Localized Carbohydrate and Essential Nutrient Deprivation as Causal Factors in Grapevine Bud Necrosis

Bud necrosis (BN) of grape is observed as an abortion and drying of one or more primordia of the developing compound winter bud. The causes are uncertain, but high vigor, and canopy shade increase BN incidence under some conditions. This work was conducted to determine if localized carbohydrate or mineral nutrient deprivation increased the incidence of BN observed with ‘Riesling’ grapes in Virginia. Changes in carbohydrate levels in ‘Riesling’ grapevines was attempted by shading and shoot tip removal. In 1994, 92% shade was applied at 20, 40, or 60 days after budbreak in one vineyard, and starting 40 days after budbreak in another vineyard for three-week periods. In 1996, a second shade experiment at 25 and 65 days after budbreak for a 40-day period was conducted. Shade reduced PPF in the fruit zone of canopies to <2% of ambient PPF in both experiments. The first experiment did not increase BN. However, the second experiment increased BN in the distal nodes of the shaded vines compared to the control vines. Shoot vigor, measured as shoot diameter and internode length at season’s end, was positively correlated with BN in shaded as well as unshaded vines. The frequency of necrotic buds was greater at nodes 5 through 16 than at nodes 1 to 4 in both shaded and unshaded vines. Levels of total nonstructural carbohydrates (TNC) measured spectrophotometrically, were not significantly affected by shade treatment. Levels of sucrose, glucose, fructose, and starch in bud, leaf, and stem tissues analyzed by high performance liquid chromatography were lower in shaded vines as determined at the time of shade cloth removal. Shoot tip removal increased BN at nodes distal to 12. Bud tissues of shoot-tipped vines had lower levels of sucrose, glucose, fructose, and starch than did the control vines. Mineral nutrient surveys conducted on ‘Riesling’ and ‘Chardonnay’ vines indicated that BN is not caused by essential nutrient deficiency. Carbohydrate surveys revealed that ‘Riesling’ buds had lower levels of sucrose compared to ‘Chardonnay’ buds. Visual starch rating revealed that ‘Chardonnay’, the BN-insensitive cultivar, had greater levels of bud starch deposits than did the BN-susceptible cultivars, ‘Riesling’, ‘Syrah’, and ‘Viognier’. Starch deposits in grape buds was negatively correlated with BN incidence. ‘Riesling’ and ‘Chardonnay’ vines showed a node position effect where in nodes 1to 6 had higher levels of starch deposits compared to nodes 7 to 13. From these studies it can be concluded that a negative correlation between carbohydrate levels in vines and BN incidence exisits.