ANNAPOLIS, MD (June 11, 2012) - Boxwoods have been an integral part of American landscapes since colonial times. They remain a very popular shrub in MidAtlantic gardens, prized for their evergreen habit and deer resistance. Last fall, however, a new threat to this mainstay of the formal garden was discovered for the first time in the US – the fungal disease called Boxwood blight, or Box blight. Since its initial discovery in Connecticut, North Carolina and Virginia in October, 2011, the disease has been found in several states in the Northeast, including in Prince George’s County, MD, and is our June “Invader of the Month”.
Boxwood blight is caused by the fungus Cylindrocladium pseudonaviculatum, also known as Calonectria pseudonaviculata. The disease has been attacking boxwoods since the 1990s in Europe. The first symptoms of the disease are leaf spots, which quickly expand to browning of entire leaves and leaf drop. Defoliation begins on the lower branches and inner canopy of the shrub, and progresses upward. A key symptom is the development of black or brown twig cankers, which look like dark streaks on the normally green stems. The fungus does not attack the roots, so infected plants will survive, but the defoliation can be so severe that the shrubs lose ornamental value.
Spores of the fungus are produced in small, white clusters on infected leaves and stems. Spores are spread by splashing water in rainfall or overhead irrigation, as well as by human activities such as pruning. Long distance spread occurs through movement of infected nursery stock and vehicles that may contain fallen infected leaves. The fungus produces tiny survival structures called microsclerotia in infected leaves that enable the pathogen to survive over the winter.
All boxwoods (Buxus species) are susceptible to boxwood blight, although some variation in disease severity among cultivars has been reported. In addition, Sarcococca (sweet box) and Japanese pachysandra (Pachysandra terminalis) have been infected under laboratory conditions.
There are other boxwood diseases that might be mistaken for Boxwood Blight. Volutella blight is a common fungal disease that causes bleaching of individual twigs and foliage, and is often associated with cold injury. The Volutella blight fungus, Volutella buxi, produces orange or pink spore structures on infected twigs and foliage, different than the white spore structures of Cylindrocladium. We have seen mixed infections of Volutella and Cylindrocladium on the same plant. Macrophoma leaf spot, caused by Macrophoma candollei, is another disease of declining foliage, but the spore structures of the pathogen look like black dots on otherwise tan leaves. Root diseases such as Phytophthora crown and root rot can cause dieback in boxwood, which might be mistaken for boxwood blight as well.
Research is currently underway to develop management recommendations for this new disease. At present, removal of infected plants and plant debris are recommended. Fungicide trials are being conducted to determine which products may be effective. Because this disease is so new to our area, the extent of the problem is not yet known. We encourage anyone who has planted boxwoods within the past 2 years to look for symptoms of boxwood blight and contact the Home and Garden Information Center (1-800-342-2507, M-F, 8AM-1PM) if boxwood blight is suspected.
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For more information about Invasive Species of Concern in Maryland, visit www.mdinvasivesp.org