Survey on Latent Infection of Canker-Causing Pathogens in Budwood and Young Trees from Almond and Prune Nurseries in California.

Almond band canker and prune Cytospora canker have become more severe in the last decade, especially in young orchards, in California. To test our hypothesis that young trees from nurseries could carry the canker-causing pathogens at latency phase to new orchards through transplanting, a multiyear survey on latent infection of canker-causing pathogens of budwood and young trees of almond and prune nurseries in California was conducted. A total of more than 1,730 samples including shoots of rootstocks and scions and grafting union cuttings were collected from 11 nurseries. A real-time quantitative PCR assay was applied to quantify the latent infection levels by six canker-causing pathogen taxa: and species of , , , , and For almond, the average incidences of latent infection caused by spp. (43.6%) and spp. (24.2%) were significantly greater than those by the other four pathogen taxa. The molecular severity (MS) of latent infection caused by spp. (3.6) was significantly greater than those caused by other pathogen taxa, except for spp. (2.6). For prune, the average incidence of latent infection caused by spp. (13.5%) was significantly higher than those caused by (1.5%) and spp. (1.3%) but not significantly higher than those caused by spp. (6.9%), spp. (6.3%), and spp. (7.7%), respectively. Moreover, the average MS values caused by spp. (3.8) and spp. (3.2) were the highest followed by those caused by (1.4), spp. (2.2), and spp. (2.3). Different almond varieties showed various levels of susceptibilities to different canker-causing pathogens. This study concluded that and are the predominant pathogen species in almond, and is the most important canker-causing pathogen species in prune in nurseries. These findings confirmed the observations of predominancy of canker-causing pathogens in almond and prune orchards in California.