What is the latest news on the boxwood blight?

What measures prevent this blight?


Answer

Boxwood blight was first detected in the United States in 2011 and is gradually spreading across the country. Connecticut was among the states to first report a heavy incidence of the disease and our area continues to be strongly affected. As of 2020, it is confirmed in 25 states and entire public gardens have been devastated.

Boxwood blight, an aggressive fungal disease caused by Calonectria pseudonaviculata, can be identified in your plants by the appearance of dark spots on the leaves in the spring or fall, following a period of rain. The pathogen is most active at around 75º F. and then becomes inactive at temperatures above 82º. The oily, leaf spots enlarge and merge, leaves turn brown or tan, followed by rapid leaf fall. Young branches may exhibit black lesions. The leaf spots and bark streaks differentiate this blight from other common boxwood diseases. Once blight is detected on your plant it cannot be cured, merely contained.

The same fungal pathogen may cause disease in other Buxaceae family plants, specifically in the genera Pachysandra and Sarcacocca. Recent trials by the American Boxwood Society have indicated that C. pseudonaviculata may also be hosted by familiar garden plants as diverse as Alchemilla molis, Arctostaphylos uva-ursi, Brunnera macrophylla, Epimedium youngianum, Galium odoratum, Geranium sanguinium, Phlox subulata, Tiarella, cordifolia, Callirhoe involucrata, Ibiris sempervirens, Mazus reptans and Vinca minor.

Federal and State Agencies and horticultural research organizations have reacted with measures to intervene in the spread of this disease. A code of best practices has been developed and is in use amongst some growers; a strategy to avoid the widespread use of fungicides is being encouraged.

The spores of this disease are sticky and, while not as easily spread by splashing water as many fungal diseases,  adhere easily to garden tools, animals, clothing and garden debris. The most common sources of infection in a garden are through the purchase of an infected plant or the use of contaminated tools, though pets and wild animals can also spread it from plant to plant. What is the home gardener to do to protect their plant investment and contain the disease on their property? The American Boxwood Society recommends the following steps.

Buy carefully

Inspect plants before purchasing, always from a reputable seller. Ask the seller if fungicide was recently applied, as it can mask symptoms. Ask about the sourcing of the plants and if the grower is part of the Boxwood Blight Cleanliness Program. Make sure that you discuss the growing habit of available boxwood species with your nursery before making your selection as leaf size, branch structure, height and width can vary considerably. Choose a cultivar that is considered less disease prone.

Keep your tools clean

Ethanol, bleach and Lysol™ are all effective in killing the fungal pathogen. Sanitize your tools and ask landscaping workers to use clean tools, free of soil and plant material from other properties, when working on boxwood in your yard. Boxwood pruning, thinning and shaping should be done at a time when conditions for transmission are at a low - a sunny and dry day when the leaves are not wet. More stringent controls (change of clothes, washing of vehicles) may be necessary in areas of known blight.

Remove affected plant material

First, confirm that your plant has boxwood blight through a lab test by your county's cooperative extension office. If the diagnosis is confirmed, you must remove affected plant materials. Cornell University Cooperative Extension recommends that you take a strong course of action and  remove all plant material as well as the top 1/2 inch of soil and replace with unrelated plant genera.  Merely removing branches of the affected plant is far less effective. Incinerate the diseased plant materials. Cover the soil with mulch after plant removal as a containment method for fungal spores remaining in the soil.

Fungicides should be used only as a last resort, according to directions and in concert with physical control measures, in areas of extensively damaged plants. This is a preventative, not a curative, measure to prevent spread of blight. Virginia Cooperative Extension Boxwood Blight Task Force, a leader in research and best practices, provides this list of fungicides for home grower's use. Your county's cooperative extension office can tell you what fungicides are licensed for use and effective in your area.

Not all boxwood problems are blight! Most are cultural and boxwood can be suffering from multiple problems simultaneously. For more information for selecting disease-resistant cultivars, keeping your boxwood strong and healthy and identifying problems, please refer to our Boxwood Guide.

Courtesy of NYBG Plant Information Service

  • Last Updated Feb 07, 2021
  • Views 500
  • Answered By Leslie Coleman

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