FACTORS ASSOCIATED WITH VARIATION IN SUSCEPTIBILITY OF GRAPEVINE PRUNING WOUNDS TO INFECTION BY EUTYPA-LATA

Pruning wounds are the only important infection site for Eutypa lata, the cause of Eutypa dieback. In 1989-1991, the susceptibility of grape vine pruning wounds inoculated with E. lata was significantly affected by date of pruning. Wound susceptibility was highest when vines were pruned early in the dormant season (November or December) and lower wh en vines were pruned later in the dormant season (January or March). W ound susceptibility declined significantly during the 28 days followin g pruning, except for fall pruning in one experiment. The decline in s usceptibility was highly correlated with an increase in suberin and li gnin deposition (r = 0.64-0.92) and with degree-day accumulation (r = 0.83-0.85). The rate of increase of suberin and lignin was also highly correlated with the rate of accumulation of degree-days over 0 C (r = 0.93-0.99). The growth of populations of nonpathogenic microorganisms on the wound surfaces also was dependent on pruning date and was corr elated with degree-day accumulation. Microorganism populations increas ed more rapidly when vines were pruned in early spring than in fall or early winter. The age of wood at the time of pruning did not signific antly affect susceptibility. Grape cultivars were significantly differ ent in their susceptibility to infection by E. lata, but their relativ e susceptibility was not always consistent between two field experimen ts.