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Description
Bud necrosis (BN) has been observed in Virginia vineyards since 1987. Riesling appears to be most susceptible. Surveys of five Riesling vineyards in 1990 revealed: (a) the percentage of affected primary buds ranged from about 20 in the least affected vineyard to nearly 100 in the most affected; (b) BN is detectable as early as July; (c) large, vigorous canes and canes from interior regions of the canopy had greater BN incidence than did weaker or more exposed canes; and (d) BN incidence was typically least among the basal four nodes of the cane. Experiments were conducted in 1991 to quantify shoot growth rate and shade effects. BN incidence was significantly and positively related to rates of shoot growth measured in the three-week period following bloom (R2 = 0.40). In another experiment, BN incidence was increased from 38 to 72 by a combination of increased shoot density (21 shoots/m of canopy) and a three-week application of shade cloth (64 light reduction) just prior to veraison. Photosynthetic photon flux density measured in fruit zones was reduced to 0.2 percent of ambient values by this combined treatment. Shoot thinning ( to 10 shoots/m of canopy) or shade cloth alone did not affect the incidence ofBN. BN in Virginia thus appears to be increased by rapid shoot growth, shade, or both.
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