Contribution of native plasmids of Pantoea vagans C9-1 to epiphytic fitness and fire blight management on apple and pear flowers and fruits

Authors: KLEIN-GORDON, JEANNIE - Michigan State University; JOHNSON, KENNETH - Oregon State University; Loper, Joyce; Stockwell, Virginia

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2023
Publication Date: 6/7/2023
Citation: Stockwell, V.O., Klein-Gordon, J.M., Johnson, K.B., Loper, J.E. 2023. Contribution of native plasmids of Pantoea vagans C9-1 to epiphytic fitness and fire blight management on apple and pear flowers and fruits. Phytopathology. https://apsjournals.apsnet.org/doi/10.1094/PHYTO-04-23-0144-SA.
DOI: https://doi.org/10.1094/PHYTO-04-23-0144-SA

Interpretive Summary: Fire blight is an important disease of pear and apple causing estimated annual losses of >$100 million in the United States. New ways are needed to control this disease. In this research, the biological control agent Pantoea vagans C9-1 was evaluated. Four different types of C9-1, varying in stress tolerance and biological control activity in lab experiments, were applied pear and apple trees. It was found that all types of C9-1 reduced the incidence of fire blight by ca. 50% and stress tolerance was only important for C9-1's survival during the summer. This research underscores the importance of conducting biological control field trials to test predictions based on lab experiments.

Technical Abstract: Pantoea vagans C9-1 (C9-1) is a biological control bacterium that is applied to apple and pear trees during bloom for suppression of fire blight, caused by Erwinia amylovora. Prior bioinformatic studies of C9-1 focused on predicted phenotypes associated with its three megaplasmids, pPag1, pPag2, and pPag3, and their putative role in environmental fitness and biocontrol mechanisms. Plasmid pPag3 is a member of the large Pantoea plasmid family (LPP-1) that is present in all Pantoea spp. and has been hypothesized to contribute to environmental colonization and persistence, whereas pPag2 is unique to C9-1. We assessed fitness of C9-1 derivatives cured of pPag2 and/or pPag3 on pear and apple flowers and fruit in experimental orchards. We also assessed the ability of C9-1 'pPag3 to reduce populations of E. amylovora on flowers and disease incidence. Previously, we determined that tolerance to stresses imposed in vitro was compromised in derivatives of C9-1 lacking pPag2 and/or pPag3; however, in this study, the loss of pPag2 and/or pPag3 did not reduce fitness of C9-1 on flowers in orchards. Over the summer, pPag3 contributed to survival of C9-1 on developing apple and pear fruit in two of five trials, whereas loss of pPag2 did not significantly affect survival of C9-1. We also found that loss of pPag3 did not affect C9-1’s ability to reduce E. amylovora populations or fire blight incidence on apple flowers. Our findings partially support prior hypotheses that LPP-1 in Pantoea species contributes to persistence on plant surfaces, but questions whether LPP-1 facilitates host colonization.