by Sally Colby
According to Tracey Olsen, plant pathologist with the Pennsylvania Department of Agriculture, the best way to control a plant disease is to never allow it to enter your operation. That’s the case with boxwood blight, or BWB, a recently discovered plant disease that’s affecting boxwood and related species.
“Boxwood isn’t native to the United States,” said Olsen, “but they’ve been here for quite a while. They’re typically planted en masse to form a hedge or as foundation plants. What people desire is tightly compacted plants, which is great if you’re the fungus Calonectria pseudonaviculata – the fungus that causes boxwood blight, or BWB.”
Olsen said that BWB is a relatively new disease; first recognized in the mid 1990s in the U.K. It was found in New Zealand in 2002, and is now widespread in Europe. In the United States, the first detection was in October 2011 in both North Carolina and Connecticut. Boxwood blight has now been confirmed in 16 states including Alabama, Connecticut, Delaware, Georgia, Kentucky, Maryland, Massachusetts, North Carolina, New Jersey, New York, Ohio, Oregon, Pennsylvania, Tennessee, Rhode Island and Virginia.
It’s unclear where the fungus originated in the world, or how or when the pathogen entered the United States. The fungus is specific to in the buxaceae family, which includes all of the boxwoods, sweet box, and pachysandra. Olsen said that BWB is most frequently seen on dwarfing English boxwoods, which are extremely compact and tight with a soft leaf.
The disease process occurs through spore transfer from diseased plants. Infective spores on plant leaves are splashed off by rain, overhead irrigation or physical contact; and the sticky nature of the spores causes them to transfer readily to another plant. Microsclerocia that form in infected tissue can survive for many years. “Leaf wetness is required,” said Olsen. “If you’re a nurseryman with overhead watering; you’re creating ideal conditions.”
BWB symptoms are distinct and fairly easy to recognize. “Spores are formed, usually in July,” said Olsen. “They’re disseminated onto a healthy leaf, penetrate the leaf, and a circular lesion forms. It also forms stem lesions. Once the leaf is completely blighted, it falls off down into the soil, or it may get hung up in the stems.”
Toward the end of summer and into fall when temperatures are cooler, the fungus becomes dormant. Plants are stressed and look bad, and there’s the potential for other diseases to move in and cause more problems. “The majority of the problems are seen from July through October,” said Olsen. “From October through June, plants will look like they had a problem at some other time.” Olsen added that if plants are placed in a hoop house for the winter, there’s opportunity for the fungus to revive under artificial heating in January or February.
Olsen said that one disease that may be mistaken for boxwood blight is Volutella stem blight. “The difference is the color of the sporodochia,” said Olsen, adding that they’re a characteristic salmon pink. “The sporodochia form on the underside of the leaf and stem, and the diagnostic or characteristic symptom for differentiating boxwood blight from Volutella is that leaves stay attached with Volutella. They dry up but stay attached to the stem.”
Tim Abbey, Penn State Extension horticulture educator, said that managing an outbreak of BWB requires significant commitment on the part of the grower. Abbey said detailed eradication plans are available on line, but outlines the basic steps for growers.
Abbey said eradication at the nursery level requires training and awareness among all employees. “All of the debris has to be treated just like the infected plant,” he said. “If the plants are being grown on soil or stones, leaves with spores have dropped down and it’s much more complicated to eradicate.”
Personnel should be trained to minimize movement of spores within the nursery, and understand that spores are sticky and easy to transmit. Although spores don’t travel far, they can potentially travel within the nursery on clothing, equipment and even animals.
Remove fallen leaves and plant debris from boxwood species and contain the material in case it’s infected. Infected plant material should be destroyed through controlled burning or burying. Composting is not recommended. Plants and/or plant material should be bagged and fully contained prior to being moved. If possible, avoid carrying plant material over from one year to the next.
If you have a nursery that’s free of BWB, how can it be kept out? Start with designated delivery area and a dedicated quarantine area for holding incoming boxwood species. “If there is an issue later, it’ll be an easier cleanup,” said Abbey. “Material will stay in quarantine for a minimum of 30 days. If you’re bringing material in in spring, sometimes plants aren’t symptomatic, especially with a new infection, so if any (potentially infected) plants are left over in mid to late summer, you’ll see symptoms then.”
Whether or not plants are infected, take precautions with watering to prevent potential transmission of spores. Abbey said that overhead irrigation can splash spores, so use drip irrigation if possible. Minimize runoff of irrigation water to prevent movement of spores in water. “Stop doing anything that’s going to move those spores around or cause them to splash up onto susceptible foliage,” he said.
Trained staff should scout all potential host plants for signs of disease. Conduct regular inspections of new plant material that’s in quarantine, and continue to inspect material throughout the summer and fall. Remember that spores are easy to transmit, so tools used on boxwood or other susceptible species should be disinfected frequently, especially between different groups or field blocks of plants. Avoid working with wet plants, and wash footwear between fields. Ideally, anyone working with boxwood species should wear clean, disposable foot covers and disposable suits. Detailed recommendations for appropriate sanitation measures for tools and clothing can be found on various states’ boxwood blight websites.
Boxwood greens are often used in Christmas decorations, so take appropriate precautions if you are bringing boxwood into your facility from outside sources. Keep cut boxwood separate from live nursery stock, and instruct workers on proper sanitation to avoid potential contamination of live nursery material.
Abbey suggests nurseries that have never had a case of BWB should request a phytosanitary certificate or a copy of Boxwood Blight Cleanliness Agreement. Growers should report any suspected cases of BWB to state diagnostic labs for accurate diagnosis.
Dealing with boxwood blight
by Sally Colby