Garden Detective: Beware of boxwood blight

Boxwood blight has hit North America, and experts fear it will ravage plants on Long Island this year.

The disease, caused by the fungus Cylindrocladium pseudonaviculatum, is aggressive, spreads quickly and doesn't respond to fungicides.

Boxwoods are extremely popular broadleaf evergreen shrubs that can be found all over Long Island, most commonly as hedges. There are many varieties, some of which grow only a foot tall and retain a globelike shape, others that are larger and have a more free-form habit, and some that are trained into topiary forms. They're popular in both formal and informal gardens and thrive in full sun to part shade. Chances are you have some growing in your yard.

The first symptoms of the fungus are the appearance of stem lesions and small, light-brown, dark-rimmed spots on leaves. The spots grow quickly and can eventually take on a bull's-eye pattern before leaves turn straw-colored and drop off the plant, leaving stems bare, which is when the stem lesions become visible.

This pathogen thrives in damp, warm weather, so if conditions are right and the pathogen is present, the spores can quickly multiply and be spread from one plant to many others by wind or rain.

What's more, the fungus can survive winter and reinfect plants the following year -- up to five years.

The blight was recognized in Europe about 10 to 20 years ago and was first identified in North America last autumn, when it was confirmed in North Carolina and Connecticut. By this past January, there had been sightings in New York State, Massachusetts, Maryland, Oregon, Rhode Island, Virginia and British Columbia.

So, what to do? Absolutely nothing will cure this disease once it has taken hold. Fungicides appear to be ineffective, so prevention is the best and only cure. And by prevention, I mean you need to prevent bringing home infected plants.

If you're set on buying boxwoods this year, check the undersides of leaves, where spores form, for the appearance of a white powdery or sticky substance. Avoid those completely, point them out to the nursery's management and wash your hands before handling other plants.

At home, allow proper spacing between plants to avoid crowding, which encourages warmth and moisture within and between plants, creating the perfect breeding ground for the fungus. And don't rely on automatic sprinkler systems, which soak the leaves. Always direct water at the soil instead. A soaker hose is the right tool for the job.

Margery Daughtrey, a plant pathologist with Cornell Cooperative Extension of Suffolk County, said it is important to be able to distinguish disease from normal seasonal changes in boxwoods: Leaves that are merely discolored (orange or bronze) do not indicate blight (that's normal during winter). Branches with dead tan leaves still clinging to them are not signs of blight, either, she stressed. What is typical of the new boxwood blight is the "dead, naked twig look" visible on the photo on this page.

Cornell officials warn that this particular blight can be impossible to distinguish from other diseases that affect boxwoods, like Volutella blight and Macrophoma blight, and they are urging Long Islanders who see any signs of the disease to bring or send a sample for testing to the Suffolk County Diagnostic Lab, 423 Griffing Ave., Riverhead, NY 11901 or call 631-727-4126.