A Bacterial 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. The disease incidence ranged from 75 to 90% in each of 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 that 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 seedborne. Additionally, although the disease symptoms share similarities with those of bacterial blight in guar, they differ in the absence of the large angular necrotic lesions on leaves that are 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 multilocus sequence analysis of the isolate indicate that it is most closely related to P. vagans, but also distinct genetically, and tentatively designated as Pantoae sp. isolate B550. [Graphic: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.