Home & Garden

Boxwood blight of concern to Kentucky owners

Leaf drop would be a sign of possible boxwood blight.
Leaf drop would be a sign of possible boxwood blight. Photo by Susan Smith-Durisek

For as long as there have been gardens, folks have been planting and generally fussing with that wonderfully stinky shrub we call boxwood.

I have boxwood on the brain at the moment for a couple of compelling reasons. The next eight weeks form that optimum season of annual garden makeover and installation, and boxwood is such a useful plant in the enterprise.

Evergreen and finely textured, boxwood has an anchoring and soothing quality that makes it such a great foil for something more fleeting and restless, anything from tulips to hostas.

It is not necessary to frame each garden bed in low, clipped box to reap its rewards. A few strategically placed boxwood can give an understated structure and unity to an entire garden bed or area.

A boxwood is happiest in a little shade, especially away from the winter sun, which can cause that sickly bronzing. It is not a plant for heavy, wet clay.

I like the classic, billowing pungent English box — if it is well grown and not sheared into gumdrops and cubes — but still it is sickly. Breeders have produced varieties that approach the form and texture of English box but are far easier to keep looking good, and they grow fast enough to have a real presence after five years.

Green Beauty looks, almost, like a big old English boxwood, and after several growing seasons it will form a mound 3 to 4 feet high and wide. Green Gem is a more compact fine globe useful for low hedging. Justin Brouwers is another superior compact growing variety, perhaps a better choice than Green Gem in a hot, sunny location.

So much for the upside of boxwood.

About a year ago, a wholesale grower in North Carolina returned from a trip to find his whole field of boxwood dead. A plant pathologist established that a serious boxwood disease entrenched for 18 years in Europe, boxwood blight, had made its way to the United States.

Its appearance in other nurseries in other states and its potential effect on the horticulture industry created an instant stir.

It has not been reported in Kentucky, but it is present in Connecticut, Massachusetts, Maryland, New York, Oregon, Pennsylvania, Rhode Island, Virginia, Ontario and British Columbia as well as a single case in Ohio.

"The probability of it being in Kentucky is very high, but we haven't found it yet," said Nicole Ward, associate extension plant pathologist at the University of Kentucky, who added the case in Ohio was found in Cincinnati.

It's important for homeowners to be on the lookout for the blight, said Ward, and it's easy to spot.

"Look for leaf drop," said Ward. "Boxwoods will turn brown, but they normally don't drop leaves. Leaf drop is the key. When you see that, make a call."

Ward recommends calling a county extension agent as soon as you think you spot the fungus "to get a proper diagnosis so extension can tract it and then to help eradicate it."

According to the U.S. Agriculture Department, boxwood is the top broadleaf evergreen sold at retail nurseries by number and value, outselling azalea and holly.

Everyone was girding for a second year of blight this season, but the heat and dryness have suppressed the blight, caused by the fungus Cylindrocladium buxicola. The spores do not travel far but are spread through water and by pruning and handling.

Sharon Douglas of the Connecticut Agricultural Experiment Station in New Haven is part of a working group tasked with studying the disease and establishing practices to manage it for growers, nurseries, landscapers and homeowners.

This involves trying to keep infected plants out of the marketplace and adopting planting, watering and pruning methods that will check its spread.

"In a more normal year it's going to be present, and we are going to have to deal with it," said Joe Bischoff, director of government relations for the Washington-based American Nursery and Landscape Association. "But it is something that can be managed. We may have to be a little smarter about who we are buying it from, making sure the producers are doing the right things." For homeowners, he said, that means buying from reputable nurseries.

So, should you put in some boxwood this fall?

If you are installing boxwood in a garden without them, "I wouldn't have any qualms; we have seen a lot of clean material," Douglas said.

If you have boxwood, especially old and valuable English or American box, the dynamic changes. The experts advise isolating new boxwood plants for at least four weeks to see whether they develop the blight.

Fallen boxwood leaves should be raked and bagged — not composted — and pruning equipment should be sterilized between plants with alcohol or a bleach solution. In addition, two other landscape plants, the ground cover pachysandra and the evergreen shrub sweetbox, are known to harbor the disease.

Of course, you can lower the risk even more by not planting fresh boxwood, pachysandra or sweetbox, at least for a year or two. If you have a landscaping crew, make sure it follows the best management practices, outlined on the Web site of Douglas' agency, Ct.gov/caes.

Lynn Batdorf, curator of the national boxwood collection at the U.S. National Arboretum, said he first saw blight in a Philadelphia area garden in 2004, though his efforts to raise the alarm then went unheeded. He said he places new boxwood plants in quarantine for two to three years before adding them to the collection to make sure they are free of blight and other diseases.

The disease appears first as brown spots on leaves that join together until the foliage looks straw-colored. Stems develop black cankers.

A blighted plant will regrow leaves after its first set has dropped, but the act of regrowing seriously depletes and stresses the plant, and the new leaves are likely to become infected. In other words, a blighted plant is not long for this world.

As with minimizing the occurrence of other boxwood fungal diseases, Batdorf advises growing boxwood in an open site that gets good air movement and not shearing the foliage, which promotes dense growth.

Scientists at the Agriculture Department are trying to improve ways to diagnose the disease, even when dormant; look at the blight resistance of existing varieties; and, long term, to breed resistant varieties.

Others are trying to determine which fungicides are most effective against it.

"Last year it took us by surprise, but now most of the growers are aware of the problem," said Douglas. "I would like to say there's no one shipping diseased plants, but there's no guarantee."