A new fungus, Cylindrocladium buxicola, which causes disease on boxwood was recently observed in Ontario. The fungus was positively identified by DNA sequence comparisons as the cause of box blight, from samples from a nursery in Southern Ontario. Whole boxwood plants required wounding for successful infection, however detached leaves did not. The ‘Green Series’ boxwood cultivar ‘Green Mountain’ was found to be the least susceptible to C. buxicola compared to other cultivars. Survival of C. buxicola in Southern Ontario was low after the winter, but was higher throughout the fall. Whole genome sequencing of two C. buxicola isolates revealed the presence of the MAT1-1 gene within a German isolate and MAT1-2 in an Ontario isolate, implying the possibility for sexual reproduction. All other 50 isolates tested were MAT1-2. The preventative use of fungicides to control box blight in nurseries, rather than curative use, is likely to provide the best management option for disease.