The Hortiholic loves to write about the many fun, positive aspects of horticulture. I like to share cool new plants or things I've learned over decades to help other gardeners become more successful. Every once in a while something comes along that isn't so much fun, but deserves attention. Boxwood blight (caused by the fungus Calonectria) falls into that category. It's important to understand this isn't the end of using boxwood, but another manageable disease.
Backstory for those that haven't heard the latest? Boxwood blight may have been in the U.S. for a few years before it was confirmed in 2011. It has now spread to 22 states. Illinois joined that less-than-elite group with two confirmed cases in northeastern Illinois earlier this year. The fungus starts with brown/black round spots (first image above) that generally run together darkening the entire leaf (right). Rather quickly the plant will defoliate leaving bare stems. These stems will show elongated black streaks on the bark (below).
We need to show restraint in jumping to the conclusion that any boxwood malady is the bad disease (cue the spooky Halloween music, heavy on the organ). There's confusion as there is another irritating, but not fatal, disease call Boxwood blight (caused by Volutella) that's been around for years. So, we need to be careful when we use the term "Boxwood blight". Volutella does not exhibit the black stem streaks or defoliation of the plant.
There's also an insect, Boxwood leafminer, that can cause browning of leaves (that could be confused with Calonectria). Leafminer is easily controllable.
As an average homeowner with even a few plants, what can you do to protect your boxwood?
Like life, education, not hysteria, is the key to success. Learn the symptoms of Boxwood: leafminer and the two very different blights.
If you have an irrigation system consider reducing the frequency of watering zones inhabited by box. This blight is spread not by insects or wind (this is great news, people), but by splashing water on an infected plant. Established box isn't water needy and doesn't require "dampening" for 5-10 minutes in the middle of the night three times a week. You don't want to get the Hortiholic started on the subject of improper use of irrigation systems. How much better to water deeply, but infrequently. 'Nough said?
Mulch to reduce splash. Early indications are that mulch may reduce the splash of spores onto lower leaves, thus reducing the likelihood of infection.
Don't prune or work in and around your boxwood early in the morning when there's dew on the leaves or anytime the foliage is wet, especially in humid weather. Again, this is a fungus that requires moisture to infect its host.
Can you live with a less formally pruned geometric shape? A "looser" plant that's not so tightly sheared will allow better air circulation, drying the foliage more quickly, reducing the opportunity for the blight to infect.
If your boxwood has symptoms wait until the plant is dry, cut a sample that includes a significant portion of stem (not just a few leaves) and place it in a double plastic bag. Hygiene is a good thing. Seal completely and bring it into Plant Health Care. A picture or two showing the overall appearance of the plant is an extremely helpful addition to the physical sample. Again, at this point chances are overwhelmingly against your plants being infected with Calonectria.
When adding boxwood to your landscape be sure to deal with reputable nurseries and garden centers that know plants and adhere to established boxwood cleanliness programs. Chalet, for example, has separate written protocols for handling boxwood for our: retail store, landscape division and growing nursery in Wisconsin.
Like people, all plants can have "issues". This newest Boxwood blight should be considered a manageable disease. After all, try to name even one substitute plant that does everything boxwood can do. There are a lot of reasons it's worth using.