Boxwood blight can be devastating to American boxwood cultivars, which are common in the Kentucky landscape. Complete defoliation can occur within a week and plants can die within a single growing season. Use of tolerant cultivars, cultural practices, and fungicides can reduce incidence and spread of boxwood blight.
Boxwood Blight Facts:
- Symptoms on leaves can appear as light or dark brown circular leaf spots with darker borders (Figure 1). These symptoms often go unobserved due to rapid defoliation. Defoliation of the lower plant canopy is often the first obvious symptom of boxwood blight (Figure 2).
- Dark brown or black streak-like lesions appear on infected stems (Figure 3).
- Favored by warm, humid weather.
- Caused by the fungus Cylindrocladium buxicola.
- The pathogen can survive on plant debris in the soil for at least 6 years.
- The disease may be spread by splashing water, wind, tools, clothing, and wet hands. Long distance movement is reliant upon the transport of infected plants, infested soil, or contaminated equipment.
Avoid Introduction of Diseased Plants:
Careful selection of healthy plant material is the first step to prevent spread.
- Inspect nursery plants carefully; do not purchase unhealthy or symptomatic plant material.
- Discuss concerns with landscape contractors before new landscapes are installed.
- Nurseries and garden centers should communicate plant history with suppliers before receipt of new material. All shipments should be inspected before unloading.
Cautiously Introduce New Plants into Established Landscapes:
Protect established landscapes, especially if valuable boxwood are on site.
- Introduce only symptom-free plants into landscapes.
- If valuable or established boxwood already exist, avoid hasty introduction of new boxwood plants. Move new plants in slowly by setting up a transitional site or quarantined area that can serve as a holding area for three weeks. If plants remain vigorous and symptom-free, they are likely safe to introduce to landscapes.
If boxwood blight is suspected, contact your local Extension agent, who may submit a sample to the UK Plant Disease Diagnostic Lab for confirmation.
If boxwood blight is confirmed, the following management options are recommended:
- Diseased boxwoods in landscapes should be removed immediately to prevent spread.
- Replant landscapes with boxwoods that have disease tolerance. Some recommended cultivars are listed in Table 1.
- Diseased boxwoods grown for commercial sale should be destroyed and not sold.
Once boxwood blight has been diagnosed in the landscape or nursery, take these steps to prevent infections to nearby healthy boxwoods:
- Increase plant spacing and prune dense shrubs/trees within the landscape to allow for air movement, reduced humidity, and rapid leaf drying.
- Minimize overhead watering and sources of leaf wetness that can increase fungal spore production.
- Fungicides do not cure boxwood blight. Use of fungicides can help protect nearby healthy plants, but residual protection lasts only 7 to 14 days.
- Homeowners can utilize fungicides containing chlorothalonil to protect plants from infection or suppress disease development. Always follow label directions when utilizing fungicides.
- Commercial growers and retail centers should contact UK Extension Agents and/or Specialists for specific fungicide recommendations.
- Boxwood Blight (PPFS-OR-W-20)
- Homeowner’s Guide to Fungicides (PPFS-GEN-07)
- Landscape Sanitation (PPFS-GEN-04)
- Susceptibility of Commercial Boxwood Varieties to Cylindrocladium buxicola (North Carolina State University)
- Best Management Practices for Boxwood Blight (Virginia Cooperative Extension)
By Kimberly Leonberger, Extension Associate, and Nicole Ward Gauthier, Extension Plant Pathologist