Invasive Species: Boxwood Blight

Boxwood Blight

(Calonectria pseudonaviculata)
*Detected in Michigan*

Boxwood blight leaf lesions on boxwood (MDARD).

Characteristic black stem lesions on boxwood (MDARD)

Excessive leaf drop from infected boxwoods that have been removed.  The surrounding plants show signs of boxwood blight (MDARD)

Why we care: Boxwood blight is a serious fungal disease that attacks several genera in the plant family Buxaceae; this includes boxwood (Buxus spp.), a popular landscape shrub along with Japanese spurge (Pachysandra terminalis), Allegheny spurge (Pachysandra procumbens) and sweet box (Sarcococca spp.). The disease can quickly defoliate boxwood plants under favorable environmental conditions in nursery production and in the landscape. Fungicides can be effective in protecting healthy plants from boxwood blight infection, but they will not cure plants that are already infected.

What is at risk? Boxwood blight is a devastating disease that has caused significant losses to homeowners and the nursery industry in states that already have the disease.  In Connecticut, the first year after detection, boxwood losses in field-grown and container nurseries exceeded $3 million. There could be a similar impact in Michigan should it become widespread. Boxwoods are one of the most popular woody plants sold in the U.S., and Michigan has the fourth largest horticulture industry in the nation.

The threat: Boxwood blight was first detected in Michigan in three separate incidences in 2018: a landscape firm, a homeowner's yard, and in holiday wreaths sold at retail stores. This fungus can spread via the transport of infected plants that may not show any symptoms at the time of shipment. Nurseries are aware of this and many participate in the Boxwood Blight Cleanliness Program that works to minimize the chance that infected plants are distributed. It may also spread on infected plant parts such as boxwood foliage in holiday greenery.   

The fungus that causes boxwood blight produces sticky spores on infected plant material and can be moved around by animals, people or splashing water.  The disease also can be spread by contaminated pruning equipment.  Infected plant debris is also a problem, because spores on the ground can remain viable for up to five years.

Pachysandra (Pachysandra terminalis), a common groundcover plant, is also a host of boxwood blight. Boxwood blight causes leaf spots on Pachysandra but does not typically cause defoliation or death of the plants. As a host, though, infected Pachysandra could serve as a reservoir for the disease in landscape settings. 

Sweet box (Sarcococca sp.), another ornamental in the boxwood family (Buxaceae), has only shown to cause disease under experimental conditions.

What could happen in Michigan? Michigan has the fourth largest horticultural industry in the US, and boxwood blight could make growing and landscaping with boxwoods challenging. As a direct result of the disease, Michigan could lose popular, highly susceptible varieties of boxwood in both the nursery and landscape industries.

What can you do?  

If you have boxwoods:

• Monitor boxwoods, pachysandra and sweet box for the disease regularly.
• Avoid overhead watering.  Use drip irrigation or water plants at the roots.
• Prune plants when conditions are dry and sanitize pruning equipment between plants or sections of plants.  Rake and remove leaf debris.
• Do not use boxwood holiday decorations near boxwoods in your landscape.
• If boxwood blight is confirmed, remove and destroy infected plants and any plant debris by double bagging the diseased material and disposing of it in a landfill or burning it on site.

• Consider alternate species, such as inkberry holly, Japanese holly, dwarf spruce, globe arborvitae, yews, or azaleas.
• Purchase boxwood plants from reputable nurseries that participate in a Boxwood Blight Cleanliness Program. 
• Buy less susceptible varieties.  Refer to this list developed by North Carolina State University.
• Isolate new boxwood plants from existing plantings for at least a month to watch for symptoms of the disease.
• Space plants to allow for good air circulation.

MORE INFORMATION:

Educational Materials:

MSUE Fact Sheet: Boxwood Blight Disease Identified in Michigan - printable PDF

Purdue Extension: Boxwood Blight

University of Illinois Extension: Boxwood Blight Look-alikes

Boxwood Blight Management Options:

The Best Management Practices: Boxwood Health for Production and Landscape Management

Send suspect samples from Michigan to MSU's Diagnostic Services Laboratory

Virginia Cooperative Extension: Boxwood Blight

Michigan Firms Enrolled in the Boxwood Cleanliness Program - printable PDF