Fungicide Recommendations for Boxwood Blight in Virginia

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

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.

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.

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. 

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.

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:

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Boxwood Leafminer

The arrival of Spring means insect pests are just around the corner. One pest to watch out for is the Boxwood Leafminer (Monarthropalpus flavus).  Although boxwood tend to have few pests, the leafminer can be a headache for growers. Damage starts small with some discolored leaves, but as populations build over a period of years, you will see defoliation, and in severe cases, plant death. Luckily, there are effective options to combat this pest.

Boxwood Leafminers are tiny, orange, gnat-like midges that have only one life cycle per year. The cycle begins each spring as the previous year’s larvae cause the boxwood leaves to blister and swell. Those larvae pupate and emerge as adults in April/May (Central Virginia). After pupating, the adults hover only inches above the boxwood because they are weak flyers and wind will blow them away. Over a 3-week period, adults emerge in waves to mate. Females complete their life cycle after they lay their eggs in the tender leaves. These eggs hatch in early summer (mid-June) and the larvae begin the cycle of growth that will conclude the following spring.

Saunders Genetics has done extensive work trying to determine the best timing and chemicals to control Boxwood Leafminer. In many programs, sprays are targeted towards adults, but because leafminer emerges in waves, you would need to spray every 2-3 days over the 3-week period, making it nearly impossible to eradicate them. Instead, control can be achieved with a single spray of a systemic insecticide after eggs hatch (mid-June). These types of insecticides remain effective until temperatures turn cold in the fall. When a spray is applied properly and thoroughly to all boxwood in a given area, you can receive control for up to 2-3 years. According to research, the most effective products are those that contain the active ingredient imidacloprid, thiomethoxam, or dinotefuran. Please note that Saunders Genetics bases this recommendation on in-house trials. We recommend growers and landscapers contact their local Extension personnel for recommendations and regulations in their area.

Although chemical control is a great option, there are also boxwood varieties available that have shown genetic resistance to Boxwood Leafminer. Saunders Genetics has done thorough research, testing over 129 cultivars. The results are listed in the chart below.

Genetic resistance is a defining feature in the NewGen™ Boxwood program. The goal of NewGen™ is to provide grower-friendly plants that can thrive when faced with pest and disease pressure. NewGen™ boxwood were chosen because they are more resistant to Boxwood Leafminer than many of the varieties currently on the market. If you are replacing or planting boxwood, look to these NewGen™ varieties which will be available for purchase in 2020.