Box blight

Box blight, caused by the fungal pathogen Calonectria pseudonaviculata, is a devastating disease affecting Buxus species worldwide. This fungal infection is highly contagious and can lead to rapid defoliation and the death of boxwood plants in hedges, topiary, and landscape plantings.

The primary host for this pathogen is the common box (Buxus sempervirens), although other Buxus species can also be susceptible. The fungus attacks both foliage and stems. Once established, the disease can spread rapidly, especially in dense plantings where microclimates are conducive to fungal growth and spore transmission.

Symptoms initially appear as brown spots on leaves, which rapidly expand and lead to foliage bleaching and subsequent drop. A diagnostic sign of box blight is the presence of black, longitudinal streaks on the stems. In humid conditions, a fuzzy, white, or light grey growth consisting of fungal spores can be observed on the underside of infected leaves.

Environmental conditions play a critical role in the development and spread of Calonectria pseudonaviculata. Warm temperatures and high humidity are the primary triggers. The spores are primarily dispersed via splashing water from rainfall or overhead irrigation, as well as by contaminated garden tools, pruning equipment, and animal movement.

Preventative measures are essential as the disease is difficult to eradicate once established. Management strategies include improving air circulation through thinning, strict sanitation by cleaning tools, and removing infected plant material. Fungicidal treatments, when used preventatively and timed correctly, can help manage the severity of the infection, but they should be combined with good horticultural practices.