Virginia Turfgrass Council – Boxwood Blight: An Emerging Disease that Keeps Landscapers and Gardeners Awake at Night

VIRGINIA TURFGRASS JOURNAL: Chuan Hong, Ph.D., Virginia Tech<br /> <br /> Boxwood has been the choice evergreen ornamental shrub in landscape designs for homes, public gardens, parks, golf courses, and other green spaces; they accentuate garden entryways, give structure to garden designs, edge garden beds, divide courtyards, designate parking areas, fill window boxes, and create labyrinths. This preference is attributed to several features of boxwood plants: low maintenance, deer resistance, and high tolerance to heavy pruning, and to lesser extent, to drought and cold conditions. This iconic landscape plant, however, is now endangered due to an emerging disease – boxwood blight caused by a fungal pathogen (Calonectria pseudonaviculata). This disease wiped out the English boxwood gardens at the Woodrow Wilson Presidential Library in 2016, invaded and disfigured historic boxwood plantings at the Tudor Place (October 2018), Longwood Gardens (August 2020), and the Colonial Williamsburg (July 2021). It also decimated many private boxwood gardens like this one that had been the family treasure for generations. There have been several lawsuits filed by highly valuable estate owners against landscaping companies, complaining on introducing the blight pathogen to their properties. To protect customers and landscaping companies themselves from getting into such legal battles, the first and foremost important step is to learn what a blighted boxwood plant and planting looks like, so proper actions may be taken to mitigate the damages to both customers and landscapers, as well as other property owners in the same neighborhood and area. Boxwood blight infection may start in the garden and landscape where newly introduced host plants including boxwood, pachysandra and sweet box are placed, or may be at first randomly scattered depending upon the means by which the blight pathogen is introduced to a property then spread locally. If the pathogen is introduced by purchasing and bringing in infected plant materials, then spread by contaminated domestic (e.g., dogs, cats) and wild animals (deer), initial infection and blight symptoms usually start low in the canopy, or at the heights at which contaminated animals most likely come in contact with boxwood plants. If the pathogen is spread by contaminated insects and birds, initial infection and blight symptoms may appear wherever the insects and birds land. If the pathogen originates from a nearby property in the neighborhood, it could be introduced by sharing contaminated garden tools, and/or by contaminated runoff water, shoes and other personal belongings. These additional means of pathogen spread often result in new infection and symptoms in lower portion of the canopy. Boxwood blight has three diagnostic symptoms: 1) leaf spots and blight, 2) black streaks on young stems and branches, and 3) rapid defoliation. Leaf symptoms begin as small, light to dark black specks that enlarge into discrete spots or diffuse blotches. The spot centers often become tan with an outer ring of black, necrotic tissue. Alternatively, entire leaves may turn completely gray or black without any discrete spot if the pathogen loads are high, plus an extended period of warm and wet conditions. This often results in leaves falling off plants within a week. Leaf symptoms develop more rapidly on English boxwood and other highly susceptible cultivars than those less susceptible ones such as NewGen Boxwood®, Little Missy. Likewise, symptoms develop more quickly on you