Boxwood Back From The Blight

Boxwood, those plush green globes and mini-hedges popular with the highest gardening elites down to the lowliest discount garden centers, fell from grace about a decade ago with the advent of a deadly disease called boxwood blight.  In the early days, photos of giant piles of dead boxwoods culled from nurseries and lush gardens browned by the blight circulated as warning stories.  But what has changed since then?

First, a little review.  Boxwood blight showed up in several east coast locations simultaneously during the summer of 2011.   Like Stonehenge and superconductivity, no one knows its exact origin, but boxwood starting dying of the strange fungus in the United Kingdom way back in the 1990’s.  The first symptoms that occur are light to dark brown, circular leaf spots with dark borders. Infected stems have dark brown to black, elongated cankers. Rapid defoliation occurs, especially in the lower canopy of the shrub.

Disease transmission primarily happens through movement of infected plant material, contaminated landscape and garden tools, and rain/irrigation splashes.  Fungal spores are spread by wind, rain or sprinklers.  Because spores are sticky, they can potentially be spread by contaminated clothing and animals, including birds.  Spores on infected leaves that have dropped can survive five years. Warm and humid conditions cause the fungus to spread quickly.  Gardeners are urged to clean their tools, never water boxwood from above and replace dead boxwoods with something else.  The fungicide recipes and regimes required to keep boxwood green resemble a cross between Baked Alaska and Gateau St. Honoré and are unsustainable.

In terms of dollars and cents, boxwood is a fairly large business, with sales of $126 million annually on 11 million plants, so it’s worthwhile for the nursery industry and government to become involved.  Adversity also inspires genius, and it has gotten some plant pathologists looking for possible solutions in tiny places.  One group of researchers found a bacterium they’ve named SSG for its size and shape (small, sage green) in the leaves of a very blight susceptible boxwood cultivar called ‘Justin Brouwers.’  In laboratory experiments, SSG interrupted the lifecycle of the blight pathogen at several stages and killed blight spores.  When sprayed on diseased leaf litter under boxwood plants, SSG reduced the blight by 90%.  Their conclusion is that SSG “offers great promise for sustainable blight management in nursery production and in the landscape.”  Other biocontrol agents being studied include another bacterium called Pseudomonas and a fungus named Trichoderma.     

But what if you want to buy the most blight-resistant boxwood you can find today?  Scientists at the US Department of Agriculture compiled data from several previous studies to find that answer.  According to their list, Buxus microphylla ‘Little Missy,’ ‘Winter Gem,’ ‘Compacta’ and ‘Green Beauty’ are among the most blight resistant types out of 131 examined.  Before buying, acquaint yourself with what boxwood blight looks like. When shopping read the labels carefully, and examine the plants with even more scrutiny.  After planting, continue to keep watch.  Hopefully, all will remain green.