Anatomical Developments and Effects of Artificial Shade on Bud Necrosis of Riesling Grapevines

Abstract

Bud necrosis (BN) destroys lateral buds of grapevine in their first year of development. The incidence of BN varies worldwide, among cultivars, and among nodes of a given shoot, but the fundamental causes remain unclear. Riesling buds were collected at 10-day intervals during the 1994 growing season to characterize temporal and anatomical features of BN. Ultrathin (1 to 5 mm) tissue sections revealed that the earliest microscopic evidence of BN appeared as groups or discrete zones of misshapen cells, in the base of the primary bud axis, above the nodal cushion. Affected cells began to show distorted or pleated cell walls 66 days after bud break, with cell compression and cell lysis apparent about 90 days after bud break. Scanning electron microscopy revealed similar lesions, and confirmed that the observed cell destruction was not an artifact of tissue preparation or microtomy. The role of shade was examined in two separate experiments to determine its effect on BN incidence and on tissue carbohydrate levels. In 1994, 92% shade cloth was erected over Riesling canopies for three-week periods, starting at either 20, 40, or 60 days after bud break in one vineyard, and starting 40 days after bud break in a second vineyard. BN incidence was unaffected. In 1996, shade was maintained for a 40-day period, starting at either 25 or 65 days after bud break. Both 1996 shade periods increased BN incidence. Shade effect on BN was more pronounced among nodes that emerged during the shade treatment, as opposed to nodes that had already appeared on the shoot prior to shade. Levels of sucrose, glucose, fructose, and starch in bud, leaf, and stem tissues were lower in shaded vines than in control vines at the time the shade treatment was terminated. Localized, transient reductions in carbohydrates or other essential substrates may be a contributing factor in grapevine BN incidence.