Boxwood blight (BB) was first identified in the U.S. in the fall of 2011 and has since been detected in at least 22 states across the country, 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 fiscal year 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 well.

Blighted leaves and distinct black stem cankers caused by boxwood blight

Mulching

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.

Fungicides

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 DMIs) control BB preventively, as does preventive sprays of chlorothalonil. Always consult current product labels before applications.

C. pseudonaviculata white sporulation

Biological control

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.

Heat therapy

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.

Boxwood affected by BB, dying from the bottom up; note black cankers and defoliation

Other

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. AmericanHort will present a webinar series later this year to discuss this research in greater detail.