Identification of Extensive Mycorrhizal Genera and Other Beneficial Microbes from Four Boxwood Cultivars with Varying Blight Resistance Levels

Background Rhizosphere microflora is important for plant growth and health. The objectives of this study were to characterize boxwood rhizosphere microbial community and understand their associations with plant disease resistance traits. Rhizosphere soil samples were collected from four cultivars with three levels of boxwood blight resistance at two geographically distant nursery locations during three seasons of 2021. Bacterial and fungal communities were characterized through DNA metabarcoding. Results Overall, Bacillus and Clonostachys were the most abundant bacterial and fungal genera, respectively; and 27 bacterial and 6 fungal core genera were also identified across all cultivars, locations, and seasons. Fungal community structure was distinct among cultivars of three resistance levels in May and August, and likewise, the bacterial community in August across the two locations; the microbial community structure in intermediate and/or susceptible cultivars was also extensively influenced by soil properties. Additionally, the microbial community composition was influenced by cultivar blight resistance level, with 25 microbial genera differing in abundance between the intermediate/tolerant and the susceptible cultivars. Furthermore, twenty-five fungal genera, each having many mycorrhizal members, were identified, with Ceratobasidium , Hyaloscypha , and Sistrema being the most abundant. The connections between presumptive mycorrhizal fungi (PMF) were 6.9–56.9% higher in the cross-kingdom network of the blight tolerant cultivar than in the other three cultivars, dependent on location. Notably, 17 PMF genera interconnected in the Oregon tolerant network, including the hub taxa Glomus and Amanita . Conclusions The boxwood rhizosphere harbored many potential mycorrhizal genera and other beneficial microbes that may play a role in promoting boxwood growth and disease resistance, providing evidence supporting boxwood as a low-maintenance plant. The structure and composition of the fungal community differed among the cultivars, suggesting that fungi are recruited and specialized in response to plant phenotypic and genotypic cues that affect plant health. Importantly, the interactions among diverse mycorrhizal fungi may contribute to each cultivar’s blight tolerance but could be disrupted by cultural practices, such as intense fungicide application. These findings provide several new perspectives that will be useful in boxwood breeding and management to enhance plant resilience to disease and environmental stress.