in Canada. It has not been regarded as a menace to the crop. Pot tests out of doors prove, however, that it causes serious reduction in the yield of grain and forage in several varieties of spring barley.

The brown stripes often induced by the virus in some plant species and varieties tend to distinguish it from other viruses associated with grasses. The most common symptoms associated with the virus, however, are the unmistakable mosaic types of chlorotic mottling, spotting, and streaking. The chlorotic areas in the leaves frequently are ashy gray to white because of the complete or nearly complete destruction of the carotinoids, or yellow pigments, as well as the chlorophyll.

The virus is transmitted easily by manual methods of inoculation at summer temperatures at Beltsville. It infects wheat, sweet corn, field corn, smooth crabgrass, and occasional seedlings of smooth brome. It has induced only local lesions in inoculated leaves of rice and tobacco. In Michigan Amber wheat seedlings, the first one or two leaves that develop systemic signs usually become almost solid ivory color or white, denoting a marked, acute phase of the disease. The seedlings are not killed. The new leaves become much less chlorotic in the chronic phase. But when infection is through the seed, only the chronic symptoms appear, beginning with the first leaf of the wheat seedlings. In Golden Giant sweet corn and U. S. 13 field corn, the virus seldom kills the seedlings, but the plants become badly stunted and usually worthless. The long yellow or bleached stripes in the foliage are like those induced by the brome mosaic virus, the cucumber mosaic viruses, and by some leafhopper- transmitted viruses that infect corn. With the small grains, cool temperatures favor strong symptoms in the infected plants.

The thermal death point of the virus in plant juice is near 154.4 F., with 10 minutes exposure. The survival time in air-dried tissue at room temperature has not been more than 40 days. The dilution end point in water is slightly beyond 10,000 times. Traces of the virus have passed the Berkefeld "N" filter, but not the Berkefeld "W" filter.

Outstanding is the ability of the virus to enter some of the seeds of the diseased barley and wheat plants. Very few viruses are seed-borne; this was the first to be discovered in a grass species. Because infected barley seeds tend to be poorly filled, many probably go with the screenings during threshing and thus tend to keep down the occurrence of the disease. Tests have failed to show the virus to overseason in the soil.

WHEAT STRIATE MOSAIC was discovered in winter wheat in South Dakota in 1950 by John T. Slykhuis, who succeeded in transmitting the virus to healthy wheat plants and reproducing the disease by means of the leafhopper Endria inimica. Attempts to transmit the virus by manual methods of inoculation have failed. The disease occurs also in Kansas and Nebraska. The extent of the losses caused by striate mosaic is not known.

In Minter and Rushmore varieties, the disease shows up in fine, light-green or yellow lines in a streaked pattern. In the early stages, the lines are in the tissues directly over the veins. In Minter, brown necrotic spots may develop later. In Rushmore and other varieties, the streaks are followed by premature yellowing and death of the leaves. The diseased plants frequently are so stunted they produce little or no seed. In the advanced stages, striate mosaic is readily confused with other yellow mosaics of wheat. The host range of the virus is not known.

BARLEY YELLOW DWARF, another new virus disease of cereals, was widespread and destructive in California in 1951. John W. Oswald and Byron R. Houston found the virus to be transmitted by five widely distributed species of aphids, the corn aphid (Rhopalosiphum maidis), the apple grain aphid (R. prunifoliae), the English grain aphid (Macrosiphum granarium), the grass aphid (M. dirhodum), and the greenbug (Toxoptera graminum). Attempts to transmit this virus by manual methods of inoculation have failed.