Differential Tolerance to Calonectria pseudonaviculata of English Boxwood Plants Associated with the Complexity of Culturable Fungal and Bacterial Endophyte Communities

Isolated boxwood endophytes have been demonstrated to effectively protect boxwood plants from infection by Calonectria pseudonaviculata (Cps). However, the roles of endophytes as communities in plant defense are not clear. Here, we demonstrated differential tolerance to Cps of English boxwood (Buxus sempervirens ‘Suffruticosa’), an iconic landscape plant and generally regarded as highly susceptible, and its link to endophyte complexity. Fifteen boxwood twig samples were collected in triplicates from three historic gardens—Colonial Williamsburg, George Washington’s Mount Vernon and River Farm, and Virginia Tech’s research farm in Virginia Beach in the summer and fall of 2019. A portion of individual samples was inoculated with Cps under controlled conditions. Significant differences in disease severity were observed among samples but not between the two seasons. Examining the endophyte cultures of the summer samples revealed that bacterial and fungal abundance was negatively and positively correlated with the disease severity. Nanopore metagenomics analysis on genomic DNA of the tolerant and susceptible group representatives confirmed the associations. Specifically, tolerant English boxwood plants had an endophyte community dominated by Bacilli and Betaproteobacteria, while susceptible ones had a distinct endophyte community dominated by Gammaproteobacteria, Actinobacteria, and diverse fungi. These findings may lead to boxwood health management innovations—devising and utilizing cultural practices to manipulate and increase the abundance and performance of beneficial endophytes for enhanced boxwood resistance to Cps

Leaf Endophyte Community Composition and Network Structures Differ Between Tolerant and Susceptible English Boxwood

Differential tolerance of English boxwood to boxwood blight has been linked to the ratio of culturable bacterial and fungal dominance in the leaf tissue of representative samples. To further understand how the whole endophyte communities may involve the tolerance of large samples, we extracted DNA from healthy leaf tissue of previously identified 28 tolerant (T), 41 moderately tolerant (M), and 21 susceptible (S) English boxwood plants, then sequenced associated bacterial and fungal amplicons using the Nanopore MinION platform. The endophyte community did not differ in diversity among the T, M, and S plants but differed in the abundance of bacteria and fungi, particularly between T and S samples. The bacterial genera Brevundimonas and Ammonifex had higher relative abundance in the T and M communities than in the S community, in which the fungal genera Botrytis and Thermothelomyces and family Chaetomiaceae were more dominant. The same results were obtained when mother and daughter samples in the T community were compared with controls in the S community, suggesting bacteria as a work force in the T community. Cooccurrence network analyses revealed that the T network had more fungal hubs but was less complex, with more positive connections than the S network, suggesting that the T community was supported by a healthier network. The resistance of English boxwood to blight is likely attributed to bacteria dominance and a synergic community network. This study is foundational to constructing synthetic communities and using whole communities of tolerant plants through vegetative propagation for microbe-modulated immunity

X. fragariae and C. cladosporioides cause strawberry blossom blight

Blossom blight was documented in some strawberry production fields in Watsonville in 1996 and 1997. Xanthomonas fragariae and Cladosporium cladosporioides were identified as the causal organisms. This is the first documentation of the two organisms causing blossom blight of strawberry in California. This is also the first report identifying C. cladosporioides as a pathogen of strawberry