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Boxwood blight arrives, but not in Marin — yet

  • Doris Gates
  • There’s a new bad guy in the Bay Area, and its name is boxwood blight. It’s a fungus that infects plants in the boxwood family, and it can do serious damage.

    All species of Buxus, one part of the family, seem to be susceptible, some more than others. Two popular varieties, American or “common” boxwood (Buxus sempervirens) and English boxwood (Buxus sempervirens ‘suffruticosa’), appear especially vulnerable. Other plants in the family, such as sweet box (Sarcococca spp.) and some spurges (Pachysandra spp.), may be hosts.

    Boxwood blight only recently arrived in California, where it has appeared in San Mateo and Santa Clara counties. To date, it has not been seen in Marin, but it is probably only a matter of time. The disease was first identified in England in the 1990s and is now found throughout Europe. It was detected in the United States in 2011 and has since spread to more than 20 states across the country.

    The fungal pathogen, Calonectria pseudonaviculata, causes boxwood blight. It thrives in mild, damp weather, especially in temperatures from 64 to 80 degrees. Spores are spread by wind and rain, and they may be carried by birds, animals, shoes, clothing, and gardening tools. Wet leaves or high humidity are required for infection.

    The disease does its damage quickly, sometimes in less than a week. First signs include circular brown spots on leaves and distinctive black streaks, or lesions, on stems. Fungus spores, either white or salmon-colored, may appear on the underside of leaves, particularly in humid weather.

    Leaves turn tan or brown, plants rapidly defoliate and dieback occurs. Roots remain healthy, but the weakened plant usually dies, sometimes the result of secondary causes. At present, there is no known cure.

    An invasion of boxwood blight can be devastating to homeowners because boxwood is widely used in home gardens for both hedges and decorative elements. A mature hedge may be relied upon for privacy or a windbreak, and replacing one can take time and money.

    So what is a gardener to do? First, although there are no known cases yet in Marin, it pays to be alert for symptoms. If you see something suspicious, bring a cutting (in a plastic bag) to the UCCE Marin Master Gardener help desk in Novato. We can assist with a diagnosis.

    Second, if you are planning new plantings, it could be wise to avoid boxwood for now. Over time, disease-resistant strains and new control techniques may be developed. If you do purchase new plants in the family, it is recommended that you isolate them for a least a month before putting them into your garden.

    Third, if or when boxwood blight hits closer to home, or if you want to be proactive, there are some defenses. Avoid overhead watering, which can help spread the disease, and don’t work with boxwood under wet conditions. Practice good sanitation, remove fallen debris, and disinfect pruning equipment frequently. A layer of mulch can help to prevent fungus spores from settling into the soil.

    Fourth, a word about fungicides. Some horticulturists believe that fungicides may be a preventative measure. Others think they suppress symptoms but don’t prevent spread of the disease. Fungicides are not a cure for diseased plants, and whether they are effective in prevention is still under study.

    Last, if a diagnosis of boxwood blight is verified, all affected plants should be removed immediately to prevent spreading. Infected material and fallen leaves should be bagged and sent to the landfill, because home composting alone is not likely to kill the disease.

    Any spores left behind can survive in leaf debris and soil for many years. So a good cleanup can mean a lot to the long-term health of your garden.