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.  

Trouble Beneath The Snow

April, and the trees are a-budding, the bulbs are a-blooming, and the lawns are a-greening – except for a few lawns, which have big brown spots.  This I know, partly because I’ve received some photos from distressed homeowners, under the title “Why Did My Lawn Die?” and partly because my own lawn has brown spots, too.  It might not surprise anyone to learn that the problem has roots in 2020, but it is odd to learn that a common phenomenon – snow – is involved.  Let me tell you about the snow molds.

A lot of fascinating fungi live hidden in lawns, waiting for  just the right conditions to show themselves, and this is certainly true of the snow molds.  We’ll focus on the two most common types, called pink and gray.  Unlike most plant diseases, which require warm or at least moderate weather, the snow molds like it chilly.  Their idea of a good place to live is in the top layers of unfrozen soil with a nice, insulating cover of snow.  The longer the snow stays in place, the more damage these diseases can cause to the grass.  So, typically, the farther north you live, the more likely are to have snow mold appear on your lawn.   

Pink snow mold, the flashier of the two, gets its common name from its mycelium (body) color and the fact that the spots of dead grass it makes often turn pink on the edges.  Sometimes these spots are dramatic, with bleached, matted grass blades surrounded by a bright Crayola-like band, other times the patches are more diffuse.  The weather we love to hate in the Northeast - repeated frosts, cold fogs, slow and drizzling rains, with temperatures from just above freezing to almost 60 F – are joy to pink snow mold.  When the sun comes out and things dry, pink packs it in.  Kentucky bluegrass and fine-leaved fescues show more resistance than other grasses.  Those of you who like a good scientific name will appreciate knowing this pathogen is technically called Microdochium nivale.   

Gray snow mold makes bigger patches of trouble, sometimes measuring from one to three feet across.  The grass will often be flattened, matted and a pale straw or grayish color.  But there is a little shred of good news here.  While its damage may be more dramatic than pink, gray often doesn’t kill the grass, and a lawn may have a better chance of recovery.  Interestingly, it takes a longer period of snow cover for gray snow mold to develop and spread than for pink, but gray snow mold is more common on home lawns.  Also, if a lasting snow doesn’t appear until well into January, the chances of a banner year for the snow molds decreases, but you won’t catch me feeling sorry for them.

What you do influences these creatures.  Avoid fertilizing your lawn late in the season, but keep mowing until growth stops.  Avoid making giant snow piles.  In spring, patches can be raked, re-seeded and lightly fertilized.