Boxwood blight is a destructive fungal disease that poses a serious threat to Buxus species, commonly known as boxwood. This infection can quickly devastate hedges and specimen plants, leading to widespread defoliation and plant death if left unmanaged. Understanding the specific organism responsible and the environmental conditions that enable its spread is the first step in protecting boxwood plants from this aggressive pathogen.
The Specific Pathogen Responsible
The organism that causes boxwood blight is a fungus currently identified as Calonectria pseudonaviculata. This fungus belongs to the phylum Ascomycota, which includes many plant pathogens. It was historically known by other names before a taxonomic reclassification established its current scientific designation. This pathogen is a highly specialized fungus that primarily targets the above-ground parts of boxwood plants, including the leaves and stems.
Identifying the Signs of Infection
The initial signs of boxwood blight are small, circular leaf spots that appear on the foliage. These spots are often tan or light brown in color, surrounded by a darker purple or black border, and sometimes an accompanying yellow halo. As the disease progresses, these spots enlarge, causing the entire leaf to turn brown and rapidly drop from the plant. This rapid defoliation is one of the most distinctive features of the disease and often starts on the lower branches, moving upward in the canopy.
A second characteristic symptom is the development of linear, dark brown to black streaks or cankers on the green stems of the plant. These stem lesions are a key diagnostic feature. The presence of both leaf spots and these dark stem streaks, combined with sudden, heavy leaf loss, strongly indicates an infection by this fungus. Repeated defoliation weakens the plant, making it susceptible to other issues, though the pathogen itself does not attack the roots.
Environmental Factors Driving Disease Development
The successful development of boxwood blight is heavily dependent on specific environmental conditions, particularly moisture and temperature. The fungus thrives in microclimates that maintain high humidity, as it requires prolonged periods of leaf wetness to germinate and infect the plant tissue. Continuous leaf wetness for 12 to 21 hours is necessary for significant lesion development. Short periods of dryness, such as three hours or more, can substantially reduce the rate of new infections.
The optimal temperature range for infection is relatively mild, typically between 64°F and 77°F (18°C to 25°C). Disease severity peaks when temperatures are around 75°F, while infection rates decline rapidly at temperatures above 84°F (29°C) or below 43°F. Poor air circulation within the plant canopy is another promoting factor, as it slows the drying process, creating the extended leaf wetness duration the fungus needs. Dense plantings or tight shearing of boxwood hedges can thus create ideal conditions for the pathogen to flourish.
Mechanisms of Pathogen Spread
The primary mechanism for the localized movement of the boxwood blight fungus is the splash dispersal of its asexual spores, called conidia. The fungus produces these conidia in sticky, slimy masses. Instead, water droplets from rain or overhead irrigation detach the conidia and splash them onto nearby healthy foliage. This water-driven dispersal explains why the disease often progresses rapidly through dense plantings or during periods of frequent rainfall.
Long-distance spread of the disease is most often facilitated by human activity and the movement of infected plant material. The fungus can be transported on contaminated nursery stock, even if the plants are not yet showing symptoms. The sticky nature of the spores allows them to adhere easily to equipment, pruning tools, clothing, and even shoes. Infected plant debris, particularly fallen leaves, remains a significant source of inoculum, as the fungus can survive in this material for up to five years.