Boxwood blight: A new disease to watch for

By Joan Allen - UConn Home & Garden Education Center
Featured Article - posted Tue., Apr. 9, 2013
Photo by Joan Allen. - Contributed Photo

Boxwood (Buxus sp.) includes some of our most popular landscape plants. They are generally easy to care for, can compliment both formal and natural landscape areas, have evergreen foliage for winter interest, and are available in a wide array of sizes and forms. While boxwood will do well in a variety of sites ranging from full sun to quite a bit of shade, occasionally they are affected by a number of pest and disease problems. There is a new boxwood disease in Connecticut that has the potential to devastate a boxwood planting in only one season if weather conditions are favorable and the fungal pathogen is introduced.

Boxwood blight was first confirmed in Connecticut in October of 2011 and has been found throughout the state. Diseased plants in the landscape so far have been associated with the introduction of new boxwood infected with the pathogen. This disease is important on boxwood in Europe, but the origin of the disease is not known.

The pathogen is the fungus Cylindrocladium pseudonaviculatum (synonym C. buxicola). Leaves and stems are affected, while roots are not. Foliar symptoms include tan to dark brown spots that may have darker borders. As the spots enlarge, zone lines may form, giving the spots a target-like appearance. As disease progresses, leaves turn straw-colored and fall from the plant. This can happen in a short period of time. Stem infections result in small dark lesions along the stem, and these can be found from the base of the plant to the tips of the shoots. English and American boxwood seem to be most susceptible, but all boxwood may become infected. Pachysandra has been confirmed as another susceptible host plant.

Spores of the fungus are produced on both leaf and stem lesions. On the leaves, white tufts (sporodochia) of spores will be produced on the undersides during moist conditions and spores are spread by wind or wind driven rain. Long-distance spread is likely via human activity such as movement of infected plants, spores that adhere to clothing or equipment, and animals and birds. The spores are sticky and will easily adhere to a surface they come into contact with. Warm, moist weather favors disease development. This fungus survives the winter and for five years or more on fallen leaves and in infected plants.

The best management strategies for this and other plant diseases are preventative. Once a plant is infected, it cannot be cured. When purchasing new boxwood, inspect the plants carefully for symptoms or sporulation. After bringing home your new plants, it is best to quarantine them for one to several months away from other boxwood before planting. Infection cannot occur under dry conditions. Promote lower humidity and dry leaves by allowing space between plants and avoiding overhead irrigation. Check boxwood for symptoms and signs (sporulation) on a regular basis. If you suspect boxwood blight, it should be confirmed as soon as possible to prevent spread to other plants.

If the disease is confirmed, sanitation is important to remove inoculum that will survive in fallen leaves and plant debris for as long as five years. Upon boxwood blight confirmation, follow the instructions from the state agriculture officials. Plants should not be composted.

If your boxwood become diseased and dies, replacement possibilities include less susceptible boxwood varieties but the best choice would be other alternatives that include dwarf cultivars of holly, inkberry, Pieris, Rhododendron, and Azalea.

Fungicide sprays may be used but in Europe they have not been very effective in areas where the disease has been present for some time. Sprays are used preventatively in combination with the other described management practices when weather is expected to be favorable for disease.

Thorough coverage of the plant is necessary. Products available to Connecticut home gardeners include those containing the active ingredients chlorothalonil and mancozeb. Look for products labeled for boxwood and read and follow all label instructions carefully.

Boxwood blight is a regulated disease in Connecticut. Please do not destroy or remove plants suspected of having the disease until it has been confirmed by a diagnostic laboratory. For information on diagnosis, contact the UConn Home & Garden Education Center (toll-free) at 877-486-6271.

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