Fruit surfaces harbor microbial communities of diverse taxa that may play an integral role in maintaining fruit quality and inhibiting pathogen invasion (Droby and Wisniewski, 2018). Studies utilizing both culture-based and high-throughput amplicon sequencing strategies have provided insights into the diversity and functionality of these microbial assemblages (Leff and Fierer, 2013; Droby and Wisniewski, 2018; Zhang et al., 2021; Abdelfattah et al., 2021; Sui et al., 2024; Ngolong Ngea et al., 2024). In recent years, microbial communities on the fruit surface of apple (Malus × domestica Borkh.) have been comprehensively characterized revealing diverse communities of bacteria and fungi (Whitehead et al., 2021) with variable composition and structure. These communities have been shown to be influenced by several factors, such as geographical location (Abdelfattah et al., 2021), fruit genotype (Liu et al., 2018), management practices in the field or during storage (Abdelfattah et al., 2020; Wassermann et al., 2019; Bartuv et al., 2023; McLaughlin et al., 2024), fruit development stages and storage periods (Zhimo et al., 2022; Sui et al., 2024; Al Riachy et al., 2024; Lane et al., 2024), tissue type within the same fruit (Abdelfattah et al., 2020, Abdelfattah et al., 2021; Bösch et al., 2021), as well as factors like orchard air quality (Schweitzer et al., 2024). Zhimo et al. (2022) characterized bacterial and fungal community assemblages and dynamics of the apple surface microbiome of different cultivars across developmental stages that revealed strong community succession and the existence of underlying universal dynamics.

The presence of core taxa in the epiphytic microbiome of apple fruit has been established despite variations observed in overall diversity and composition (Abdelfattah et al., 2021; Zhimo et al., 2022). In this regard, Abdelfattah et al. (2021) reported the presence of two bacterial and six fungal core genera, while Zhimo et al. (2022) found 15 bacterial and 35 fungal core taxa of successional microbes on apples. Notably, the yeast-like fungus Aureobasidium spp., was found to consistently occur in high relative abundance in both studies under different climate conditions in multiple fruit cultivars, as well as during different stages of fruit development and storage conditions, including duration. Members of this genus, especially A. pullulans, are known for their ability to colonize plant tissues as both epiphytes and endophytes and exhibit significant intra-specific diversity. In general, Aureobasidium is noted for its adaptability to diverse environmental conditions, including fluctuations in temperature, humidity, and nutrient availability (Di Francesco et al., 2023). The genus is also recognized for its ability to produce a wide variety of natural compounds, including volatile organic compounds (VOCs), melanin, pullulan, β-glucan and siderophores (Wang et al., 2009; Muramatsu et al., 2012; Di Francesco et al., 2015; Jiang et al., 2016; Don et al., 2021). Its wide occurrence on different hosts and inhibitory activity against a wide range of plant pathogens have established Aureobasidium as a prime biological control agent (BCA) for the management of pre- and postharvest diseases of fruit crops (Ippolito et al., 2000; Schena et al., 2003; Vero et al., 2009; Zhang et al., 2010; Mari et al., 2012; Remolif et al., 2024). Field applications of A. pullulans were reported to decrease pathogen inoculum while promoting the colonization of beneficial microbes on fruit surfaces, resulting in lower infection levels during storage (Shi et al., 2022). However, the above studies have shown that the highest level of biological control is obtained using a postharvest application of Aureobasidium spp.

Despite the recognition of Aureobasidium spp. as an effective biocontrol agent, several gaps remain in our understanding of the impact of introducing large numbers of Aureobasidium spp. yeast cells on fruit surfaces in the field or after harvest on the composition of the fruit surface microbiome, and how its composition is correlated with reduced rates of postharvest decay. More specifically, how does the application of a BCA, such as Aureobasidium spp., influence the abundance and diversity of other beneficial microbial taxa and microbial pathogens over time? How does a BCA establish itself on the fruit surface and how does it interact with other microbial taxa present in the epiphytic fruit microbiome during different stages of fruit development, ripening, and storage, and how do these interactions contribute to disease suppression? Understanding the dynamics of the interactions that occur in the epiphytic microbiome of fruit is crucial for optimizing the application of a BCA for enhanced efficacy.

The present study was conducted to address these questions. More specifically, the objective is to characterize Aureobasidium spp. present on apple fruit surfaces and evaluate how preharvest and postharvest applications of the strains of Aureobasidium spp. that were the most antagonistic against Penicillium expansum influence the assembly dynamics of the fruit microbial community. Additionally, the study investigated the ecological processes driving microbial interactions and examined how these factors contribute to protecting fruit from postharvest decay during extended periods of cold storage.