A vascular wilt disease caused by a new bacterial pathogen of guar belonging to the genus Pantoea identified in Texas

Guar [Cyamopsis tetragonoloba (L.) Taub] or cluster bean is a legume plant that is cultivated mainly for its galactomannan (guar gum) extracted from the seed endosperm. In 2021, a disease outbreak among germplasms in the Texas A&M AgriLife Research Guar Breeding Program located in Vernon Texas was observed. Symptoms of the disease were similar to bacterial blight caused by Xanthomonas axonopodis pv. cyamopsidis and included black necrotic streaks on stems and petioles, vascular necrosis, wilting, and eventual death of affected plants. Incidence of the disease ranged from seventy-five to ninety percent in each the affected germplasms. Diagnosis of affected germplasms consistently recovered bacteria that were identified on the basis of their 16S rRNA partial gene sequence as belonging to the genus Pantoea. Two independent pathogenicity assays and the satisfaction of Koch’s postulate under greenhouse experimental conditions determined the Pantoea isolates were the causal agent of the disease. Investigations of the disease etiology also led to the discovery that the Pantoea sp. can be seed-borne. And although the disease symptoms share similarities with those of bacterial blight in guar, it differs in the absence of the large angular necrotic lesions on leaves that is characteristic of the later stage of the bacterial blight infection. However, infection by the Pantoea sp. similarly resulted in the eventual death of affected plants. Whole-genome pairwise alignments and multi-locus sequence analysis of the isolate indicate it is most closely related to P. vagans, but also distinct genetically, and tentatively designated as Pantoae sp. isolate B550.