<i>Calonectria pseudonaviculata</i> (<i>Cps</i>) poses an increasing threat to boxwood, a major nursery crop and iconic landscape plant worldwide. Here, we report on a potent biocontrol agent that produces small sage green (SSG) colonies on potato dextrose agar. SSG is a bacterial strain recovered from Justin Brouwers boxwood leaves with unusual response to <i>Cps</i> inoculation. Water-soaked symptoms developed on leaves 2 days after inoculation then disappeared a few days later. This endophyte affected several major steps of the boxwood blight disease cycle. SSG at 10⁷ cfu/mL lysed all conidia in mixed broth culture. SSG at 10⁸ cfu/mL reduced blight incidence by &gt;98% when applied one day before or 3 h after boxwood were inoculated with <i>Cps</i>. Its control efficacy decreased with decreasing bacterial concentration to 10³ cfu/mL and increasing lead time up to 20 days. When applied on diseased leaf litter under boxwood plants, SSG reduced <i>Cps</i> sporulation and consequently mitigated blight incidence by 90%. SSG was identified as a new member of the <i>Burkholderia cepacia</i> complex with distinct characters from known clinical strains. With these protective, curative, and sanitizing properties, this Burkholderia endophyte offers great promise for sustainable blight management at production and in the landscape.Published version