Grapevine trunk diseases threaten grape production worldwide. In California, preventative practices (delayed pruning, fungicide applications after pruning) that reduce infections of pruning wounds by pathogen spores produced with rain are timed during the dormant season, when high spore-trap counts of the causal fungi are reported from studies in mature, symptomatic vineyards. Similar studies in young, asymptomatic vineyards are lacking. Given infrequent usage by California growers of preventative practices in young vineyards, empirical data on spore detection may provide convincing evidence to start preventing trunk diseases before symptoms appear. Active and passive spore traps were examined after rain from December to March for four years (2013–2017) in six young, asymptomatic (<5-years-old in year 1) and six mature, symptomatic (13–18-years-old in year 1) vineyards, in Lodi and Napa, California, USA wine-grape regions. We compared detection of spores of causal fungi of Botryosphaeria-, Eutypa-, and Phomopsis diebacks, which are common and widespread trunk diseases in California. From 769 samples, we isolated 15 species in culture, with DNA sequencing of 2 bar-coding loci for species-level identification. Detections (defined as presence of a species at least once per timepoint per site) were sparse in years 1 and 2, and so statistical analyses of detections pooled across timepoints and sites were restricted to years 3 and 4. Most common among all 12 sites and both trap types were Diplodia seriata and Diaporthe chamaeropis. Our novel species-level detections of six Diaporthe species fill a gap in the knowledge of which species can spread during California's dormant season. Overall, detections in young sites in years 3 and 4 (10 and 29 detections, respectively) were significantly lower than those of mature sites (31 and 57 detections, respectively). The presence of spores in young sites, even with no symptomatic vines, suggests preventative practices are needed in young vineyards. Nonetheless, our findings of more detections in mature sites reinforces higher usage by growers of preventative practices in mature, symptomatic vineyards.