THE RELATIONSHIP BETWEEN MINERAL NUTRITION AND LATE-SEASON BUNCH STEM NECROSIS OF CABERNET SAUVIGNON (VITIS VINIFERA L.) GRAPEVINES
Late-season Bunch Stem Necrosis (BSN) is observed as a necrosis of the cluster stem (rachis) that leads to shriveling of berries on the affected portion of the cluster. Field experiments were conducted over three years at two vineyards in northern Virginia to examine relationships between specific nutrients and the incidence of BSN of Cabernet Sauvignon grapevines. Nutrients, used alone or in combination, included nitrogen, magnesium, and calcium. During the 1997 and 1998 seasons at Leesburg vineyard, applications of nitrogen, magnesium, and calcium produced little change in bloom-time petiole mineral concentration. Fertilizer treatments appeared to have no effect on BSN incidence, but the incidence of BSN was less than or equal 1% in the control plots each year. During the 1996 season at Winchester vineyard, bloom-time leaf petiole and vÃ©raison rachis nitrogen concentration of unfertilized (control) vines were 0.80% and 1.16%, respectively. The corresponding control BSN incidence was 41% at harvest time. Application of nitrogen fertilizer at 112 kg/ha actual nitrogen increased bloom-time leaf petiole and vÃ©raison cluster stem nitrogen concentration to 1.85% and 2.18%, respectively. 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 nitrogen levels in 1997. During the 1998 season, bloom-time leaf petiole and vÃ©raison rachis nitrogen concentration of unfertilized vines were 0.88% and 0.98%, respectively. The corresponding BSN incidence was 23% at harvest time. Application of nitrogen fertilizer again increased bloom-time leaf petiole and vÃ©raison rachis nitrogen concentration to 1.18% and 1.34%, respectively. Corresponding BSN was reduced to 3% at harvest time. Magnesium and calcium had no impact on BSN incidence; however, BSN symptoms were reduced when either was combined with nitrogen fertilizer. The relationship between mineral nutrition and BSN incidence at Leesburg was inconclusive. The BSN of Cabernet Sauvignon at Winchester was, however, positively associated with depressed bloom-time petiole total nitrogen concentrations. VÃ©raison rachis analysis consistently revealed an increase in nitrogen concentration due to application of nitrogen fertilizer. VÃ©raison tissue analysis may be a good diagnostic tool of vine nitrogen status. Magnesium and calcium appeared not to be involved in the disorder. The results illustrate that BSN-prone vineyards should be individually examined for nutrient imbalance or other stresses that may be contributing to BSN.
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