Disease hitting boxwood shrubs in Oregon gardens

A disease that attacks boxwood shrubs could potentially become a serious problem in residential and public gardens in Western Oregon.

Boxwood blight, which first showed up in nurseries in Oregon in 2011, is making inroads in the Portland area, according to Jay W. Pscheidt, a plant pathology specialist with Oregon State University's Extension Service.

"The message to homeowners is that if they notice something unusual going on with their boxwood they've never seen before, it could be boxwood blight," Pscheidt said.

The fungus is active during warm, wet weather, splashing from plant to plant by rain. Even though this spring has been relatively dry, overhead watering causes the same conditions so the disease can spread like any other year. Using a soaker hose will help. The sticky fungal spores easily attach to clothing, pruners or other equipment and then transfer to other boxwoods, so avoid working with wet plants.

Boxwood blight (Calonectriapseudonaviculata) causes spots on leaves and stems and eventually defoliates and kills the plants. Stem lesions show up as wide black streaks; on leaves they appear as round spots.

There is no cure, but prevention measures can help keep the disease from spreading, he said. The most effective method is to inspect purchased plants and keep them in an area away from already planted boxwood for two weeks or more. If the shrubs don't show spots, they can be planted.

"That might be hard," Pscheidt said, "but the fungus can be present and the plant hasn't shown symptoms yet."

Boxwood blight emerged in Europe in the early 1990s and quickly spread. Eradication has been unsuccessful, he said. The fungus most likely hitched a ride on plants shipped from Europe to North America. By 2011, the disease had been found in nurseries and landscapes in several East Coast states, three nurseries in Oregon and four in British Columbia, Canada.

According to Pscheidt, wholesale nurseries in Oregon, which consider boxwood a bread and butter crop, have kept the disease mostly in check with a nursery cleanliness program administered through the Oregon Department of Agriculture. But the disease has jumped into some landscapes in Portland and Coos Bay. Now that it's made inroads into gardens, cooperation from homeowners is essential.

Kym Pokorny works with the Oregon State University Extension Service. Contact (541) 737-3380, kym.pokorny@oregonstate.edu, on Twitter @OregonStateExt.

Beat the blight

• Here are Pscheidt's recommendations for preventing the spread of boxwood blight:
• Inspect new plants and isolate them from other boxwoods for at least two weeks before planting.
• Avoid overhead watering. Instead, use a soaker hose.
• Don't work with wet boxwood.
• Disinfect pruners and hedge clippers after use.
• Never compost infected plants.
• If you have a diseased plant, place it in a plastic bag and dispose of it in the garbage bin rather than yard debris recycling bin.
• Buy less-susceptible cultivars. The fungus seems to prefer the most common boxwood known as American or English boxwood (Buxus sempervirens). Instead, look for B. microphylla 'Wintergreen,' B microphylla 'Franklin's Gem,' B. sempervirens 'North Star,' B. microphylla 'Wedding Ring' and B. harlandii 'Richard.'