On the Vinifera variety Mission and the hybrid grapevine Baco 22A, growing on their own roots, spring growth was delayed by grapevine leafroll virus. Cane and root growth of Mission vines was considerably reduced by the virus, but it had little effect on growth of Baco 22A. Leafroll virus reduced fruit yield of Mission vines by an average of 66 percent and sugar content of grapes by 30 percent, corresponding figures for Baco 22A being 44 percent and 9 percent respectively. Tartaric acid content of grapes at harvest was not appreciably affected by the disease. Pigment concentration in wine made from grapes of infected vines of the red-fruited Mission variety was approximately half that in wine made from grapes of healthy vines.
Field surveys of vineyards in the major vine-growing districts indicate that grapevine fanleaf and leafroll viruses are widespread and of high incidence in New Zealand, and that enation disease, which is possibly caused by a strain of fanleaf virus, and yellow mosaic, a strain of fanleaf virus, are present but of low incidence. Field diagnoses of fanleaf and leafroll viruses have been confirmed by transmission to indicator varieties.
Virus infection was demonstrated in kumara, the New Zealand sweetpotato (1pomoea batatas (L.) Lamk.) . Leaf symptoms were widespread in young plants of the popular variety 'Owairaka Red', but they did not persist during most of the growing season. Virus symptoms were eliminated from this and seven other varieties by heat treatment followed by shoot tip culture.
Because of the high incidence and serious effect of grapevine fanleaf and leafroll viruses on vines in New Zealand, virus-free material is needed for n::!w plantings and replacement of existing infected vines.Virus-free vines of a number of varieties have been introduced from California, and heat treatment has been applied to all important local varieties.Vines of 18 varieties have been treated at 100°F for periods ranging from 56 to 141 days. Of 167 vines raised from shoot tips of heat-treated plants, 45 have been screened for virus infection. Four were found to be infected with fanleaf and 9 with leafroll virus.
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