Niche-Based Priority Effects Predict Microbe Resistance toErwinia amylovorain Pear Nectar

ABSTRACT Fire blight is a devastating disease affecting pome fruit trees that is caused by Erwinia amylovora and leads to substantial annual losses worldwide. While antibiotic-based management approaches like streptomycin can be effective, there are concerns over evolved resistance of the pathogen and non-target effects on beneficial microbes and insects. Using microbial biological control agents (mBCAs) to combat fire blight has promise, but variable performance necessitates the discovery of more effective solutions. Here we used a niche-based predictive framework to assess the strength of priority effects exerted by prospective mBCAs, and the mechanisms behind growth suppression in floral nectar. Through in vitro and in vivo assays, we show that antagonist impacts on nectar pH and sucrose concentration were the primary predictors of priority effects. Surprisingly, overlap in amino acid use, and the degree of phylogenetic relatedness between mBCA and Erwinia did not significantly predict pathogen suppression in vitro, suggesting that competition for limited shared resources played a lesser role than alterations in the chemical environment created by the initial colonizing species. We also failed to detect an association between our measures of in vitro and in vivo Erwinia suppression, suggesting other mechanisms may dictate mBCA establishment and efficacy in flowers, including priming of host defenses. Competing Interest Statement The authors have declared no competing interest. Footnotes Funding: This work was supported by grants from the California Pear Advisory Board (RV), USDA (2017-67012-26104 and NE1501; RS and RV, respectively), University of California Davis (Start-up; RV), and Washington State University BIOAg Program (Proj. 168; RS and DC). AAF was supported through an NSF REU site award (DBI-1950299). RS also acknowledges support from the Utah Agricultural Experiment Station (Proj. 1466) and Utah State University. This manuscript has been approved as journal paper number 9680. e-Xtra: Supplementary material is available online. The author(s) declare no conflict of interest