DISTRIBUTION AND RETENTION OF CLEISTOTHECIA OF UNCINULA NECATOR ON THE BARK OF GRAPEVINES

Clesitothecia of Uncinula necator were dispersed by late summer and early autumn rain to the bark of grapevines. Rain-dispersed ascocarps accumulated rapidly on bark during a 10-wk period and were then retained on bark through subsequent rain events between leaf abscission and budbreak the folowing spring. The density of populations on bark was significantly correlated (r = 0.72-0.88) with catches of cleistothecia in filter-paper funnels attached to the trunk of grapevines. The density of populations on bark during winter was not correlated with several measures of disease incidence and severity from the previous growing season. We concluded that while incidence and severity may determine the potential population available for dispersal, rain events determine the actual efficiency of transfer from infected organs to the bark of the vine. The percentage of ascocarps that reacted positively with the fluorescent vital stain fluorescein diacetate ranged from 50 to 62% and did not change significantly during overwintering until cleistothecia began to dehisce in spring. Therefore, the density and viability of populations of cleistothecia on bark at the time of budbreak appear to have been determined at the time of leaf fall the previous autumn and were not modified by subsequent environmental conditions. The densest aggregations of cleistothecia occurred on the cordons of cordon-trained vines, with successively lower densities occurring on the bark of the upper and lower trunks. The pruning and training system of vines of Vitis labrusca cv. Concord did not affect the density of populations of cleistothecia on bark.