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Contribution of native plasmids to fitness and fire blight biocontrol efficacy of Pantoea vagans strain C9-1

Jeannie Klein: Department of Plant Pathology, University of Florida

<div><em>Pantoea</em> <em>vagans </em>strain C9-1 is a registered biocontrol agent for fire blight of pear and apple caused by <em>Erwinia amylovora</em>. C9-1 suppresses growth of <em>E. amylovora</em> on stigmas via competition and antibiosis, aided by gene products encoded on its three native megaplasmids. pPag3 is a member of the Large <em>Pantoea</em> Plasmid (LPP-1) plasmid group and was proposed to play a critical role in environmental fitness. We cured C9-1 of two plasmids: pPag2 (166 kb), pPag3 (530 kb), and both pPag2 and pPag3, and evaluated colonization and survival of C9-1 and derivatives from bloom to fruit harvest. In experimental orchards, loss of pPag2 and/or pPag3 did not affect establishment, growth, or survival on apple and pear flowers through petal fall compared to the wild type C9-1. Loss of pPag2 did not affect survival on fruit. However, population sizes of C9-1 lacking pPag3 were lower than C9-1 on apple fruit in two of five trials. We conducted disease efficacy trials with C9-1 and a derivative lacking pPag3. C9-1 cured of pPag3 reduced the incidence of disease to levels similar to the wild-type C9-1. Overall, loss of pPag3 (LPP-1) did not reduce epiphytic fitness on apple and pear flowers or affect C9-1's ability to reduce disease incidence; nonetheless, C9-1 derivatives without pPag3 occasionally exhibited reduced survival on maturing fruit, approximately one month after application to flowers.</div>