A reduction in vine vigor of cabernet sauvignon (Vitis vinifera L.) decreases bunch stem necrosis

Bunch stem necrosis (BSN) is a physiological disorder in grapes, which results in unripe shrivelled berries with poor quality attributes. Past research into this area is conflicting with many different hypotheses being proposed to explain the disorder. Vines on SO4 rootstock, a high vigor rootstock, have demonstrated high incidence of BSN and the published literature has also indicated that shading of vines at a particular time may influence BSN incidence. In this study, it was proposed that there was a positive relationship between vine vigor and BSN incidence due to a shift in the source-sink relationship between the developing fruit and vine growth for key mineral elements such as calcium, potassium and magnesium. Treatments to manipulate shoot vigor and the source-sink relationship were applied to Cabernet Sauvignon grape vines growing on SO4 rootstock. Treatments included root pruning to reduce vigor, heading back of canes and the application of a reflective mulch (Extenday (TM)) to increase vigor, 50% shading for three weeks prior to flowering and 50% shading for three weeks after flowering. A strong positive relationship between vigor and BSN incidence was determined in the first growing season. In the following season, where all treatments were reapplied except root pruning, the relationship was not as strong but was still positive. Root pruning still reduced BSN and vigor in the second season, just as heading back of the canes and the reflective mulch increased vigor and BSN incidence. Environmental conditions between the two seasons were very different and this may have influenced the observed responses. Neither shade treatment significantly changed the vigor of the vines in either season. Shade prior to flowering reduced BSN incidence in both seasons. However, shade after flowering only influenced the BSN incidence in the second season where the BSN incidence was increased. It is proposed that the shade treatments had a direct affect on the source-sink mechanism rather than manipulating the vigor of the vine. These results provide evidence that some management practices are available to manage this disorder but their effectiveness will be influenced by the prevailing environmental conditions especially around the time of flowering.