Boxwood Blight Initiatives
HRI exists to help the horticulture industry. When an important crop like boxwood, the most popular evergreen shrub in the US, faces critical pest and disease attacks, we commit to finding solutions.
Outreach is one of our best skills. For example, we distilled the latest research findings and offer best management practices to help production mitigate the risk and spread of boxwood blight. Our team provides information as it becomes available, especially on boxwood blight and emerging pests like box tree moth
Research is our highest priority, in the form of direct funding and leveraging partners for additional dollars that benefit our industry. We recently launched an initiative to evaluate boxwood cultivars for tolerance to boxwood blight that will help the industry determine what varieties to breed, grow, sell, and plant.
Download the Boxwood Blight Best Management Practices Guidelines for Production and for Landscape.
Bonus content includes one-page BMP highlights for printing and sharing.
New Best Management Practices for Boxwood Health in the Landscape Fact Sheet Available
Resource Expands HRI’s Growing Boxwood Blight Library
The Horticultural Research Institute (HRI) released a set of recommended best management practices (BMPs) aimed at dealing with boxwood blight in the landscape. The new BMPs came at the request of industry stakeholders following new findings from ongoing boxwood blight research.
As boxwood blight discoveries continue to increase, so does concern over the long-term success of the most popular broadleaf evergreen, boxwood. The BMPs are meant to provide guidance to landscape managers regarding practices and their risk of boxwood blight. They are suggested for voluntary adoption; none are mandatory or legally enforceable.
Topics address practices to help train personnel, the importance of scouting, reducing the risk of introduction to a property, preventing spread of a confirmed infected area, replanting, and sanitation. All recommendations were developed by a working group convened by HRI consisting of National Plant Board representatives and landscape managers. Furthermore, the National Association of Landscape Professionals (NALP) endorsed the resulting BMPs.
“Boxwood blight continues to grow as a threat to established landscapes across North America,” states Gary Knosher, HRI President. “Boxwood is the most popular broadleaf evergreen sold in the United States and has significant presence in landscapes and historic gardens. Creating voluntary management practices for boxwood health to guide landscape managers was a natural next step in HRI’s commitment in helping the industry mitigate the spread of the disease.”
The current BMPs for nursery production were updated as well to reflect more recent findings by the research community. For example, 70% alcohol has been confirmed to kill conidia, a type of spore, within seconds. This is a critically important and cost-effective tool to incorporate into sanitation practices such as cleaning pruning tools between blocks and/or properties.
HRI is invested in helping the industry understand the complex nature of boxwood health. To aid growers, we partnered with the Oregon Association of Nurseries and hosted a one-day, sold-out workshop in February 2020. The workshop emphasized the importance of BMPs and cultivar selection to best managed boxwood blight. Presentation materials from this workshop are available here.
HRI proudly supports scientific research and students for the advancement of the environmental horticultural industry. Outreach is a key component to HRI’s mission. HRI was established by industry leaders on the premise that no one could better direct needed research to advance horticulture than the very people who work in it, day in and day out.
HRI Launches New Boxwood Variety Evaluation Initiative
HRI has recently relaunched its Emerging Issues: Boxwood Blight Fund and will use this fund to coordinate development efforts for a standardized evaluation of boxwood varieties’ tolerance of or susceptibility to boxwood blight.
In 2012, in response to confirmations of boxwood blight in the United States, HRI established the Emerging Issues: Boxwood Blight Fund and dedicated over $16,000 towards research on mitigation efforts. Since then, HRI has leveraged these funds to gain an additional $2.7 million through multiple funding sources including the USDA-ARS Floriculture and Nursery Research Initiative and USDA-APHIS Farm Bill. HRI continues to directly fund boxwood blight research, including two projects funded in 2018.
In addition to needed mitigation research, the industry needs to better understand which boxwood varieties exhibit tolerance or susceptibility. HRI is using the re-established Boxwood Blight Fund to help establish a standardized testing protocol. Previous research has shown potential variability among results. For example, a variety may show high tolerance in one site but moderate tolerance at another site. Ultimately, HRI hopes to endorse a labeling system indicating a variety’s tolerance level to guide better choices by nursery growers, retail customers, and landscape managers. Both the Virginia Nursery & Landscape Association and the West Virginia Nursery & Landscape Association have committed $10,000 toward this work.
To make a charitable contribution in support of the Boxwood Fund, please contact HRI at 614-487-1117. Make checks payable to HRI at 2130 Stella Ct; Columbus, OH 43215 and include a memo line: Boxwood Blight.
Reclaiming Boxwood from Boxwood Blight
Back in the late 2000s and early 2010s, before boxwood blight became a household name among nurseries, growers reported a slump in sales of boxwood and plentiful inventory. This was during the period where the great recession was in full swing and new housing construction was at low levels. Fast-forward a few years, and it’s a whole different story.
The recession ended, and the U.S. real estate market rebounded. Growers now report an inability to meet consumers’ demands for boxwood—a good scenario! Boxwood is the #1 woody plant sold in the U.S. In 2014, the USDA National Agricultural Statistics Service estimated the value of U.S. boxwood production at over $126 million, beating azalea, holly, hydrangea, arborvitae, and many others. Even some greenhouse producers are handling boxwood at certain times of the year.
With demand for boxwood at an all-time high, it’s unfortunate that growers must contend with boxwood blight, the impact of which is increasingly felt in production. The costs to grow boxwood have reached an all-time high as well for most growers.
While other diseases and pests threaten boxwoods—such as boxwood leafminer, Volutella, and mites—boxwood blight, caused by Calonectria pseudonaviculata, is the primary concern in the US at the moment. This was confirmed at the recent American Boxwood Society (ABS) meeting, where new developments in boxwood blight research were featured. Some in the industry are concerned that the U.S. is on the cusp of exponential boxwood blight spread.
“The American Boxwood Society was overwhelmed at the response to the Boxwood Blight meeting in Beltsville on February 20, 2018,” commented Bennett Saunders, President of ABS. “It is obvious to the Society that the industry is very interested in fighting this disease. Indeed, the talks centered on keeping the disease out, but there is an increasing realization that in the future we will manage the disease through better pruning techniques, better ground cover management, more resistant cultivars, and other practices.
As this disease becomes better managed, we see a continued strong demand for this ‘Aristocrat of Plants’, in spite of the higher cost of production.”
To date, boxwood blight has been positively identified in 25 states. However, consensus among researchers is that it is likely present throughout the contiguous U.S. wherever there is a sizable population of boxwood. Any state where boxwoods are used in the landscape, but the disease hasn’t been reported, may just not have been identified yet. In any event, incidence of boxwood blight is increasing, both in production and landscape settings, as well as in cut greenery. Several reports emerged during the 2017 holiday season of boxwood blight-infected cuttings used and sold in wreaths and greenery in floral arrangements; horticultural inspectors issued quarantines on some of this material.
Margery Daughtrey, Senior Extension Associate with Cornell University specializing in ornamental plant pathology, noted, “In any given locality, the fungus that causes boxwood blight is usually moved only short distances, mostly by rain splash, or wind-driven rain…but we help it out in its distribution by inadvertently moving infected plants over large distances via the nursery trade. Diseased boxwood leaves and cankered shoots travel via B&B material, gallon containers, and even in Christmas wreaths.”
In parts of Europe, where boxwood blight has become almost ubiquitous in production and landscapes, treatment with fungicides is the norm. Eradication is not their primary control strategy, since the disease is widespread. The U.S. may get to that point – or maybe not. Our industry strategy is to keep boxwood blight out of areas, especially historic gardens - relying on a “start clean, stay clean” mantra. Time will tell how successful we are. In the interim, research continues.
Though boxwood blight was only identified in the U.S. in Fall 2011, a tremendous amount of research has been done since then. From 2014 to 2017, USDA APHIS has dedicated over $2.7 million to boxwood blight research through the Farm Bill, Section 10007. Additional research funds have come from the Floriculture and Nursery Research Initiative program through USDA ARS and the IR-4 Ornamental Horticulture Program with funding from USDA APHIS and USDA NIFA. The Horticultural Research Institute (HRI) sponsors research directly in tandem with its education and advocacy efforts.
Early research efforts focused on fungicide efficacy trials. Since then the research has morphed into the investigation of novel control strategies and longer-term solutions.
Dr. Chuan Hong, Virginia Tech, is coordinating a number of valuable research projects with a “you got questions, we got answers” attitude.
One example is the evaluation of mulch to help prevent spread in landscape situations. The spores of C. pseudonaviculata are very large and sticky; therefore, they do not readily spread via air currents. Tools, boots, rain splash, animal movement (even cats and dogs!), and infected plant material are prime ways boxwood blight is transmitted. Dr. Hong has found that mulching can effectively suppress disease through reduction in rain splash transmission and possibly by a change in the microbiome. Dr. Anton Baudoin, Virginia Tech, is considering the impact of mulch in production.
Dr. Hong is also working with Dr. Len Coop, Oregon State University on development of a boxwood blight forecasting model. The model can be found online now but should only be used as reference at this point. Future validations are needed and will enable it to more accurately serve as a guide for fungicide applications. Release of a mobile app is expected later this year.
Regarding fungicides, Dr. Jim LaMondia, Connecticut Agricultural Experiment Station, has been testing the efficacy of commercially available products. Dr. LaMondia’s work has indicated some potential, early curative effects from certain demethylation inhibitor (DMIs, also known as sterol biosynthesis inhibitors or FRAC code 3) fungicides. More work is needed here, since many DMIs regulate plant growth or can cause severe plant injury.
Dr. LaMondia is also considering the impacts of alternate host plants, such as Pachysandra and Sarcococca, on disease development. Pachysandra production is different than boxwood and is not considered a likely source of boxwood blight. However, Pachysandra can serve as a reservoir for the pathogen in landscape settings.
Sanitation should always be a consideration – regardless of what disease is at play - for both producers and landscape managers. Several products are effective, such as ethanol, bleach, Lysol™, and Zerotol™. Ethanol, in particular, has been shown to effectively kill C. pseudonaviculata microsclerotia. This is great news because microsclerotia are masses of hardened fungal material capable of tolerating extreme weather conditions for an extended period of time. Microsclerotium formation is one way boxwood blight can persist in the soil for a very long time.
Other key research areas include biocontrol options, betterment of diagnostic tools (maybe even one suitable for field use!), the disease cycle, and a breeding program.
Finding tolerant boxwood varieties is a critical need identified by many throughout our industry. Mark Sellew, President of Pride’s Corner Farm, added, “At Prides Corner Farms we still grow and sell susceptible varieties but have dramatically changed our cultural practices to minimize disease pressure from limited access to the plants, to careful water management and finally to an aggressive chemical program with multiple MOA’s.”
“I believe our industry has to do a better job of policing ourselves, particularly when it comes to growing very susceptible varieties like Buxus suffruticosa. This plant, in my opinion, should be absolutely banned. It is highly susceptible and is most likely spreading the disease and jeopardizing the future market potential of the entire genus of Buxus.”
Once boxwood blight was discovered in the U.S, HRI acted quickly and established a fund for boxwood blight research, making some of the first fungicide efficacy trials possible. Since then, HRI has continued support of the industry by leveraging these funds for additional support and the development of a best management practices (BMP) document through collaboration with AmericanHort and the National Plant Board. The BMPs help gold standard growers ensure clean boxwood production.
One of HRI’s latest projects is working with researchers towards a standardized protocol where boxwood varieties can be assessed for boxwood blight tolerance. Previous studies have been conducted to evaluate boxwood varieties in terms of tolerance and susceptibility; however, the studies varied in methodology and, in some cases, results. Due to this variability, a more streamlined approach is desired. The boxwood blight research fund was reopened in 2018 and is now accepting donations to help guide the industry towards production of boxwood blight tolerant varieties.
Jill M. Calabro, Ph.D. is science and research programs director at the Horticultural Research Institute, the AmericanHort research and scholarship foundation. The Horticultural Research Institute (HRI) has provided more than $7.5 million in funds to research projects covering a broad range of production, environmental, and business issues important to the green industry since 1962. Nearly $11 million is committed to the endowment by individuals, corporations, and associations. For more information about HRI, its grant-funded research, or programming, visit www.hriresearch.org or call 614-884-1155.
New Boxwood Blight Best Management Practices Released
WASHINGTON, DC and COLUMBUS, OH—September 15, 2017—A lot can change in five years, especially in terms of research. Back in 2012, the US nursery production world was still reeling from the discovery of boxwood blight the year prior. The Horticultural Research Institute (HRI) quickly established a fund dedicated to boxwood blight research and a boxwood blight working group composed of industry leaders and National Plant Board (NPB) representatives. A best management practices document for nursery production was produced out of this collaboration.
Subsequent research is still in progress, but enough new information exists to warrant a revision of the best management practices released back in 2012. AmericanHort and HRI, in conjunction with the NPB, are pleased to announce that the revised, updated Boxwood Blight Best Management Practices, version 2.0, is now available online at HRIResearch.org.
Two closely related fungi, Calonectria pseudonaviculata and C. henricotiae, can cause boxwood blight. Currently only one, C. pseudonaviculata, has a known presence in the US. This is a good thing. Both, however, are present in the EU; so, the threat remains for C. henricotiae to come to the US. Despite being the same genus (Calonectria), these two species have some key differences, such as different responses to temperatures and fungicides.
While researchers know of only three host plants for boxwood blight, boxwood (Buxus), Pachysandra, and sweet box (Sarcococca), the economic impact is potentially quite large, with over $20 million per year in US retail sales. Impacts to landscapes are immeasurable. Boxwood blight has been confirmed in 24 states, with Illinois and Missouri being among the latest detections. There are a few states yet with significant boxwood production where the disease hasn’t been confirmed, such as Texas and Louisiana.
Since 2012, the industry has rallied behind a coalition of researchers dedicated to finding the best management strategies for this disease. Resources to support these research efforts have come from a few different sources, including directly from the industry through the Horticultural Research Institute’s (HRI) grants program, the IR-4 Ornamental Horticulture Program administered by USDA-NFA, and from Farm Bill, Section 10007 monies administered by USDA-APHIS. Section 10007 has allocated nearly $3 million alone, all towards boxwood blight!
Early research focused on the basics of boxwood blight management, such as fungicide efficacy studies and cultivar resistance evaluations. Recent focus areas have included (but are not limited to) long distance spread of the disease, pathogen survival in soil, insect transmission, biological control agents, risk mapping and disease forecasting, temperature impacts on disease development and pathogen survival, impacts of mulch in landscapes on disease development, and the use of heat therapy in boxwood propagation.
Dr. Chuan Hong, plant pathologist at Virginia Tech and the lead researcher of key boxwood blight projects, commented, “Boxwood blight risk is mounting for horticultural production facilities, especially those near an infested landscape site, now in 24 states. This BMP update with the latest research from the Farm Bill projects was well timed and will serve individual growers and the horticulture industry as a whole very well.”
To access the latest version of the Boxwood BMPs, please visit the AmericanHort Knowledge Center. To contribute to the continued research of this disease, its impacts, and its solutions, please donate to the Horticultural Research Institute at HRIResearch.org/Donate.
The Horticultural Research Institute (HRI), the research affiliate of AmericanHort, has provided over $7 million in funds since 1962 to research projects covering a broad range of production, environmental, and business issues important to the green industry. Providing best management practices for pest and disease management and prevention is one example of how HRI benefits the horticultural industry. Over $10 million is committed to the endowment by individuals, corporations, and associations. For more information about HRI, its grant-funded research, scholarships, or programming, visit hriresearch.org or contact Jennifer Gray at 614.884.1155.