Boxwood blight identification and quarantine

I don't grow boxwood at home in Melville -- deer browse them hard in zone 7a and I moved away from formal hedging after the blight hit Long Island -- so the bulk of this guide is sourced from Connecticut Agricultural Experiment Station (the lab that first confirmed U.S. boxwood blight in 2011), NC.

I don't grow boxwood at home in Melville — deer browse them hard in zone 7a and I moved away from formal hedging after the blight hit Long Island — so the bulk of this guide is sourced from Connecticut Agricultural Experiment Station (the lab that first confirmed U.S. boxwood blight in 2011), NC State Extension, Penn State Extension, Clemson HGIC, and USDA APHIS.

Boxwood blight arrived in the United States in 2011 and spread to 27 states within five years. If you grow boxwood in the mid-Atlantic or Southeast, it is a genuine threat. The disease is not a slow decline — under humid conditions, it can defoliate a healthy shrub in under a week.

The pathogen

Boxwood blight is caused by the fungus Calonectria pseudonaviculata (syn. Cylindrocladium pseudonaviculatum). Per NC State Extension, the fungus produces two types of spores: conidia (asexual spores for short-range spread) and microsclerotia (thick-walled survival structures that persist in soil and plant debris for years).

The microsclerotia are the persistence mechanism that makes this disease so difficult to eradicate. Per Penn State Extension, they can remain viable in soil for at least five years, meaning a replanting in the same site with susceptible boxwood is very likely to fail.

A related fungus, Calonectria henricotiae, was identified in Europe in 2013 and has been detected in the United States. Per USDA APHIS, both species cause identical symptoms and are treated identically from a management standpoint.