Studies of Grape Leafroll in California

¹ Crops Research Division, Agricultural Research Service, U. S. De-Pathology: University of California, Davis, California.
² Department of Plant Pathology, University of California, Davis, California.
³ Department of Viticulture and Enology, University of California, Davis, California.

Transmission experiments from Mission vines to Emperor indicators showed that Mission can Be infected with the grape leafroll virus. Foliar symptoms of leafroll are more distinct in Mission than in Emperor, and appear sooner following inoculation. Inoculation of mature red-fruited Emperor will produce fruit symptoms, or inoculation of Mission cuttings will produce leaf symptoms of the leafroll virus within 6 months.

A white fruit color in varieties that normally produce red fruit, a premature reddening of leaves in varieties that produce red or black fruit, a downward rolling of the leaf margins, and an early burning of grape foliage were all shown by graft transmission tests to Emperor and Mission indicator vines fo be symptoms of grape leafroll.

Surveys based on leafroll symptoms indicate that the incidence of leafroll may be 80 percent or more in many California vineyards, particularly wine-grape vineyards in the coastal counties, though not all California vineyards are so seriously affected and some are completely free of the disease.

Indexing of a small sample of wine-grape varieties from coastal vineyards indicated that over half of the vines were infected with grape leafroll virus.

Indexing of vines of grape rootstocks and grape varieties and selections growing on rootstocks obtained from experimental plantings at Davis and Oakville, California, showed that about 34 percent of the vines were infected with grape leafroll virus.

Problems arising from leafroll virus infection and factors influencing the spread of leafroll are discussed.