Comparisons of Thompson Seedless Clones for Raisin Production

Abstract

A three-year study was conducted to compare three Thompson Seedless clones, 1A, Urla-HT, and Enns-HT. 1A is a University of California non-registered selection with mild grapevine leafroll; Urla-HT and Enns-HT are Turkish and California selections, respectively, heat-treated and registered virus-free. 1A and Urla-HT were compared with both cane- and long-spur pruning to determine if Urla-HT was more fruitful with spur pruning as reported. Enns-HT was compared under cane pruning only. Orthogonal comparisons of 1A vs. Urla-HT (all pruning treatments) and 1A vs. Enns-HT (cane pruning only) found 1A to produce higher raisin yields than Urla-HT or Enns-HT. This was attributed to the heavier berries and clusters of 1A. Additionally, 1A was more fruitful than Urla-HT, a contradiction to Urla's reported high fruitfulness characteristics. The larger vine size of Enns-HT confirmed field observations of vine growth but was accompanied by lower productivity as compared to 1A. The better performance of 1A, in spite of virus presence, suggests possible genetic differences in these clonal comparisons.