Growers must work vigilantly to keep the fungus that causes boxwood blight out of their nurseries

Boxwood is one of the most popular landscaping plants around. It’s evergreen and pretty, it provides natural privacy, it’s easy to maintain and it can be shaped into attractive ornamental topiaries. In fact, so popular is boxwood that it’s the best-selling broadleaf evergreen in the United States — and Oregon is by far the largest producer.

But over the past decade or so, even while sales have climbed, boxwood and the nurseries who grow it have been trying to fend off a stubborn threat. It comes in fungal form, quietly but quickly. It shows up as dark spots on boxwood’s green leaves and diamond-shaped lesions on its stems. It fries leaves to a straw-colored crisp and can defoliate plants in less than a week, decimating home landscapes and treasured public gardens.

The culprit? Boxwood blight.

First discovered stateside in 2011 — it likely hitched a ride from Europe — boxwood blight is a fungal disease that spreads via waterborne spores that get splashed onto nearby boxwood. The spores can also be transmitted unwittingly when infected plants are transported among nurseries. Soil, equipment, animals and even clothing can be contaminated with boxwood blight spores.

The fungus also produces long-lived structures (microsclerotia), which can survive for multiple years in soil and organic debris. An outbreak of boxwood blight can wreak havoc on nurseries who count boxwood as one of their primary offerings.

“Depending on the scale, the impact can be quite significant,” said Jerry Weiland, a research plant pathologist with the USDA-ARS Horticultural Crops Research Laboratory in Corvallis, Oregon. “If there’s a nursery where 80% of it is boxwood and they get blight, the impact is a big deal. I know of at least three growers who have lost probably in excess of a quarter-million dollars each.”

Ever since boxwood blight was first detected in the U.S., efforts have been underway to slow its spread and keep it in check. Here in Oregon, where boxwood is a crucial crop for the nursery industry, experts from Oregon State University (OSU) and the Oregon Department of Agriculture have been working diligently to arm nurseries with the knowledge and tools they need to keep those dark spots from wreaking havoc on one of the state’s major agricultural industries.

“One of the most important things is prevention, and one of the main ways we do that is through education,” said Luisa Santamaria, an associate professor and OSU Extension plant pathologist who has been working on boxwood blight education and information for the nursery industry since 2016.

Boxwood’s big numbers

Oregon’s nursery industry has been riding a bit of a COVID-19 wave the past two years, with gross annual sales hitting $1.9 billion in 2021 — up nearly double over 2019’s $1 billion total. Helping fuel that boost is boxwood. Nationwide, boxwood sales topped $141 million last year, and in Oregon — far and away the leading boxwood producer in the country — nurseries have only been selling more and more.

According to a 2021 study in the Journal of Environmental Horticulture, Oregon tallied boxwood sales of $21.9 million in 2009, $23.1 million in 2014 and $27.1 million in 2019, putting Oregon on top with about 20% of all the boxwood in the U.S. The No. 2 state is California, which produces about 10% of the boxwood in the country, with sales of $15 million in 2019.

So, it’s no surprise that a threat like boxwood blight gets a fair amount of attention in Oregon.

“In terms of the challenges associated with boxwood blight, it is still huge,” said Chris Lee, shipping and farm manager at Eshraghi Nursery in Hillsboro, Oregon. “It is more persistent than many other stresses, as there is no way to eradicate the pathogen without destroying the host plant. Coupled with the consistent, high demand for boxwood, boxwood blight keeps us more vigilant than other stresses.”

Lee said the nursery sells about 65,000 boxwood plants annually, totaling more than $800,000, and its on-site inventory tops 340,000 boxwood of various sizes, varieties and growing stages.

“Boxwood is an important plant at Eshraghi Nursery,” Lee said. “Our specialty remains grafted Japanese maples and grafted conifers, but we grow an expansive catalogue to meet customer expectations. Boxwood is a common plant that our customers often use for efficient shipping, reducing freight percentages on landed costs.”

Fighting blight

Keeping boxwood blight at bay is no small labor. Thankfully, Oregon’s dry summers, which the fungus doesn’t like, help slow it down, but the damp springs and falls are more challenging.

Weiland said the main thing nurseries can do to avoid blight is to not bring in stock from other nurseries. In the past four years, he’s surveyed about 20 Oregon nurseries, 16 of which had boxwood blight; 10 of those got it by unknowingly bringing in infected plants.

“That’s the No. 1 route for [boxwood blight] to end up in a nursery, because a lot of times, it’s hard to see the symptoms on plants when they come in,” Weiland said.

Ross Dumdi, plant health manager for the West Coast for Bailey Nurseries, said Bailey has been vigilant about boxwood blight. Since the disease’s arrival, Bailey has been adamant about having a closed loop system. Bailey propagates and grows their own boxwood, so nothing comes in from outside, either on boxwood or other host plants.

Additionally, Dumdi said Bailey does not permit customers or tours to come to the nursery because there’s always a chance that sticky boxwood blight spores may be tagging along. Bailey also doesn’t allow customers to return boxwood plants to the nursery and makes sure those customers dispose of them appropriately, usually by burning them.

Eshraghi runs a similarly tight ship, with propagation cuttings sourced only from in-house, traceable material. Proper irrigation and drainage management are important at the nursery as well, as is proper sterilization during pruning and fertilizing.

Weiland said overwatering can be a big contributor to boxwood blight’s spread. The disease requires about five hours to infect a plant, so if there’s moisture on the leaves for that time period or longer, blight spores are more likely to settle in.

“A lot of growers could irrigate less than they are,” Weiland said, adding that avoiding overhead watering can also help dissuade boxwood blight.

Boxwood plants should also be spaced farther apart to help discourage the spread of boxwood blight. If they’re packed closely together, the canopy may look healthy, but underneath, which is harder to access, may be a different story.

Compliance for the win

To help nurseries in their fight against blight, both OSU and the Oregon Department of Agriculture (ODA) offer educational and other resources. The ODA’s Boxwood Blight Cleanliness Program (BBCP) is a voluntary program that tracks propagation practices, where plants come from and where they are located in the nursery, scouting protocols, water management and sanitation. ODA inspectors also visit nurseries and inspect stock.

Kara Mills, program lead for the ODA’s Nursery and Christmas Tree Program, said the BBCP takes a systems approach and essentially looks at a nursery as if it were a building with a series of fire doors that help with containment. She said eight nurseries are involved in the program at present, including Bailey and Eshraghi.

Lee said participating in the program is rigorous and includes training employees on what to look for, scouting boxwood monthly and other practices.

“Being part of this compliance program is not easy,” Lee said. “It is more work and the program is rigid. However, the payoff of knowing we are doing our best is essential. Our customers deserve that.”

Mills is hopeful that more nurseries will participate in the BBCP to help keep boxwood blight under control. She said the program is all about supporting the nursery industry, not adding an extra burden to it.

“What we really want to do is reduce boxwood blight in our nurseries,” Mills said. “If there are nurseries out there that want to do that in a program with lots of support, that’s what we’re here for.”

Lessons learned

Beyond the BBCP, OSU has made education a big part of its role in the battle against boxwood blight. Santamaria has developed educational materials in English and Spanish to help workers learn about boxwood blight, proper sanitation and sterilization, and what to do if they see signs of the disease. She also does on-site trainings and offers online educational opportunities, including a certification program.

“I would really like to see more nurseries getting involved in getting their workforces educated,” Santamaria said. “It’s very important and brings a lot of value.”

Although there are some fungicides available to help treat boxwood blight, and though there are some less-resistant cultivars available or in development, boxwood blight isn’t likely to disappear anytime soon. As a result, it will be important for nurseries to continue to employ best practices, educate their employees and stay vigilant.

Lee said Oregon has been a leader in the nursery industry in general, but it’s also stepped up in the charge against boxwood blight.

“Boxwood blight has been a challenge to our industry on a national level,” he said. “I am not surprised at how nurseries, especially in Oregon, have risen to this challenge. The diligence of Oregon Association of Nurseries and the ODA needs to be applauded.” 

Jon Bell is an Oregon freelance journalist who writes about everything from Mt. Hood and craft beer to real estate and the great outdoors. His website is www.jbellink.com.

From the January 2023 issue of Digger magazine | Download PDF