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Abstract
Center rot of onion is a bacterial disease caused by diverse plant-pathogenic Pantoea species that leads to substantial losses in both field and in storage. Pantoea stewartii subspecies indologenes (Psi), known to cause disease in millets, was recently identified as a causal agent of center rot in onion. The Psi isolates have not been well-characterized and their transmission risk to onions in cropping systems where millets precede onion planting is less understood. In this study, we characterized 17 Psi strains isolated from various hosts and classified them into two distinct pathovars based on their distinctive pathogenicity profiles. We also evaluated the survival of onion-pathogenic Psi in corn and millet residues and assessed its potential transmission risk to onions. Although the pathogen was present in soil at the time of onion transplanting, the risk of transmission to the crop was low. We further investigated the role of exopolysaccharide production, flagellar motility, quorum sensing and pathogenicity-related genes in the survival of onion-pathogenic Psi. Exopolysaccharide production seemed to play a minor role in survival in pearl millet residue, while the other targeted genetic factors showed no contribution to survival in millet residue nor in bare soil. Additionally, we explored strategies to incorporate biocontrol agents (BCAs) into bactericide spray programs for center rot management. Our aim to enhance the efficacy of BCAs by favoring early colonization of onion foliage had limited effect. Onion plots pre-treated with peroxygen before BCA application exhibited similar disease levels in both foliage and bulbs as compared to those treated with BCAs alone. Findings from our study to integrate Bacillus species with copper to manage center rot indicated that, under low disease pressure, rotation interval between Bacillus species and copper could be extended to 10-14 days without compromising disease control efficacy. However, tank-mix applications of Bacillus with copper offered no additional benefits, hence, rotation would be preferrable than tank-mixing these products. Overall, our findings elucidated taxonomic standing of Psi, its survival on crop residues and emphasized the importance of integrated-management program with BCAs and copper-based bactericides for effective center rot management.