Results of the studies showed that the spread of Pierce's disease in the plot areas was similar to that in other vineyards outside the plots. The incidence of disease in the valley, however, was declining rapidly each season. The occurrence of diseased vines in the plots was not related to previous season's infections and was apparently independent of cultural or control practices. There was no evidence of spread from diseased vines in the plot to other vines. Pierce's disease spread into the plot from outside, and there was no evidence of a greater concentration of diseased vines about the margins than elsewhere in the plots.

The total populations of vectors, as sampled by sweeping with nets at intervals through the growing season, showed a positive correlation with the incidence of disease in areas of about 100 acres or more, but not so in smaller areas. There was no apparent difference in the number of diseased vines in plot areas with a grass cover crop after mid-July, a normal practice in many irrigated vineyards of the valley, and in those where weeds had been killed by cultivation and oil sprays.

Observations on the feeding habits of green and redheaded sharpshooters indicated that they feed only occasionally on vines, and then primarily on canes that had grown to the ground among the grass. But in experiments where the shoots were kept on 7-foot trellises, the vines actually developed a little higher percentage of disease than in adjacent plots of control vines where the shoots were allowed to grow to the ground.

Attempts to control Pierce's disease by controlling the vectors with chemical sprays or dusts have failed to show any decrease in the incidence of disease. Although experiments in control have given mostly negative results, a great deal of knowledge about the disease its hosts, vectors, and manner of spread has been accumulated. The disease is no longer a mystery.

THREE OTHER VIRUS DISEASES Of grapevines have been spread into some new plantings in California by vineyardists who choose their propagating wood indiscriminately.

The diseases are commonly known as white Emperor, vine mosaic, and fanleaf. They have become increasingly important as old plantings are being replaced on phylloxera-resistant stocks and as new plantings increase. White Emperor is a disease of the variety Emperor, a late red table grape grown mostly in Fresno, Tulare, and Kern Counties. Vine mosaic and fanleaf have developed primarily in varieties of wine grape.

WHITE EMPEROR, the grape that will not color, has no market value for table grapes and very little value as culls for making alcohol. Normal Emperor fruit colors to a bright red, and the clusters of grapes are very attractive. To meet specific market standards, the fruit must be sweet and must color to a minimum standard.

Vineyardists have been concerned by the fact that fruit on certain individual Emperor vines does not color properly by harvesttime. Symptoms show in the fruit and leaves. At maturity, the fruit of diseased vines is usually low in sugar and may vary in color from a pale greenish yellow to pink. Early in the season, leaves of diseased vines are darker and thicker than normal. They are wrinkled along the vines and curled down around the margins. As they age they become reddish to bronze in color, and the rolling at the margins becomes very prominent. The tissue between the five large veins gradually turns yellow as if sun-scorched, while that adjacent to the veins becomes reddish brown.

The disease has been studied by H. P. Olmo, at Davis, and A. D. Rizzi, of the California Agricultural Extension Service. They reported that individual "white" vines would continue to produce white grapes and "red" vines to produce red fruit over a period of years, and that progeny vines would produce white or red fruit respectively. Harmon and Snyder of the Department of Agriculture, working in Fresno, showed in carefully mapped vineyard plots that over a 10-year period white Emperor did not spread.

The cause of the disease of Emperor grapevines is probably a virus. Harmon and Snyder showed by some intergrafting experiments that the white type of Emperor carries an infectious factor that will transmit to red Emperors and cause them to turn white.

VINE MOSAIC, first reported in California in 1945, occurs in many of the vineyards in the northern part of the State. Only one vineyard in southern California is known to have the disease. This form of mosaic, called panachure in France, has been reported in several European countries Portugal, France, Spain, Switzerland, Italy, and Czechoslovakia and South Africa. Reports of grape disorders mention two mosaic diseases white mosaic, which is called vine mosaic in California, and true mosaic, which has not been found in California.

Bright-yellow leaves on the spring growth is an outstanding symptom of the California form of vine mosaic. Some of the yellow leaves turn white; as they age they burn about the margins and dry up. Others develop some green color. Later-formed leaves may show different kinds and degrees of mottling or may appear normal. Chlorosis and mottling in many yellow and cream-colored patterns are usually present in leaves of diseased vines most of the growing season. The cream-colored areas occur in bands about the large leaf veins, in irregular blotches, or in speckles as if splattered over the leaf surface. Alight green mottling often develops in young leaves and disappears as the leaves age. In some leaves chlorosis appears to have leaked out of the veins into the surrounding tissue. Varieties such as Pinot Blanc, Sauvignon Blanc, French Colombard, and Folle Blanche fail to set fruit. Sometimes the blossoms shell after bloom or set mostly seedless berries.

Mosaic is caused by a virus that is easily transmitted from diseased to healthy vines by grafting. We have little evidence to indicate that the disease spreads in the field.

FANLEAF WAS FIRST recognized in the variety Pinot Chardonnay in Santa Clara Valley in 1948. It has since been found in several new and some old vineyards of a number of varieties. The disease is known in several European countries under different names in France, degenerescence infectieuse; in Italy, arricciamento and roncet; and in Portugal, urticado. The disease also occurs in Germany, Switzerland, and Sicily.

Symptoms generally characteristic of fanleaf are a gradual dwarfing of the vine; early-season dwarfing of shoots; deformity of shoots and leaves; mottling of leaves on early-season shoot growth; and shelling of flowers from the clusters, or the setting of mostly seedless berries. Internodes on early-season growth fail to elongate. Later growth tends to zigzag at the nodes as the the shoots lengthen. Some of the nodes have two or more buds and are enlarged. The leaves on young shoots stand upright along the axes and do not open out as early as do leaves on normal vines. Leaves are variously deformed. On the young shoots they show mottling. The petiolar sinuses, normally narrow, open and spread out, and the margins straighten or even bend upward in deformed leaves. The angle of sinus opening sometimes has exceeded more than 200 degrees. The five main veins of the leaf become gathered together toward the midrib, like a partly closed fan. That symptom suggests the name fanleaf. Marginal sinuses may be opened deeply so that the leaves appear tattered; or they may be closed, and the leaves lopsided.

Fanleaf is also caused by a virus that is transmitted by grafting. It does not appear to spread naturally in the vineyards of California.

THE CONTROL of all three of these diseases white Emperor, vine mosaic, and fanleaf can be accomplished by the careful selection of healthy propagating wood of both the rootstock and the scion varieties. There is no cure for diseased vines. Healthy stock should be replanted in their place.

W. B. HEWITT completed his graduate studies at the University of California in the fall of 1936. Since then he has been teaching plant pathology in the College of Agriculture in the same institution.