Stopping aggressive boxwood blight

Miranda Ganci, College of Agriculture and Life Sciences graduate student in plant pathology, has a clear vision of her future career. “I see myself working as an extension agent in order to assist growers with disease identification and management,” she says. “Additionally, I am interested in working in the crop protection industry in a role in which I could assist plant breeders with developing disease resistance in crops.”

She’s already playing that role. Ganci, who is from Hickory and expects to receive her N.C. State University master’s degree in 2014, is studying ways to design mitigation strategies against box blight, an aggressive disease that threatens the economic viability of the boxwood industry. That research was the subject of her presentation in the poster competition at the 2013 Graduate Student Research symposium in April.

Her entry was titled “Evaluating the Role of Microsclerotia in the Disease Cycle of Boxwood Blight, Caused by the Fungus Cylindrocladium buxicola.” Her objective was to determine the survivability of the pathogen when infected plant material was stored and exposed to the different environmental and site conditions where boxwood debris is commonly found in commercial boxwood fields and nurseries.

Box blight is a foliar disease characterized by circular black to brown leaf lesions and elongated black stem lesions on infected boxwood, with infection leading to severe dieback and defoliation of the plant. North Carolina is the largest producer of boxwood in the United States. So there’s a definite economic value of the research to the state’s nursery industry, as the information can be used to optimize cultural control recommendations to improve management of the disease.

“Our research is focused on the biology, epidemiology (change in intensity of the disease over time and space) and management of C. buxicola,” says Ganci, who received her 2012 CALS bachelor’s degree in plant and soil science, with an agroecology concentration. “The fungal pathogen is the causal agent of the foliar disease, box blight, on boxwood and other Buxus species.”

Ganci is also part of a team conducting cultivar susceptibility trials to identify boxwood cultivars that are resistant to box blight.

“The purpose of identifying cultivars that are resistant to box blight is to provide growers with cultivar options that will not be severely affected by infection of the fungal pathogen that causes box blight. Consequently, these cultivars will not have significant damage to the aesthetic value of their foliage if they are infected,” she explains. “Additionally, we are investigating the role of microsclerotia (a resistant overwintering structure that can germinate and lead to reproduction of the fungus) in the disease cycle of box blight.”

Microsclerotia form within dying plant debris, and they can survive for months to years in plant debris or in the soil. Microsclerotia are a source of inoculum (fungal structures that initiate continuation of the disease cycle) which can endure harsh environmental conditions and contribute to the long term survival of the pathogen.

“If we understand more about the biology of reproduction of the pathogen, then we can try to design efficacious management practices that interrupt or reduce the production of inoculum, which subsequently can reduce or eliminate disease severity on boxwood,” she says.

The findings presented on her symposium poster indicate that the pathogen is most likely to survive as microsclerotia in plant debris maintained within the subsurface of soil-less potting media and less likely to survive on the surface of field soil – and that dry conditions could decrease the viability of C. buxicola. Meanwhile, her data show that the microslerotia in plant material in the subsurface of field soil, surface of soil-less potting media and subsurface of soil-less potting media are capable of surviving over winter in western North Carolina. This is information that can be useful to improve management of the disease.

Box blight was detected on boxwood in a North Carolina the fall of 2011.

“The causal agent, C. buxicola, can be very aggressive in the fall and consequently thousands of boxwood plants were infected. These plants had to be destroyed to attempt to prevent further spread of the disease,” Ganci says.

“Additionally, the plants had lost most of their leaves due to the infection so they were definitely not sellable. Many Buxus species and cultivars are susceptible, including the most widely grown cultivars English and American boxwood, B. sempervirens ‘Suffruticosa’ and ‘American,’ respectively.”

Research on the management of box blight is a priority because the boxwood industry has significant economic impact, she says, “and N.C. State University has a role to protect the viability of boxwood production in this state.”

Ganci notes that, according to the USDA 2009 Census of Horticultural Specialties, the value of boxwood plants sold annually is more than $100 million.

“We want to identify disease resistant cultivars and disease management techniques that will eliminate or reduce the economic loss that growers will experience if their nurseries are infected with C. buxicola,” she adds.

Ganci emphasizes that Dr. Kelly Ivors, CALS associate professor and Extension plant pathologist, and Dr. Mike Benson, CALS plant pathology professor, “have made this project possible by providing mentorship and expertise and by securing funding so we can implement our research projects.”

And, ultimately, growers will be the primary beneficiaries of the results of their research, she says.

For that reason, the researchers regularly interact with growers to identify their needs and concerns about box blight. The landscape industry and consumers also benefit from the research, she says, because they have a vested interest in healthy boxwood plants.

“Boxwood are popular for their utility in the landscape. They can thrive in many soil types and climatic conditions. They do not require much maintenance, and they do not have many serious disease problems. One of the primary selling points for boxwood is that they stay green all winter long even in colder regions. They are also highly deer resistant,” Ganci says.

Boxwood are commonly shaped into hedges and used in topiary gardens, she says. However, they are common in their free form in the landscape, as well.

“Field-grown boxwood are simply larger and older than container grown boxwood. Field-grown plants are much more expensive because the growers have invested a lot of time and resources into them. Additionally, management techniques may differ for field-grown plants and container plants.”

Box blight is the first pathogen of boxwood to seriously threaten the economic viability of the boxwood industry, she notes, because it can spread very quickly and it is very aggressive, especially on the most widely grown cultivars.

“These are some of the reasons that researchers in the United States and across the globe are collaborating to quickly design disease mitigation strategies.”

With a focus on both disease management and disease resistance, Ganci and her fellow scientists have made significant findings in cultivar susceptibility trials on 23 commercial cultivars.

“We identified four cultivars that displayed partial resistance under field conditions due to minimal disease symptom development,’’ she says. “We will further explore characteristics of the partially resistant cultivars to identify information that could potentially be used in boxwood breeding programs.”

— Terri Leith

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