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Boxwood blight spreads as researchers seek answers
Staff Report | February 1, 2017
Boxwood Blight Shrub

The spread of the disease among one of America’s most popular shrubs has been nonstop for more than five years now.
Photo: Georgia Department of Agriculture

Boxwood blight continues to spread across the United States as researchers look for effective ways to stop the pathogen.

The fungus and its sticky spores, which survive in soil and plant debris, spread easily. Though seldom killing the boxwood, the disease blights – and eventually defoliates – the shrub.

The University of Illinois Plant Clinic recently announced the presence of boxwood blight in Illinois. The clinic received two boxwood samples late last year from landscape installations in northeast Illinois. Both were positive for boxwood blight.

Staff members at the University of Illinois Extension say they’re confident the infected plants were not from an Illinois nursery.

First identified in the United States in the fall of 2011, boxwood blight has now been detected in at least 22 states, according to the Horticultural Research Institute (HRI), in both nursery and landscape settings.

HRI says two closely related fungi, Calonectria pseudonaviculata and C. henricotiae, cause boxwood blight on three plants: boxwood (Buxus), Pachysandra, and sweet box (Sarcococca).

Since 2012, boxwood blight research has been supported through the farm bill. In the fiscal year ended Sept. 30, 2016, funding exceeded $486,000, HRI says, supporting research collaborations among IR-4, the Connecticut Agricultural Experiment Station, Cornell University, Hood College, Oregon State University, North Carolina State University, Virginia Tech and USDA.

HRI notes that research activities thus far have been “heavily focused on management measures.” Here’s a brief summary of scientists’ best advice and areas of inquiry thus far in fighting boxwood blight:

Sanitation and disinfection

Sanitation and disinfection are two critical components of all disease control programs. Soil in nursery beds and benches can harbor boxwood blight 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 laboratory 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 at Virginia Tech. Studies at the Connecticut Agricultural Experiment Station have shown bleach to be effective against the spores as well.

Mulching

Mulching can help or hinder development of a plant disease, depending on the system, HRI says. When it comes to boxwood blight, mulch theoretically should reduce disease development by reducing pathogen dispersal via rain splash – a primary cause of boxwood blight’s spread.

Researchers at Virginia Tech are investigating the potential mitigation of boxwood blight using various mulches. Studies are being conducted in operational nurseries and residential landscapes to gauge the impact on disease development.

Fungicides

Few data exist on what fungicides best control boxwood blight. Dr. Jim LaMondia at the Connecticut Agricultural Experiment Station 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, HRI reports. Propiconazole, in particular, shows promise for early curative control. In fact, most triazole fungicides (also known as DMI’s) control boxwood blight “preventively,” as does a preventive spray of chlorothalonil.

Biological control

Dr. JoAnne Crouch, USDA-ARS, and Dr. Chuan Hong at Virginia Tech are screening large numbers of microorganisms to identify candidates for biological control of boxwood blight.

Endophytes – microorganisms that live in or on a plant without causing disease – have been associated with boxwood leaves and roots. Often biological control agents have complex interactions with plants, HRI says, 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 boxwood blight during propagation. After exposure to hot water at 117.5 degrees Fahrenheit, 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.

AmericanHort and its research affiliate, , say other components of current research collaborations include a breeding program to identify disease resistant varieties and exploration into fungicide sensitivity.

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