2018 ASHS Annual Conference

Pistachio bushy top syndrome (PBTS) has affected pistachio (Pistacia vera L.) orchards planted on an interspecies clonal rootstock (‘UCB-1’) between 2011 and 2016 in California, Arizona, and New Mexico. PBTS is caused by concurrent infection by two plant-pathogenic Rhodococcus spp., both of which harbor virulence genes typically housed on a linear plasmid. Because of the unprecedented nature of PBTS, no research-based data are available to predict the long-term productivity of PBTS-affected trees in orchards, and affected orchards have largely been removed due to a concern of future economic productivity. The objectives of this work include: i) phenotypic characterization of PTBS symptomatic and asymptomatic trees entering maturity (i.e. yield, frequency of suckering, bark morphology, trunk caliper, total scaffold caliper diameter), ii) assessment of variability within symptomatic and asymptomatic populations for phenotypic characteristic estimates, and iii) determination of endo- and epiphytic populations of PBTS Rhodococcus isolates associated with foliage of rootstocks on symptomatic and asymptomatic trees. Phenotypic data on trees entering their 8^th leaf indicate that PBTS-symptomatic trees are both smaller and more variable in size then asymptomatic trees, and have over 70% reduction in yield. Symptomatic trees also exhibit greater suckering than asymptomatic trees and unique bark morphology on the rootstock. PBTS-affected trees had significantly more blank nuts and higher variability in edible yield than asymptomatic trees. Epiphytic populations PBTS Rhodococcus spp. were prevalent on both symptomatic and asymptomatic trees, indicating bacterial transmission within the field. Endophytic populations were only detected on symptomatic trees, suggesting that infectivity in the field is limited. The higher phenotypic variability in symptomatic than asymptomatic trees suggests that PBTS-affected trees exhibit varying levels of disease severity. The current study illustrates that PBTS causes reduction in yield and nut value at harvest. The economic cost of PBTS is compounded by the long-term land use opportunity cost and tree maintenance costs associated with pistachio, a crop that typically has its first economic yield 7 years after planting.