Responses of Cabernet franc Grapevines to Minimal Pruning and Virus Infection

Abstract

Minimal pruning of cordon-trained vines (MPCT) was superimposed for five years on established spur-or cane-pruned Cabernet franc vines of varying virus disease status, either healthy or infected by graft inoculation with two combinations of leafroll and yellow speckle, one from Sultana clone H₄ and one from Sultana clone H₅. The irrigated Cabernet franc readily adapted to the MPCT system without major changes in production, fruit maturation, and total vine size. Effects on vine development and morphology produced vines with the ideal balance between mature shoot growth and production which obviated the need to prune (i.e., zero pruning) as vine size was controlled by abscission of immature shoot growth. The lower productivity and vigor of cane-pruned vines was maintained when converted to the MPCT system. Compared to spur-pruned vines, MPCT vines developed during the five years a large permanent frame of old wood forming a substantial carbohydrate reserve, but had smaller root systems and reduced shoot growth. MPCT vines had more than double the number of shoots which had fewer and more closely spaced nodes. Vines of similar vigor had the same number of mature nodes, regardless of pruning system. Virus disease effects were amplified with the MPCT system. Both the H₄ and H₅ inocula produced symptoms of leafroll and yellow speckle and reduced yield by 9% and 23% and total vine size by 6% and 16%, respectively. Production was significantly lower than control in all seasons for H₅ but in only one season with H₄. The correlation between yield and weight of one-year-wood was low for pruning and inocula treatments. This indicated that one-year-wood is not a suitable measure of the vine's capacity for production. The significant correlation between yield and total vine size or yield and old wood indicates that this is a more appropriate measurement of vine vigor and production capacity. A non-destructive assessment of this parameter is required.