Effect of canopy manipulation on rot incidence and rot metabolites of White Riesling (Vitis vinifera L.) grapes

Abstract

Several experiments were conducted to determine the influence of modifying the grapevine microclimate on the incidence of fruit rot and selected rot metabolites of Virginia grown White Riesling (<i>Vitis vinifera</i> L.) grapes. Due to the relatively high incidence of rot observed in these studies, an evaluation of the effects of rot on the sensory evaluation of juice and on selected aroma components was also undertaken.

The effect of shoot topping to 10 or 20 nodes or the application of ethephon on the incidence of grape rot was measured for three seasons. Canopy modification by both topping and ethephon treatment increased sunlight penetration into the fruiting zone. Fruit rot incidence and the concentrations of rot-associated metabolites were significantly and mutually correlated. Fruit rots and rot metabolites were greatest with control and ethephon-treated vines and were least with vines whose shoots were topped to 10 nodes.

In a separate three-year study, two to four leaves were removed from the fruiting zone of grapes grown on two training systems. Selective leaf removal generally increased sunlight penetration into the grapevine canopy. The incidence of Botrytis and sour rot as well as the concentrations of rot organism metabolites in harvested fruit were reduced by leaf removal.

The effect of grape sour rot on general fruit chemistry, free volatile terpenes, potentially free volatile terpenes, selected polyols and rot metabolites was undertaken. Grape sour rot increased °Brix, glucose to fructose ratio, titratable acidity, tartaric acid, glycerol and gluconic acid. Grape sour rot reduced the concentrations of geraniol, nerol and linalool, while increasing oxidation products, aromatic alcohols and long chained alcohols.

Grape juice sensory analysis appeared to be influenced by the severe fruit rot which occurred in both the control and to a lesser extent in canopy manipulated vines in both studies. The importance of fruit rot was demonstrated by both the production of odors, metabolites, and the destruction of native grape aroma constituents.

Reductions in the incidence of fruit rot by selective leaf removal and shoot topping were demonstrated and were probably due to several microclimate factors that collectively increased evaporative potential and spray penetration into the fruit zone. The results illustrate that remedial canopy management practices that improve fruit zone porosity can mitigate the severe disease pressure that exists when rot-prone cultivars are grown in humid grape growing regions. The inability to reduce fruit rots further than noted in these studies may be due to the complex nature of fruit rots observed here, compared to those (principally Botrytis rot) examined in the majority of other canopy management studies.