Abstract
Caustic chemicals (Endothall, Wilthin and ammonium thiosulfate) were applied at bloom to French-American hybrid grape cultivars (‘Seyval blanc’, ‘Vidal blanc’, and ‘Chambourcin’) to evaluate their potential as chemical thinners. The chemicals caused phytotoxicity to leaves and clusters resulting in brown, dead spots on leaves and brown areas of the clusters. Injury symptoms increased as concentration of Endothall increased. ‘Vidal blanc’ was the most sensitive cultivar to Endothall and ‘Chambourcin’ was the most sensitive to ammonium thiosulfate (ATS).
Hand cluster thinning at bloom consistently increased cluster weight, berry number per cluster, juice soluble solids and pH and decreased cluster number and yield of all cultivars with the exception that yield of ‘Seyval blanc’ was reduced only one year. Generally as concentration of Endothall increased, thinning increased.
Endothall at 1.92 ml-L-1 caused reductions in yield and increases in juice soluble solids and pH that were very similar to hand-thinning. Wilthin at 1.92 ml-L-1 generally produced a response similar to hand-thinning, but improvement in juice quality was not as consistent. Ammonium thiosulfate caused excessive leaf injury and crop reduction under warm and humid conditions.
Key Words:
