Shrubs may be trembling by doorsteps across North America as an aggressive fungus disease of boxwood invades the continent.
Boxwood blight, caused by a Cylindrocladium fungus, was unknown to science before 2000 but has now spread through Europe and New Zealand. In October, U.S. authorities confirmed that the blight had jumped continents, with infections confirmed in North Carolina and Connecticut. By mid-January, with growers and pathologists on alert, the fungus had turned up in at least five more states — Virginia, Maryland, Rhode Island, Massachusetts and Oregon — and British Columbia.
The blight starts with spots on leaves and black streaks on twigs. Within a few weeks, a plump shrub can turn into a clump of bare sticks.
“I’ve never poured diesel fuel on a boxwood, but if I did, that’s what it would look like,” says Lynn R. Batdorf, curator of the as-yet-uninfected boxwood collection at the National Arboretum in Washington, D.C.
Younger plants die. Older plants do survive and regrow their leaves for at least several cycles of attack by the fungus, but “it ruins completely your topiary or your hedge,” says plant pathologist Béatrice Henricot of the Royal Horticultural Society’s Garden Wisley in England.
Once the fungus, referred to as either C. pseudonaviculatum or C. buxicola, strikes a garden, pathologists recommend drastic measures to fight long-lasting fungal residue. In North Carolina, one grower burned 15,000 infected boxwood plants and 15,000 uninfected ones, burying the ashes and going out of business, says plant pathologist Kelly Ivors, based at North Carolina State University’s Mills River lab.
Just where this blight fungus originated is a puzzle. Since boxwood plants haven’t evolved much resistance to the fungus, it may come from an unfamiliar enemy outside the plants’ native range, speculates plant pathologist Sharon M. Douglas of the Connecticut Agricultural Experiment Station in New Haven.
The fungus is particularly difficult to trace because all samples found so far belong to just two lineages of clones, adds her experiment station colleague Robert E. Marra. It’s hard match any particular outbreak with the region it came from because fungi in so many places are virtually identical.
Efforts to learn more about the fungus by coaxing it to reproduce sexually have failed. Like many fungi, it has two sets of mating-related genes, and needs to find a partner that differs sufficiently across both sets. “It may be that this fungus is so successful it doesn’t need sex,” says Pedro Crous, director of the Fungal Biodiversity Centre in Utrecht, the Netherlands.
Despite the fungus’ success, fans may still find reasons to keep growing boxwood: low appeal to nibbling deer, a possible life span of more than 300 years, the cinnamon smell of its flowers, the leaf pungency that Oliver Wendell Holmes called “the fragrance of eternity.” (Batdorf does acknowledge that “other people with a different preference might compare it to cat urine.”)
For unshakeable enthusiasts, he points out that even after years of the blight, boxwood still grows in European gardens.
