Boxwood blight was first discovered in Europe in the mid 1990s and is now widespread there. North America escaped confirmed cases until late October 2011. Since that time, it has been found in most East Coast states, Ohio and Oregon, as well as the Canadian provinces of British Columbia, Ontario and Quebec.
Boxwood blight is caused by a fungus, Cylindrocladium buxicola. The fungus has also been referred to by two other Latin names, Cylindrocladium pseudonaviculatum or Calonectria pseudonaviculata.
Plant species within the genera buxus, pachysandra and sarcococca have been reported as hosts to this fungus. However, there is limited information about the role pachysandra (spurge) and sarcococca (sweetbox) species play as possible vectors for initiating the disease in boxwood. Although the full host-range of this fungus has not been finalized, it is believed that only plants within the family buxaceae can be infected by the pathogen.
Research conducted at North Carolina State University in 2012 indicated a wide range in susceptibility of boxwood cultivars to boxwood blight, however B. sempervirens types were more susceptible in general, with Buxus sempervirens ‘Suffruticosa’ (English boxwood) and Buxus sempervirens ‘American’ (common or American boxwood) especially susceptible..
- Dark- or light-brown, circular leaf spots, often with darker margins. Leaf spots may grow together to eventually cover the entire leaf.
- Dark stem cankers or black streaks on stems
- Straw- to bronze-colored, blighted foliage
- Leaf drop
- Less commonly, white fuzzy masses consisting of numerous clumps of spores may be observed on infected stems and leaves with a hand lens under high humidity.
In container boxwood, sometimes only the lower foliage and stems become infected, leaving the tops green and making the plant appear top-heavy. On large field-grown or landscape plants, only one section of the plant closest to the ground on the shaded side will be blighted.
Blighting and defoliation can occur rapidly, with complete leaf loss under warm (64°F to 80°F) and humid conditions. Due to the shady and moist conditions during propagating, young boxwood plants are especially at risk if the pathogen is unintentionally introduced into the growing area.
Even after severe defoliation, root systems of boxwood-blight-infected plants remain healthy and intact, unlike roots infected with Phytophthora. The stems of infected boxwood typically remain green under the outer bark until a secondary invader or opportunistic pathogen attacks this tissue and eventually kills the plant.
How It Spreads
The primary way this disease spread throughout Europe was the movement of infected plants and cuttings. Another significant way this disease spreads locally (nurseries and landscapes) is through contaminated tools and workers as well as by movement of boxwood debris (especially fallen leaves).
What To Do
Identifying measures for preventing and managing boxwood blight in commercial nursery and field settings is a work-in-progress; researchers are in the process of evaluating fungicides and sanitizers, as well as identifying resistant boxwood cultivars. All plants infected with boxwood blight should be destroyed, as the chance of further spreading this fungus is highly probable. Once introduced, limiting movement of this sticky, contagious fungus is very difficult and will only be accomplished by always following good sanitation practices.
Information courtesy of North Carolina State University