Citation: Poleatewich, A.; Backman, P.; Nolen, H. Evaluation of Endospore-Forming Bacteria for Suppression of Postharvest Decay of Apple Fruit. Microorganisms 2023, 11, 81. https://doi.org/10.3390/ microorganisms11010081 Academic Editors: Tomislav Cernava, Beibei Ge and Kyungseok Park Received: 28 November 2022 Revised: 16 December 2022 Accepted: 22 December 2022 Published: 28 December 2022 Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). microorganisms Article Evaluation of Endospore-Forming Bacteria for Suppression of Postharvest Decay of Apple Fruit Anissa Poleatewich 1, * , Paul Backman 2 and Haley Nolen 3 1 Department of Agriculture, Nutrition, and Food Systems, University of New Hampshire, Durham, NH 03824, USA 2 Department of Plant Pathology and Environmental Microbiology, Penn State University, University Park, PA 16802, USA 3 Department of Molecular, Cellular, and Biomedical Sciences, University of New Hampshire, Durham, NH 03824, USA * Correspondence: anissa.poleatewich@unh.edu Abstract: The use of microbial biocontrol agents for control of postharvest disease has been the subject of intensive research over the past three decades resulting in commercialization of several biocontrol products. The objective of this research was to test endospore-forming bacteria collected from apple leaves for suppression of bitter rot and blue mold on apple. Bacteria were collected from abandoned, low-input, organic, and conventionally managed orchards in Pennsylvania and were screened for their ability to produce endospores, hydrolyze chitin, reduce pathogen growth in vitro, and suppress postharvest disease in vivo. Several isolates reduced bitter rot lesion size on ‘Rome Beauty’ from 40–89% compared to untreated controls. Bacillus megaterium isolates, A3-6 and Ae-1, resulted in the greatest suppression of bitter rot lesion size. One isolate, A3-2, suppressed blue mold lesion size. Scanning electron microscopy of inoculated apple wounds suggests parasitism as a mode of action explains the suppression of bitter rot lesion size by isolate A3-6. Of the top seventeen isolates exhibiting biocontrol potential, 70% were collected from abandoned or unmanaged locations. This research demonstrates abandoned apple orchards can be a source of new biocontrol agents for control of postharvest diseases of apple. Keywords: Bacillus; bitter rot; blue mold; biological control; apple 1. Introduction Production of fresh fruits and vegetables presents a unique challenge as most produce is harvested over a relatively short period of time and then stored for weeks to months to avoid exceeding demand and to provide consumers with a product year-round. Postharvest decay by microorganisms represents a significant limitation in our ability to store fruit [1]. The economic losses incurred from storage diseases are considerable given the initial investment related to crop production and management of pathogens and pests in the field prior to harvest. To date, the use of synthetic fungicides (as pre- or postharvest treatments) remains an important strategy for managing postharvest decays [2,3]. Development of fungicide resistant strains has been documented for several postharvest pathogens [4,5]. Human health concerns, due to pesticide residues on surfaces directly consumed as food, have also limited the available options for control of postharvest pathogens [6,7]. Several countries have banned or significantly restricted postharvest applications of fungicides [3,8]. As a result, there is an ongoing effort to develop alternative control methods to reduce fungicide dependence, reduce environmental risks, and improve consumer confidence in food safety. Considerable attention has focused on the use of biocontrol for management of posthar- vest diseases of fruits and vegetables, and there have been several review articles on the Microorganisms 2023, 11, 81. https://doi.org/10.3390/microorganisms11010081 https://www.mdpi.com/journal/microorganisms