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Poster: Diseases of Plants: New & Emerging Diseases


Root and shoot susceptibility of peach/almond rootstock to pistachio bushy top syndrome isolates of Rhodococcus spp.
E. Fichner (1), S. Dhaouadi (2) (1) University of California Divisision of Agriculture and Natural Resources, U.S.A.; (2) University of California Division of Agriculture and Natural Resources, U.S.A.

Approximately 30,000 acres of UCB-1 clonal pistachio rootstock planted in California from 2011- 2014 were affected with pistachio bushy top syndrome (PBTS) caused by two phytopathogenic bacteria highly related to Rhodococcus fascians (Rf) and Rhodococcus corynebacteriodes (Rc). Orchards with high disease incidence were removed and replanted to pistachio or almond. The susceptibility of almond rootstock to PBTS isolates is unknown. Objectives include determination of the susceptibility of peach/almond rootstock to PBTS isolates and the influence of root and shoot infections on symptom development and host colonization. Either roots or shoots of clonal ‘Hansen 536’ rootstock were dipped in a bacterial suspension containing both PBTS isolates. Uninoculated control plants were dipped in buffer solution. Plant growth metrics and shoot and root biomass were determined 8 weeks after inoculation, and leaves were sampled for endo- and epiphytic populations of Rf and Rc. Root-inoculated plants were stunted and exhibited increased node density in comparison to control plants. Foliar-inoculated plants remained asymptomatic, but foliar inoculation resulted in endophytic colonization of leaves. Root-inoculations resulted in aboveground plant colonization. The results demonstrate that ‘Hanson 536’ rootstock is susceptible to the PBTS isolates. The results indicate a need for field evaluation for disease in PBTS-affected orchards replanted to almond.