Boxwoods are a useful and popular element of many Virginia gardens, but boxwood blight, a serious fungal disease first discovered in the United States in 2011, poses a threat to Virginia’s boxwoods.
Mary Ann Hansen, manager of the VT Plant Disease Clinic, recently visited the Extension Master Gardener State Office to share information on boxwood blight and offer suggestions for controlling the spread of this serious disease in Virginia.
“Boxwood blight has been present in Europe for many years, but it was first discovered in the United States (and Virginia) in 2011. Many Virginia counties have had positive identifications of boxwood blight since then,” says Hansen.
“The earliest symptom of boxwood blight you might notice on your plants is leaf spotting. You’ll see circular or irregular leaf spots with a dark border and a light colored or brown center,” says Hansen. “You’ll also see black stripes on stems, which are very diagnostic.”
Boxwood foliage affected by boxwood blight. Image credit: Mary Ann Hansen and Boxwood Blight Taskforce
However, the first symptom many people notice is severe defoliation of their boxwoods, with leaves dropping off the bottom parts of the plant in particular. “It’s usually pretty sudden and severe, and that’s when people know they’ve got a problem.” says Hansen.
Boxwood blight is caused by the fungus Calonectria pseudonaviculata, which affects all members of the boxwood plant family, including Boxwood, Japanese spurge, Allegheny spurge, and sweetbox. The fungus produces structures called microsclerotia, which can survive on dead leaves for a long time, making boxwood blight easy to spread through diseased leaves.
Unlike many other fungal diseases, boxwood blight is not spread through the wind. The most common way boxwood blight has spread so far is through introduction of diseased plants, but it can also be spread by blowing leaves or animals or humans brushing up against diseased plants.
If left untreated, boxwood blight will eventually kill an infected plant.
According to Hansen, controlling the disease requires a multi pronged approach which includes:
- Avoid introducing the disease on infected plants or plant matter
- Buy plants from a reputable source (e.g. you can ask if the grower is in the Boxwood Blight Cleanliness Program)
- Consider planting resistant varieties (for information on resistant varieties, visit: http://plantpathology.ces.ncsu.edu/pp-ornamentals/; Note that while resistant boxwood cultivars are available, these plants can still spread the disease to the non-resistant boxwoods you may have planted)
- Sanitize tools, hoses, vehicles, equipment between blocks of plants or between properties (this is particularly important for landscapers who may travel between properties)
- Sanitize by dipping into 70% ethanol or Lysol Disinfectant Spray Brand III (wiping off tools is not effective)
- Make sure landscapers you hire have a plan for preventing introduction of boxwood blight to your property
Fungicides can be used to protect plants from infection if applied preventatively, however they will not eradicate the disease from plants or soil, and this option can be expensive and time consuming.
Hansen also notes that other diseases are easy to confuse with boxwood blight, including volutella blight. If you believe that you may have identified boxwood blight, you can submit a sample to the plant disease clinic through your local Virginia Cooperative Extension Office. Tips on collecting samples for the VT Plant Disease Clinic are below.
Tips on submitting potential boxwood blight samples to the VT Plant Disease Clinic:
- Collect symptomatic branch samples with some leaves still attached (early in symptom development)
- Submit soil and root samples so that we can check for other common boxwood diseases
- Double bag in sealed plastic bags and submit through your local VCE office to the VT Plant Disease Clinic
This article is based on Mary Ann Hansen’s March 8, 2018 Extension Master Gardener Webinar, a recording of which is available for current Virginia Master Gardeners here: https://blogs.lt.vt.edu/mastergardener/current-master-gardeners/sample-page/webinars/