Effect of cluster exposure, berry contact and cultivar on cuticular membrane formation and occurrence of bunch rot (Botrytis cinerea PERS.: FR.) with 3 Vitis vinifera L. cultivars
Leaf removal treatments were applied to Riesling during 1990 and 1991,and to Cabernet franc and Optima in 1991. Shaded and exposed clusters of Riesling were sampled prior to veraison and shaded and exposed clusters of all 3 cultivars were sampled prior to their respective harvest dates. Clusters were sampled to examine the effect of berry exposure, berry contact, time of sampling, and cultivar on cuticular membrane formation and the occurrence of bunch rot (Botrytis cinerea PERS.: FR.). Non-contact and contact cuticle proper and epicuticular wax berry samples were examined qualitatively using cryogenic scanning electron miscroscopy and quantitatively using enzymatic separation and chloroform extraction. Exposed Riesling samples had 18.6 and 35.0 % more epicuticular wax and cuticle proper respectively than shaded samples. Non-contact Riesling samples had 15.7 and 35.0 % more epicuticular wax and cuticle proper than contact samples, and although significant, epicuticular wax and cuticle proper weights of Riesling increased by only 5.7 and 4.5 % respectively, between veraison and harvest. Although exposed cuticular membrane samples from all 3 cultivars had more epicuticular wax and cuticle proper than shaded samples, large differences between cultivars were present. Clusters from the hand leaf removal (i.e. exposed) treatment of all 3 cultivars examined had significantly less bunch rot than clusters from the control (i.e. shaded). Cabernet franc was the most tolerant and Optima the most susceptible of the 3 cultivars to bunch rot. Time of sampling and differences between years also influenced the occurrence of bunch rot for Riesling with less rot present at veraison compared to harvest and less rot present at the 1990 compared to the 1991 harvest. Exposure, cultivar and level of contact within the cluster are all important factors in the cuticular membrane formation process and contribute greatly to determining the overall susceptibility of a grape cultivar to bunch rot when grown in a temperate climate
