Boxwood blight is a fungal disease that has become a concern for gardeners and landscapers who rely on popular landscaping shrubs. This pathogen causes rapid decline in plant health. Understanding the specific plants it targets and the ways it moves is important for protecting your landscape from this threat.
Identifying the Susceptible Host Plants
The primary target of boxwood blight is the Buxus genus, which includes all true boxwood shrubs used for hedges and ornamental plantings. The fungus is host-specific, meaning it only actively infects plants within the Buxaceae family. American Boxwood and English Boxwood cultivars (Buxus sempervirens) are considered highly susceptible to the disease.
Other members of the Buxaceae family are also true hosts and can serve as reservoirs for the fungus. These include Pachysandra (Japanese Spurge or Allegheny Spurge) and Sarcococca (Sweetbox). Some Asian boxwood species, such as Buxus microphylla and its hybrids, often show higher levels of resistance, though they are not immune to infection.
Potential for Transmission to Other Garden Plants
The fungus that causes boxwood blight does not actively infect non-host plants like roses, hydrangeas, conifers, or vegetable garden crops. The disease’s narrow host range limits its ability to invade and colonize plant tissues outside of the Buxaceae family.
While a rose bush cannot become infected, its leaves or stems can become temporarily contaminated with the sticky fungal spores. These non-host plants act as passive carriers, allowing the spores to persist on their surfaces for a period of time. If these spores are then transferred to a susceptible host like a boxwood via water splash or contaminated tools, they can initiate a new infection. This mechanism of spore harborage is a major factor in the pathogen’s ability to move throughout a garden.
Recognizing the Signs of Infection
Identifying the disease on susceptible plants involves looking for a distinct set of visual cues. The first signs are often dark, circular leaf spots that can have a slightly darker border. These spots quickly enlarge, leading to leaves turning brown or tan before they rapidly drop off the plant.
A hallmark symptom that helps distinguish boxwood blight from other issues is the development of black streaks or cankers on the green stems. These lesions are elongated and often dark brown to black in color, and they can be found even after the leaves have defoliated. The resulting bare, defoliated appearance, especially from the bottom of the plant upward, is a strong indication of a severe boxwood blight infection.
Preventing the Spread of Boxwood Blight
Preventing the movement of the pathogen relies heavily on diligent sanitation and changes to common gardening practices. Since the fungal spores are heavy and easily spread by water, moving infected material, and contaminated equipment, controlling these vectors is important. Disinfecting all pruning shears, clippers, and other tools is important after working on or near any susceptible plant, using a solution like bleach or commercial disinfectant.
Avoiding overhead irrigation is another important step, as water splash is a primary mechanism for moving spores from the soil and infected leaves onto healthy foliage. Using drip irrigation or carefully watering at the base of the plants can significantly reduce the risk of spread. Finally, any infected plant debris, including fallen leaves, should be removed, double-bagged, and disposed of in municipal waste rather than being placed in a compost pile, where the fungus can survive for years.