Boxwood Blight Best Management Practices
Boxwood Blight Info Sheet
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.
Researchers Looking Outside the Box for Solutions
WASHINGTON, DC and COLUMBUS, OH—January 26,
2017—Boxwood blight (BB) continues to spread throughout the US. The University
of Illinois Plant Clinic recently announced its presence in Illinois. The
clinic received two boxwood samples late 2016. Both were from northeast
Illinois and were collected from recent landscape installations. USDA APHIS
confirmed that both tested positive for BB. Univeristy of Illinois Extension
personnel are confident that the infected plants did not likely originate from
an Illinois nursery.
First identified in the US in the fall of
2011, BB has since been detected in at least 22 states across the US, in both
nursery and landscape settings. Two closely related fungi, Calonectria pseudonaviculata
and C. henricotiae, cause boxwood blight on three plants: boxwood (Buxus), Pachysandra, and sweet box (Sarcococca).
Since 2012, BB research has been supported
through the Horticulture Title of the Farm Bill, Section 10007. In FY 2016,
this funding exceeded $486,000 and supports research collaborations among IR-4,
the Connecticut Agricultural Experiment Station (CAES), Cornell University,
Hood College, Oregon State University, North Carolina State University (NCSU),
Virginia Tech (VT), and USDA-ARS.
The Horticultural Research Institute, a
leading force in the boxwood blight fight, continues to monitor, support, and
communicate BB research activities to the industry. Research activities have
been heavily focused on management measures:
Sanitation and Disinfection
Sanitation and disinfection are two critical
components of all disease control programs. Soil in nursery beds and benches
can harbor BB microsclerotia, overwintering structures in dead leaves that will
be viable for years. Several commercially available sanitizers, such as ethanol
and bleach, were evaluated for their efficacy against microsclerotia in lab
studies. Ethanol was very effective in Dr. Nina Shishkoff’s (USDA-ARS) trials
and completely killed conidia in leaf debris in studies conducted by Norm Dart,
VT. Studies at CAES have shown bleach to be effective against BB spores as
Mulching can either help or hinder
development of a plant disease, depending on the system. When it comes to BB,
mulch theoretically should reduce disease development by reducing pathogen
dispersal via rain splash (a primary cause of BB spread). Researchers at VT are
investigating the potential mitigation of BB using various mulches. Studies are
being conducted in operational nurseries and residential landscapes to best
gauge impact on disease development.
Few data exist on what fungicides best
control BB. Dr. Jim LaMondia at CAES is comparing commercially available
products and has identified some that are effective. A number of products were
highlighted for their control activity, including pyraclostrobin and
propiconazole. Both products have demonstrated good preventive control.
Propiconazole, in particular, shows promise for early curative control. In
fact, most triazole fungicides (also known as DMI’s) control BB preventively,
as does preventive sprays of chlorothalonil. Always consult current product
labels before applications.
Dr. JoAnne Crouch, USDA-ARS, and Dr. Chuan
Hong, VT, are screening large numbers of microorganisms in the hopes of finding
candidates for biological control of BB. Endophytes, microorganisms that live
in or on a plant without causing disease, have been identified associated with
boxwood leaves and roots. Often biological control agents have complex
interactions with plants; so, this work can be tedious and time consuming.
Thermal inactivation, or heat therapy, has
been used successfully in the past to eliminate pathogens from woody propagated
plant tissue. Dr. Marc Cubeta’s lab at NCSU is conducting experiments to
examine the response of C. pseudonaviculata and boxwood varieties
to treatment with hot water to manage BB during propagation. After exposure to
hot water at 47.5 °C (117.5°F), conidia are either killed or impaired in their
ability to cause infection. Preliminary results suggest that certain boxwood
varieties are still able to root after exposure to these temperatures. Several
varieties are being screened, and studies are ongoing to determine whether the
pathogen can be eliminated from infected cuttings in this manner.
A breeding program to identify disease
resistant varieties and exploration into fungicide sensitivity are two other
key components of this collaboration.
AmericanHort and its research affiliate, the
Horticultural Research Institute, supported funding of this research through the
Farm Bill Section 10007. This collaboration is coordinated by the IR-4 Program,
a USDA-sponsored entity to increase the specialty crop industry’s access to
labeled pest management products.
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. 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 www.hriresearch.org or contact Jennifer
Gray at 614.884.1155.
Margery Daughtrey, Cornell University.
Photo One: Calonectria pseudonaviculata spores
Photo Two: Boxwood blight in the landscape
Photo Three: Blighted leaves and distinct
black stem cankers caused by BB
Photo Four: C. pseudonaviculata white
Photo Five: Characteristic circular, black
lesions caused by the BB fungus
Photo Six: Close-up of sporulation on the
underside of a leaf shows the orangey sporulation of Volutella buxi and
the white sporulation of C. pseudonaviculata, for comparison
Photo Seven: Boxwood dying from the bottom
up, from BB. Note black cankers and defoliation