Abstract
The purpose of this study was to determine if Endothall could reliably thin ‘Concord’, ‘Chancellor’, and ‘Seyval blanc’ grapevines while improving fruit composition and maintaining acceptable levels of phytotoxicity. Endothall was applied to runoff at 1.9 and 3.8 ml-L-1 and was targeted at clusters or whole vines. Applications were made either at 75% bloom for ‘Chancellor’ and ‘Seyval’ or 25%, 75%, and 100% bloom for ‘Concord’. Vines were rated for phytotoxicity. Yields and fruit composition were recorded at harvest. All cultivars experienced leaf phytotoxicity relative to Endothall concentration. However, the amount of leaf area affected did not cause any reductions in fruit soluble solids, such that all Endothall applications resulted in either the same or greater fruit soluble solids than the untreated vines. For ‘Concord’, thinning was possible over a 3 — to 4-day period ranging from 25% to 100% bloom, with 75% bloom being the most reliable time for thinning. Endothall increased fruit soluble solids in ‘Concord’ in 2 out of 3 years, and consistently thinned ‘Seyval’ when applied at 75% bloom. Those reductions in yield were associated mostly with smaller cluster weights and, to a lesser extent, fewer clusters per vine. An increase of as much as 2.9° Brix was achieved with the use of Endothall for thinning ‘Seyval’. The highest concentration of Endothall thinned ‘Chancellor’ and increased Brix in 2 out of 3 years with fruit soluble solids increasing as much as 5.3° Brix.