Beat the blight

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A new comprehensive study ranks boxwood cultivars for blight resistance.

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October 2, 2020

© Ruud Morijn | Adobe Stock

Boxwood is a workhorse you can always count on: Tough, adaptable, evergreen, deer-resistant and beautiful. Sheared or shaggy and available in many sizes and forms, there are few gardens where boxwood doesn’t fit right in. No wonder U.S. boxwood sales are around $126 million annually, a significant source of revenue for growers and nurseries.

But the plant’s fate has come into question since boxwood blight reached the U.S. in 2011. The disease has now spread to 28 states, posing a significant threat to established boxwood plantings in landscapes and historical gardens as well as nursery inventory. Researchers and growers have quickly responded to better understand this pathogen, determine the susceptibility of various boxwood varieties already on the market and develop more resistant cultivars.

A new paper, “Ranking Resistance of Buxus Cultivars to Boxwood Blight — An Integrated Analysis,” was published in the June 2020 Journal of Environmental Horticulture. The paper documents a study conducted by researchers from the U.S. Department of Agriculture, Agricultural Research Services and Rutgers University who assembled data from prior studies as well as new data to create the most comprehensive, statistically consistent ranking to date of Buxus cultivars to boxwood blight.

A comprehensive study

Researchers used 289 samples from the National Boxwood Collection at the U.S. National Arboretum, one of the most complete collections of boxwood in the world with 700 varieties.

“The National Arboretum is one of the primary groups involved in breeding resistant boxwood, we’ve been screening for resistance for several years,” says Dr. Margaret Pooler, a supervisory research geneticist with the U.S. National Arboretum and USDA’s Agricultural Research Service.

From those initial 289 samples, the researchers chose the 65 most resistant varieties for further study and statistically merged them with data of prior studies of boxwood resistance.

“Instead of publishing yet another disease resistance comparison, we decided to compare our data to that of other studies and put them together to create a bigger study that compiles all the data. We undertook this study because there were several groups who had completed studies to screen for resistance among cultivars but the results of these studies were inconsistent. So we took a statistical approach to tease out what was causing this variability. We looked at all the studies we could find that had data, or we obtained unpublished data from other researchers to conduct a meta-analysis. We wanted to figure out why the results were different and compiled the prior research with our own, so all the data is summarized in one place,” she says.

Chuanxue (Chuan) Hong, professor of plant pathology at Virginia Tech, who summarized some of the unpublished data for the study says, “I think it’s a very important paper because it provides information that can help growers and those in the nursery industry supply less susceptible boxwood plants to their customers. If we can lower the risk of boxwood blight by using more resistant cultivars and practicing good plant maintenance, everyone will have better success. The nursery industry will have fewer returns and everyone downstream will be more successful, and this has a national impact in reducing boxwood blight. Boxwood is important to the American landscape, it is irreplaceable. It’s important to carry the boxwood heritage forward.”

B. microphylla var. japonica ‘Green Beauty’
Photo courtesy of Monrovia

Key findings

The report provides a susceptibility estimate for 131 varieties of boxwood. While no variety showed complete resistance to boxwood blight, some were less susceptible to the disease than others. The ten least susceptible boxwood listed in the study are:

  • B. sinica var. insularis 60705 (Korean littleleaf boxwood)
  • B. microphylla ‘Little Missy’
  • B. microphylla var. japonica ‘Winter Gem’
  • B. SB17 (parent plant of NewGen Freedom)
  • B. microphylla ‘Compacta’
  • B. microphylla var. japonica ‘Green Beauty’
  • B. sempervirens 43877
  • B. microphylla ‘Northern Emerald’
  • B. microphylla var. japonica 4227
  • B. sinica var. insularis ‘Wee Willie’

The full article, including the ranking of 131 boxwood varieties, is available at http://bit.ly/boxwood-ranking.

“Our key findings are that there is a lot of variability among boxwood types to the blight and there is nothing that is truly resistant, at least among the plants we have in the U.S. The different plants we looked at have varying degrees of susceptibility. So right now, everything that we know of will get blight if you hit it hard enough with the pathogen,” Pooler says.

Bennett Saunders, general manager of Saunders Genetics, a boxwood grower that is evaluating boxwood varieties for blight, says, “The sempervirens cultivars are very susceptible and we have to get away from those cultivars. Generally, the microphyllas are the most resistant to blight.”

In reviewing this new paper, he says, “The Sheridan series of boxwood are scattered in the middle as far as boxwood blight.” These were developed at Sheridan Nurseries in Ontario in the 1950s and are the crosses resulting in B. sinica var. insularis and include the varieties ‘Green Gem’, ‘Green Velvet’, ‘Green Mountain’ and others. Sheridan cultivars are the most-grown boxwood in the country, and ‘Green Velvet’ might be the most grown boxwood in the U.S. and it has moderate susceptibility to blight.”

The study did not evaluate how a plant’s form or architecture factors into blight susceptibility. Saunders provides his field observations on this aspect: “If a plant is more upright and open it tends to be more resistant because of better airflow. In most cases the disease originates at the bottom of the plant due to water splashing spores onto the plant and then the disease has to grow up through the plant. One upright sempervirens that we’ve seen a little less blight on is ‘Dee Runk.’ The disease seems to have trouble climbing that plant but under intense pressure it will succumb.”

Symptoms of boxwood blight begin as dark leaf spots.
Photo by M. Grabowski, Minnesota Department of Agriculture

A way forward

Will a truly blight-resistant boxwood ever be developed? That remains unclear for a number of reasons, Saunders says.

“There are landscapes full of susceptible cultivars carrying the disease,” he says.

But there are options to reduce favorable blight conditions.

“If we reduce the numbers of susceptible cultivars in our gardens, replant with more resistant cultivars and use best management practices, we will minimize the chances of ever seeing blight. Maybe the plants don’t have to be totally resistant if we diligently follow best management practices, some of which are very basic, such as mulching boxwood. Chuan Hong has determined that mulching can reduce blight lesions by as much as 96%,” he says.

Saunders notes that more testing is warranted since “some of the plants in the study only had one test done on them and those might benefit from having more testing done.”

B. microphylla var. japonica ‘Winter Gem’
Photo courtesy of Proven Winners

Hong says the green industry will benefit from additional trials.

“Those in the industry should push for further evaluations, not only of newer cultivars since the last studies were done, but also of how the most resistant cultivars from this paper perform in different locations around the country. This additional data will further substantiate the existing studies on susceptibility or resistance of the cultivars and help determine whether the cultivars perform differently in different environments,” Hong says.

Pooler expects the evaluations to continue.

“We definitely will evaluate new cultivars that have come out since this study. We don’t know what a truly resistant boxwood looks like. That’s what we would call a negative control and would be one extreme of our study,” she says. “We have plenty of plants to use at the susceptible end but having resistant cultivars as the gold standard; that is still pretty hard to come by. We’re waiting for what comes out from breeders in the U.S. and Europe to see how they stand up to strains of the pathogen. Our number-one objective is to breed new plants and we actively seek out and obtain new cultivars.”

The U.S. National Arboretum has made hundreds of crosses and have thousands of plants in poly houses, Pooler says.

This represents the differences in susceptibility among varieties of boxwood whose leaves were inoculated with the boxwood blight fungus during the trials.
Photo courtesy of Margaret Pooler

“Some look promising in terms of ornamental characteristics. It’s a matter of getting them tested in the lab and to see how they do in the landscape. We’re doing genetic studies of resistance that will open doors to anyone who wants to breed new cultivars if we can get a handle on the genetics,” she adds.

A collaborative research project has also been established to combat boxwood blight under a Specialty Crop Research Initiative (SCRI) grant from the U.S. Department of Agriculture.

“This project is a collaboration between all the key players in government, industry and academia. It includes pathologists, breeders and even economists, so it’s taking a multi-prong approach,” Pooler says.

Through the SCRI project Hong hopes to produce a digital screening tool that will help to objectively evaluate cultivars.

“It’s an exciting project,” says Pooler. “We’re hoping it will help us find some breakthroughs in finding resistant boxwood cultivars.”

Martha Swiss is a garden writer and designer in Pittsburgh, Pennsylvania; https://plantsomejoy.com