Response of 'Thompson Seedless' Grapevines to the Timing of Preharvest Irrigation Cut-Off

Abstract

The effects of preharvest irrigation timing on 9Thompson Seedless9 was studied for a 5-year duration on a fine sandy loam soil location and for 1 year on a sandy soil location. An early (early July) and a late (early August) irrigation cut-off were compared through the maturation and harvest period each year. Measurable treatment differences included soil moisture tension values, shoot growth, berry size, total soluble solids per berry, cluster rot incidence, and raisin quality. Total vine yields were not affected. The important effects of the more prolonged period of higher soil moisture tensions from the early irrigation cut-off were reduced shoot growth and smaller berry size. Berry size reduction was more immediate in the sandy soil location — in the first year as compared to the 3rd year in the fine sandy loam soil. The early cut-off treatment effects tended to be accumulative in the successive years of study, resulting in a lower accumulation of total soluble solids per berry and lower raisin grades in addition to smaller berries in the 5th year of study.