Natural infection by Colletotrichum spp. and the subsequent development of ripe rot were observed in susceptible grape (Vitis vinifera) clusters either protected or exposed to environmental conditions and naturally occurring inoculum by the application or removal of paper bags at various phenological stages at two Mid-Atlantic vineyards. During each of the three experimental seasons, most treatments with grape clusters that were exposed between veraison and harvest developed significantly more severe ripe rot than clusters that were exposed during only the early season or protected throughout the entire season. Spore traps were placed in one vineyard over two seasons and were analyzed via quantitative PCR. DNA of the Colletotrichum acutatum and C. gloeosporioides species complexes was detected from the bloom to the harvest stage, with a higher quantity of C. acutatum DNA than C. gloeosporioides DNA. From ripe rot symptomatic clusters, 417 isolates were collected, and a multilocus phylogenetic analysis of 51 representative isolates identified six Colletotrichum spp., with C. fioriniae (C. acutatum complex) being the most frequently isolated. Weather data were also monitored, and ripe rot-conducive conditions were observed at multiple times throughout each season. In summary, only clusters that were exposed to inoculum and environmental conditions in the late season developed severe ripe rot. The data collected in this study suggest that grape clusters have ontogenic susceptibility to ripe rot, becoming more susceptible as they mature, in contrast to the susceptible bloom stage for other fruit rotting diseases of grapevine.
Keywords: Colletotrichum; Vitis vinifera; fruit rot; grape; ontogenic susceptibility; ripe rot.