Hello fellow readers, I recently learned that Boxwood Blight might be coming to town from Patrick LePore, an associate from Bartlett Tree Experts, who alerted me that it’s arrived in Central New Jersey. He was on his way to our client’s Bob and Kathy of Piscataway to assess their boxwood, and thankfully, there’s no evidence of it there.
How to diagnose Boxwood Blight
To diagnose Boxwood Blight, Patrick inspects plants for small black leaf lesions or cankers on stems that look like staining or black streaking. “Symptoms don’t show until mid-to-late August, and once they are visible, it’s too late,” he explains. There is no cure for plants once they become infected, and the fungus can persist in leaf droppings, mulch, and soil for five to six years. “Prevention is key,” advises Patrick, who sites Virginia Tech as the most up to date in spearheading research to remedy Boxwood Blight.
Boxwood Blight is caused by a fungus, Calonectria pseudonaviculata, and has been impacting plants in the UK for over twenty years. It arrived in the US in 2011 and has spread to over twenty-three states. It’s also called Shoot Blight as water and wind propel spores, thereby spreading quickly, making groupings of boxwood highly susceptible. Unlike other fungal boxwood diseases such as volutella where leaves stay on the plant, Boxwood Blight defoliates the plant in as rapidly as one week from infection while the branches remain green. Per the USDA’s Diagnostic Fact Sheet, it’s been found in some cultivars of three species of Buxus, specifically Common or European Boxwood (B. sempervirens), Japanese Boxwood (B. microphylla), and Korean Boxwood (B. sinica var. insularis). Pachysandra and Sweetbox (Sarcococca) can be hosts for the spores.
How to prevent Boxwood Blight
Patrick advises homeowners not to sheer boxwoods, never ideal for plants, and talk to their landscapers about the sanitation of equipment, including wheelbarrows, to avoid spreading the sticky spores from house to house. The thing is, it can be transported on clothing and boots as well. Expecting workers to change their clothing, let alone sanitize equipment, is unlikely– another sound case for one of my biggest pet peeves – sheering plants into meatballs.
Be sure not to overwater boxwood, as they prefer well-drained soil on the dry side and avoid overhead irrigation. Also, be sure not to over-mulch or allow mulch to touch lower branches or trunks.
If you are suspicious you may have a diseased plant, send a photograph to your local extension office. Quarantine the infected plant by removing it with a garbage bag, vacuum the leaf debris and mulch below, and toss it into the trash – not the compost pile as Boxwood Blight is not a fun guy and will readily spread. Garden Dilemmas? AskMaryStone@gmail.com
Click through to another dilemma Boxwood Leaf Miner
Cultivars most resistant to Boxwood Blight
Virginia Tech offers a PDF of Publication that includes a listing of boxwood cultivars most resistant and least resistant to Boxwood Blight. Of those listed, boxwood I often use in landscape designs that are moderately resistant boxwood includes:
B. microphylla’ Winter Gem’, B. sempervirens ‘Fastigiata,’ and B.’ Green Gem.’ And, the most familiar of those boxwoods considered most resistant are B. sinica var. insularis ‘Nana’ and B. microphylla var. japonica ‘Green Beauty.