boxwood blight

Fungicide Recommendations for Boxwood Blight in Virginia

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By: Dr. Keith Yoder - Retired Plant Pathologist, Virginia Tech

Following the severe boxwood blight year in the Mid-Atlantic region in 2018, nursery growers and landscape care professionals should diligently follow prescribed Best Management Practices (BMPs) as a first line of defense to avoid introducing the boxwood blight fungus or, where blight is already present, to manage it most effectively while reducing the risk of spreading it to new locations.

Fungicides are an essential part of the BMPs. They protect boxwood and reduce the spread of blight by preventing germination and infection by the fungal spores. Examples of protectant fungicides listed in the Virginia Tech Nursery crops Pest Management Guide (PMG, page 5-7) are chlorothalonil (Daconil Weather Stik, 1.4 pt/100 gal) and mancozeb (Dithane 75DF Rainshield 1.5 lb/100 gal). Some fungicides are locally systemic (within the leaf) and offer specific modes of action. Recommended examples, also listed in the PMG, include fludioxonil (Medallion WDG2-4 oz/100 gal), thiophanate-methyl (Cleary 3336 WP 1.5 lb/100 gal), boscalid + pyraclostrobin (Pageant 12-18 oz/100 gal), and tebuconazole (Torque 10 fl oz/100 gal). Because the systemic classes of fungicides tend to be at risk for development of resistance, it is generally recommended (and often specified on the label) that they should be alternated and tank-mixed with broad-spectrum protectant fungicides such as chlorothalonil or mancozeb. Spectro 90WDG (1.5 lb/100 gal) is a combination product that contains both chlorothalonil and thiophanate-methyl. The above list is provided as recommended examples. We feel these can be blended into an effective season-long program, but this list is not meant to exclude similarly labeled formulations and other effective products.

In practice, a rotational schedule of very effective fungicides containing both protectants and systemic products that would have minimal chance of developing resistance might look something like this:

Week 1: Chlorothalonil (Daconil Weather Stik, 1.4 pt/100 gal).

Week 3: Mancozeb (Dithane 75DF Rainshield 1.5 lb/100 gal) + Tebuconazole (Torque 10 fl oz/100 gal).

Week 5: Chlorothalonil (Daconil Weather Stik, 1.4 pt/100 gal).

Week 7: Mancozeb (Dithane 75DF Rainshield 1.5 lb/100 gal). + Pyraclostrobin (Insignia 8-16 oz/100 gal).

Week 9: Start back with the Week 1 rotation. If need be, there are other classes of systemic fungicides that could be substituted into the rotation.

Fungicide applications should begin in the spring when conditions become favorable for infection: Infection periods occur at temperatures of 60-77°F with extended wetting and high humidity. The residual activity of fungicides is affected by amount of rainfall, weathering and degradation, and must be reapplied at regular intervals while the risk of blight infection is present. However, they must not be applied more frequently than recommended treatment intervals, or applied in excess of rates specified on the product label. Always read and follow product labels for safety instructions regarding mixing, handling, compatibility with other chemicals, application methods, re-entry intervals and limitations on amounts of product per acre per year.

Because blight spores can stick to tools, equipment, spray hoses, etc., sanitize all equipment, shoes, gloves, etc., used in tending and treating boxwood to prevent spread of fungal inoculum to healthy plantings. For a list of sanitizer recommendations, refer to the Boxwood Blight Task Force website. It is recommended that vehicles that may have been exposed to the boxwood blight fungus be thoroughly washed of debris (e.g. cleaned at a carwash) to avoid spreading the fungus from one planting to another.

A note about spray equipment: At Saunders Brothers we spray our nursery blocks with an “air-assisted” sprayer (pictured above). We chose this sprayer and like it because it gives good penetration of the spray by spraying straight down into the plants. Many airblast sprayers are characterized by the strong sideways airflow, which could blow detached leaves and plant debris from row to row, possibly spreading disease.

To be noted: While Saunders Genetics has worked with nursery growers who have successfully used the pesticides listed above, we make no guarantees or promises, nor express any opinions, concerning the effectiveness or safety of these or any other pesticides.  Always read the labels and other product information from the manufacturer and discuss the proper use and application of products with appropriate company representatives or acknowledged experts.

Basic Biology and Management of Boxwood Blight

Since Boxwood Blight was found in the United States in 2011, researchers and growers have been diligently working to understand this pathogen. Although the disease has spread to 28 states since its initial identification in North Carolina, there have been many breakthroughs in the understanding of the biology of the disease and how to control it.

Map of states with known cases of Boxwood Blight as of 2018

Map of states with known cases of Boxwood Blight as of 2018

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Boxwood Blight is a fungal disease caused by the pathogen Cylindrocladium buxicola (syn. Calonectria pseudonaviculata). The pathogen attacks the foliage of boxwood, first appearing as black or dark brown spots on the leaves. In a few days, those spots will develop yellow to brown rings around them and cover the leaf. Infected leaves fall off the plant in a matter of a week or so, and stems near infected leaves will develop streaked black lesions or cankers.

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The disease is most prevalent when environmental conditions favor warm temperatures between 60° - 80° F and high humidity. Leaves that are continually wet from constant irrigation, prolonged rain, or high humidity are prime targets for the pathogen. When conducive conditions subside, the fungus will go dormant in the form of black streaks in the limbs of affected plants or debris. The fungus can lie inactive for long periods of time and reappear when conditions become conducive again.

Here you can see the north side of the plant getting the disease first. There is almost an exact line because the north side gets less sunlight and dries more slowly.

Here you can see the north side of the plant getting the disease first. There is almost an exact line because the north side gets less sunlight and dries more slowly.

The Boxwood Blight spore is heavy and sticky, and is typically introduced through contact with infected plants, debris, tools and equipment, clothing, animals, water splash, or other means of direct contact.

Currently, the only known hosts of the pathogen are members of the Buxaceae family, Buxus sp. (Boxwood), Sarcococca sp. (Sweetbox), and Pachysandra sp (Spurge).

With a better understanding of the disease lifecycle and its optimal conditions, there are many practical ways to prevent and control the spread.

Because the spores can spread from plant to plant on clothing, tools, or splashing water, anyone working around boxwood should start with clean clothes, shoes and tools. Make sure to properly clean tools and equipment with disinfecting agents like alcohol, bleach, or Lysol before and after use in boxwood. If you are a landscaper moving from site to site, you should use a disinfectant on clothing and shoes, or use disposable pant and boot covers between sites. Starting with and maintaining clean equipment can remove the pathogen from ever entering a new site. 

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Spores of the pathogen can also spread through water so it is best to avoid overhead irrigation when possible. Facilitating an irrigation schedule that allows just a 3 hour period of dryness for the leaves can significantly reduce the spread of the disease. When there are periods of dry foliage, the leaves are far less likely to become infected, even if the spores are introduced. Never use overhead irrigation in the late afternoon because leaves will remain wet all night. Maintain good air flow when planting and pruning the plants to ensure leaf drying. Recent studies have also shown that mulching every two years or so with at least an inch of mulch around the base of the plant can lessen water splashing.

Studies have shown that, when tested in a lab, all boxwood cultivars show some susceptibility to blight. However, many field tests have shown significant differences in tolerance between cultivars. We have observed certain cultivars, particularly the microphylla, insularis, and harlandii species show strong tolerance compared to those with sempervirens genetics. Shape and structure of the boxwood also affect each plant’s tolerance. Open, more upright plants tend to be more tolerant while short, compact cultivars seem to be more susceptible. We believe this is because the shorter plants that are close to the ground get more water splashing on them. Research is ongoing to discover additional cultivars that will be tolerant to boxwood blight. 

A photo of  Buxus  ‘SB 108’ NewGen Independence™ next to an American Boxwood, planted on the same site for 3 years.

A photo of Buxus ‘SB 108’ NewGen Independence™ next to an American Boxwood, planted on the same site for 3 years.

The exciting results we have seen with varietal tolerance inspired us to create the NewGen™ Boxwood brand. We believe the combination of tolerant varieties and best management practices like those we have outlined are the future of an industry that doesn’t fear Boxwood Blight. For more information check out our education page on Boxwood Blight and subscribe for monthly newsletters filled with the most relevant and up-to-date information and research on boxwood.

 More examples of Boxwood Blight damage and examples on how the pathogen moves:

Who is NewGen™?

Easily one of the most recognized garden plants, boxwood (Buxus sp.) have been gracing formal gardens for thousands of years and are considered one of the oldest ornamental plants. As slow-growing evergreen shrubs, these plants have always been a symbol of a gardener’s commitment and investment to a green space. Boxwood have held their reputation for many years because they are low maintenance, provide structure to a garden, and have few pests and disease pressures.Unfortunately, this reputation took a hard hit in 2011 with the introduction of Boxwood Blight in the U.S. and many growers became concerned about the beloved plant.

Saunders Brothers in Piney River, VA, a family-run nursery with a rich history of boxwood production, responded to this threat by investing in research to find a solution. Their experience in trials for resistance to Boxwood Leafminer, helped them to work with researchers and universities to begin testing for varietal tolerance of Boxwood Blight. After testing over 150 varieties, it was clear certain varieties excelled. Saunders Brothers was excited to have found solutions to a potentially devastating disease. They felt they needed to share these genetics with the industry. This sparked the creation of NewGen™ Boxwood.

NewGen™ aims to be the standard bearer of a distinctively better family of boxwood. This new generation of plants promises better tolerance of Boxwood Blight, better resistance to Boxwood Leafminer, and a WOW factor in the landscape.

“To meet the threat of rising pest and disease pressures, as well as maintain boxwood as the foundation of the landscape, we felt we needed a new approach,” says Bennett Saunders, General Manager of Saunders Genetics. “We’ve focused our attention on new and improved genetics, more thorough testing methods and protocols, partnering with like-minded growers, and establishing a distinguished brand identity. We believe NewGen™ represents the future of boxwood for the industry.”

By maintaining bio-secure testing methods and standards, we hope to continue making introductions of superior plants that maintain brand standards. We aim to establish a regional network of premier licensed growers and propagators to share these plants in landscapes all over the United States. We will dedicate 2019 to introducing the program to the industry and its customers, building production on our two initial selections. Plants are scheduled to be available for sale in the spring of 2020. At this time, we are partnering with 4 growers to distribute the product line: Overdevest Nurseries, NJ; Prides Corner Farms, CT; Saunders Brothers Nursery, VA; and Willoway Nurseries, OH.

NewGen™ Boxwood intends to raise the bar for boxwood to meet the changing dynamics of a new generation of the American garden and gardener.

Pictured is the NewGen™ team, Lindsay Day (left) Marketing Coordinator and Bennett Saunders (right) General Manager of Saunders Genetics.

Pictured is the NewGen™ team, Lindsay Day (left) Marketing Coordinator and Bennett Saunders (right) General Manager of Saunders Genetics.