A new study shows some varieties are exhibiting tolerance to the plant pathogen.
A recent report from the North Carolina State University Cooperative Extension showed considerable variability in the susceptibility of boxwoods (Buxus spp.) to boxwood blight. This fungus is often referred to as Cylindrocladium buxicola, the name it was given in the U.K., where it was first observed. It is also sometimes referred to as C. pseudonaviculatum.
Boxwood blight is only recently introduced to North America. The first confirmed finding was in October 2011. The disease can cause significant leaf drop and stem lesions creating bare and brown patches. It has been discovered in 10 states (Conn., Ohio, Ore., Mass., Md., N.C., N.Y., R.I., Pa. and Va.) and three Canadian provinces (BC, ON, QC).
The researchers evaluated 23 varieties of boxwood at the Mountain Horticultural Crops Research and Extension Center in Mills River, N.C., during summer 2012. They trialed several species of boxwood grown as ornamentals: B. microphylla, B. sempervirens and B. sineca. The sempervirens types tended to be more susceptible to blight, said Dr. Kelly Ivors, associate professor and extension specialist with the Department of Plant Pathology at N.C. State University, one of the researchers involved in the study.
“Essentially, the main point we found was the two most common boxwood varieties grown in U.S. are most susceptible to the disease,” Ivors said. “We almost have a monoculture of these two types that helps the fungus get around. When we looked at microphylla, those types tend to be more tolerant.”
B. sempervirens ‘Suffruticosa’ was found to be the most susceptible to boxwood blight. Unfortunately, Suffruticosa (common boxwood or English boxwood) is the mostly commonly planted boxwood variety in the U.S. However, nine of the 23 varieties trialed were determined to be “moderately tolerant” to “tolerant.” The most tolerant is B. microphylla var. japonica ‘Green Beauty.’
However, not all sempervirens varieties were classified susceptible or highly susceptible. One takeaway from the study is that plant canopy is very important.
“A couple of the sempervirens species that are tall and upright, like the ‘Dee Runk’ are becoming more popular,” Ivors said. “They tend to be more tolerant because they are upright and tall. They have more airspace in their canopy. The resistance is dependent on genetics and the density of the canopy.”
The results of the research lead Ivors to anticipate a change in the varieties of boxwood that will be propagated.
“Homeowners like English and American boxwood, but they are harder to grow commercially because they don’t have a lot of disease resistance,” she said. “In another 10-20 years, I think they will become like an heirloom variety.
“As we see boxwood blight established in landscapes, growing and field production of these more tolerant varieties will probably become a key strategy for some growers because they will be able to plant these more tolerant varieties in landscapes that already have boxwood blight and they won’t defoliate,” Ivors said. “If you can start putting some of these varieties out in the field now, in four to 10 years, you’ll be able to sell these to those particular areas where BB has become a big problem.”
The N.C. State group of researchers is currently working on another boxwood blight-related project. Ivors said the group wants to be able to provide recommendations for commercially available fungicides that commercial growers could use for preventing infection in production areas.
The data is still being finalized, and she estimates the research will be ready to be published in about a month.