Reduction of bunch stem necrosis of Cabernet Sauvignon by increased tissue nitrogen concentration

Field experiments were conducted over three years at two vineyards in northern Virginia to examine relationships between specific nutrients and the incidence of bunch stem necrosis (BSN) of Cabernet Sauvignon. Nitrogen, magnesium, acid calcium were applied alone or in combination. Only one of the vineyards (Winchester) showed appreciable BSN incidence during the study period. During the 1996 season at Winchester vineyard, bloom-time leaf petiole and veraison rachis nitrogen concentration of unfertilized (control) vines were 0.80% and 1.16%, respectively. The corresponding control BSN incidence was 41% at harvest time. Seasonally split application of nitrogen fertilizer at 112 kg/ha actual nitrogen increased bloom-time leaf petiole and veraison cluster stem nitrogen concentration to 1.85% and 1.82%, respectively, in the nitrogen plots. The corresponding BSN incidence was reduced to 14% at harvest time. BSN symptoms were not as pronounced during the 1997 season; however, all treatments, including the control plots had elevated tissue nitrogen levels in 1997. Application of nitrogen fertilizer in 1998 was associated with bloom-time leaf petiole and veraison rachis nitrogen concentrations of 1.18% and 1.34%, respectively. Corresponding BSN was reduced to 3% at harvest time, versus 17% to 23% in treatments that did not receive nitrogen. Magnesium and calcium had minimal (1997) or no impact on BSN incidence. Measures of canopy density, cane pruning weights, crop yield, and fruit chemistry suggest that the ameliorating effects of nitrogen on BSN incidence were directly related to increased tissue nitrogen concentration, and not an indirect effect of vigor stimulation or crop ripening rate. A low incidence of BSN at the second vineyard precluded a definitive explanation of prior BSN expression at that location. We conclude that low tissue [N] may be one cause of BSN, and that vineyards should be examined individually for contributing factors.