First Report of Rhodococcus Isolates Causing Pistachio Bushy Top Syndrome on ‘UCB-1’ Rootstock in California and Arizona

Abstract

‘UCB-1’ (Pistacia atlantica × Pistacia integerrima) rootstock is a hybrid cultivar widely used by the U.S. pistachio industry. In the last three years, a large number of micropropagated UCB-1 pistachio rootstocks planted in California and Arizona orchards exhibited shortened internodes, stunted growth, swollen lateral buds, bushy/bunchy growth, stem galls with multiple buds, and twisted roots with minimal lateral branching. Field T-budding success in affected orchards was reduced to approximately 30% with unusual bark cracking often observed around the bud-union. The percentage of abnormal rootstocks within affected orchards varied from 10 to 90%. We have termed the cumulative symptoms “pistachio bushy top syndrome” (PBTS) to describe these affected trees. Two isolates, both containing virulence factors from the phytopathogen Rhodococcus fascians, were identified on symptomatic trees in field and nursery samples. Micropropagated UCB-1 trees inoculated with the Rhodococcus isolates exhibited stunted growth, shortened internode length, swollen lateral buds, sylleptic branching, and differences in root morphology, compared with control UCB-1 trees. To our knowledge, this is the first report of Rhodococcus isolates, related to Rhodococcus fascians, causing disease on a commercial tree crop and the results presented indicate that this organism is responsible at least in part for PBTS in California and Arizona.

Functional Genomics Insights Into the Pathogenicity, Habitat Fitness, and Mechanisms Modifying Plant Development of Rhodococcus sp. PBTS1 and PBTS2

“…In contrast, PBTS2 isolates grouped together with R. fascians (Stamler et al, 2015b). In agreement with the similarities between the symptoms associated with the leafy gall and PBTS syndromes, in both PBTS1 and PBTS2, the presence was demonstrated by PCR of the fasD gene, encoding an isopentenyl transferase that mediates the rate-limiting step of cytokinin biosynthesis (Crespi et al, 1992), the fasA gene coding for a P450 monooxygenase involved in zeatin production (Pertry et al, 2010; Genbank accession MF278335 for PBTS2 fasA), and vic (Stamler et al, 2015b). Although the presence of fasD is 100% correlated with virulence in leafy gall inducers (e.g., Crespi et al, 1992;Stange et al, 1996;Nikolaeva et al, 2009;Creason et al, 2014), its role in the PBTS strains remains to be determined.…”

“…We reassessed the pathogenicity of the PBTS strains on "UCB-1" plants and on two Nicotiana species and tested the hypothesis that the fas genes in the PBTS strains are easily lost during subculturing in rich medium, possibly the reason for their absence from the genome sequences (Randall et al, 2018;Vereecke, 2018). Additionally, based on the consistent co-isolation of both species from "UCB-1" trees exhibiting PBTS symptoms (Stamler et al, 2015a,b), we postulated that both isolates would not compete with each other for resources and would possibly exhibit complementary activities that would allow them to grow well in each other's presence which might be a prerequisite to synergize for optimal symptom formation (Stamler et al, 2015b). To this end, we mined the chromosomes of PBTS1 and PBTS2 to predict the occurrence of particular metabolic activities and phenotypes that might be important to thrive in their habitats, to associate with each other, and to modify plant development.…”

“…Furthermore, affected "UCB-1" plants exhibit a strongly reduced capacity to accept a Pistacia vera graft and in the unlikely event that grafting is successful, cracked gall-like tissue carrying ectopic shoots develops at the graft junctions with a weakened stock-scion union as a consequence. Altogether, these disease aspects prevent the plants from going into production (Stamler et al, 2015b). The causative agents of PBTS were designated Rhodococcus isolate 1 and Rhodococcus isolate 2 (Stamler et al, 2015b), but here will be referred to as PBTS1 and PBTS2, respectively.…”

“…Altogether, these disease aspects prevent the plants from going into production (Stamler et al, 2015b). The causative agents of PBTS were designated Rhodococcus isolate 1 and Rhodococcus isolate 2 (Stamler et al, 2015b), but here will be referred to as PBTS1 and PBTS2, respectively. 16S rRNA gene sequence analysis of several PBTS1 isolates obtained from symptomatic "UCB-1" trees, showed that they cluster together with R. corynebacterioides.…”

“…They have also been described as the cause of various infections in patients (Al Akhrass et al, 2012;Kitamura et al, 2012;Vergidis et al, 2017;Khalil et al, 2019) and as cancer killers exhibiting potent anti-malignancy activity (Zhou et al, 2017). In contrast, PBTS2 isolates grouped together with R. fascians (Stamler et al, 2015b). In agreement with the similarities between the symptoms associated with the leafy gall and PBTS syndromes, in both PBTS1 and PBTS2, the presence was demonstrated by PCR of the fasD gene, encoding an isopentenyl transferase that mediates the rate-limiting step of cytokinin biosynthesis (Crespi et al, 1992), the fasA gene coding for a P450 monooxygenase involved in zeatin production (Pertry et al, 2010; Genbank accession MF278335 for PBTS2 fasA), and vic (Stamler et al, 2015b).…”