Sphaerulina leaf spot of boxwood

The disease is caused by the fungus Sphaerulina westendorpii, a common pathogen responsible for leaf spotting in various ornamental Buxus species. It is a persistent fungal threat that thrives in humid environments and can severely compromise the aesthetic integrity of boxwood hedges.

The primary host for this fungus is Buxus sempervirens (Common boxwood). The pathogen attacks the foliage, causing chlorosis and premature leaf drop, which often leaves the plant susceptible to secondary stress factors and potential dieback.

Symptoms typically manifest as small, tan to brown lesions on the leaf surfaces. As the infection progresses, these spots expand, often displaying a dark brown border. Eventually, small, dark fruiting bodies (pycnidia) develop within these necrotic spots, indicating the fungus is ready to release spores.

Development and spread are heavily dependent on environmental factors, particularly extended periods of leaf wetness and high humidity. The spores are primarily dispersed via splashing rainwater and wind, infiltrating the dense foliage of boxwood plants where airflow is restricted.

Effective management requires a multi-faceted approach, emphasizing both cultural and chemical control strategies. Proper spacing and thinning of the canopy are essential to improve air circulation and reduce the environmental favorability for the pathogen's development.

Chemical intervention involves the use of protective and systemic fungicides, especially during the active growth phase in spring and early summer. Removing fallen leaves from the base of the plants is critical to reducing the inoculum source and preventing reinfection in the following growing season.