Boxwood Blight Information for Commercial Nurseries

(Calonectria pseudonaviculata)

Commercial Nurseries and Professional Growers

The following is a short list of Best Management Practices (BMPs) to avoid/mitigate Boxwood Blight in a commercial nursery or greenhouse setting.

• Only receive liners/ cuttings/ plants from known reputable sources. Propagate boxwood in-house if possible.

• Take precautions that employees/ managers/ inspectors come to work with no Boxwood Blight on clothes, tools, and vehicles.

• Manage traffic into nursery and between blocks within nursery to allow cleanup, whether you think you have Boxwood Blight or not. A Boxwood Blight hotspot might exist for days, months, or years before a nursery knows it exists. 

• Avoid more susceptible boxwood cultivars.

• Avoid overhead irrigation on boxwood, particularly at night.

• Spray all boxwood pre-emptively and preventatively for Boxwood Blight regularly throughout the season.

• Do not accept returned boxwood, particularly ones which have been on ground in plant yards.

• Do not allow landscapers into your nursery, as they may be carrying Boxwood Blight.

• Prune early in the season, before ideal Boxwood Blight temperatures, to avoid spreading Boxwood Blight.

• If Boxwood Blight is found in your nursery, react quickly and decisively to clean it up as quickly as possible.

Commercial Nurseries and Professional Growers

Commercial nurseries must have a zero-tolerance for Boxwood Blight.  If nurseries ship plants with blight, they are contributing to and spreading the problem.  For this reason, fungicide applications are an essential part of BMP’s for nursery production.  They protect boxwood and reduce the spread of blight by preventing germination and infection by the fungal spores. 

Boxwood Blight might arrive at a nursery in any number of ways, including animals and birds carrying the fungus.  A nursery could be Boxwood Blight-free for years, but in the one case where an animal carries the spores into a nursery, and favorable weather follows, an infection could develop and then spread quickly.  Unless a disease scout traveled to this infected area, the disease might go weeks, months, or even years unnoticed.  However, if this area was sprayed on a two-week spray schedule, the odds are very good that the fungus would never infect the plants.  A persistent spray program would smother the disease, and the nursery would keep its blemish-free status.

To date, no cultivars that have been tested in the United States are completely resistant to Boxwood Blight.  All cultivars, even the most resistant ones, have been shown to exhibit symptoms of Boxwood Blight when subjected to extremely conducive conditions.  Even the most resistant cultivars with minimal symptoms might carry Boxwood Blight into a landscape that is otherwise free of Boxwood Blight.  For this reason, all boxwood, including more resistant cultivars in commercial production should be sprayed preventatively.

A possible exception to this would be newly arrived plants from an outside boxwood source.  In this case, the boxwood should be clearly isolated by 100’ or more and left unsprayed for a period of 60 days during a period of blight-conducive temperatures and rain.  It would probably take 30 days for any fungicides on the boxwood to break down.  After the 30 days is up, scouts should carefully check the new plants for blight symptoms. 

Examples of protectant fungicides listed in the 2020 Virginia Tech Nursery Crops Pest Management Guide (PMG, page 5-4) 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. 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:

Saunders Brothers, Inc uses an air-assist sprayer for better penetration of fungicide withing the plant.

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. The residual activity of fungicides is affected by amount of rainfall, weathering, and degradation, and they 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 at higher rates than 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 car wash) to avoid spreading the fungus from one planting to another.

The type of sprayer that nurseries use for prevention of Boxwood Blight is important.   One type of sprayer that seems to be effective is an “air-assisted” sprayer.  It gives good penetration of the spray by spraying straight down into the plants. “Airblast” sprayers, commonly used in the industry and characterized by the strong sideways airflow, 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, no guarantees or promises, nor any opinions concerning the effectiveness or safety of these or any other pesticides are made.  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.

If suspected:

• Take samples to testing labs and limit access to area until results are obtained.

If confirmed:

• Take great care not to further spread disease.

• Do cleanup on dry, sunny days when foliage is dry.

• Consider fungicide applications before and during cleanup to control spread.

• Wear rubber boots and disposable clothing during cleanup to lessen contamination of clothing and shoes.

• Cut and place the infected plants in a plastic bag and take care to catch as much leaf litter as possible.

• Clean up as much leaf debris as possible. Roots are not believed to carry Boxwood Blight.           

• Debris should be burned or taken to landfill with care not to let leaves escape.

• After cleanup, wash skin, launder or dispose of outer clothing, and wash boots.

• Increase airflow in and around remaining plants by thinning and removing lowest limbs.

• Try not to allow any debris to be buried as the disease can survive in the soil for long periods of time.

• All tools and equipment should be thoroughly cleaned when moved between different gardens or areas.

• Mulch to lessen the splashing of spores.

• Monitor boxwood in times of optimal conditions for new infections and flare-ups.

• If severe outbreak of Boxwood Blight occurs on a site that lacks proper air movement, considers other options.

Provide proper air flow around the base of the plants by pruning low hanging branches. This reduces water splashing into the plant and allows leaves to dry.

If you suspect the disease in the landscape, you should take great care in collecting samples by double bagging them and taking them to the local Extension office or another testing lab. The area should be roped off or temporarily fenced off until the results are obtained.

If Boxwood Blight is confirmed, you must be diligent and careful in cleanup. Extreme care should be taken to avoid infecting other plants with debris or plant contact. Choose a time to cleanup when it is sunny and when foliage is dry. Again, the plant is less likely to be sporulating when foliage is dry. Consider applying fungicides to any area where you are working to kill any spores that might be present.

Always wear some sort of disposable suit, like disposable Tyvek® coveralls. Also, wear rubber boots as they can easily be cleaned. If possible, put a plastic bag over the infected plant and cut the plant off at the base. Take care to catch as much leaf litter as possible. Then, clean up any leaves that fall before disturbing the soil as you do not want to incorporate diseased debris into the soil. After all debris is cleaned up, remove the roots. All debris should be bagged or covered to eliminate the possibility of it blowing or falling out. Do not compost the debris. Burning the debris is the best option when possible.

Rubber boots can easily be cleaned after being worn in a landscape with boxwood.

After the cleanup is complete, remove outer clothing and launder or dispose of it. Laundering with detergent and hot water will kill spores. Boots should be thoroughly washed, and all tools and equipment should be carefully cleaned as well. Prune any other boxwood in the landscape to increase airflow to maintain dry foliage. Remove any limbs that touch the ground and might get rain splash, and be sure plants are properly mulched. Continue to monitor any boxwood during environmental conditions conducive to infection.

Since 2011 when Boxwood Blight was first discovered in the United States, there has been a great deal of research and much has been learned regarding control and living with the disease. The early assumption was that any landscape that is infected by Boxwood Blight would need to be destroyed. If the landscape is primarily Buxus ‘Suffruticosa’ (English), which is very susceptible to Boxwood Blight, then that would likely be true. At a minimum one should remove any plants in the vicinity of the infected plant(s) and closely monitor all other plants. However, there are landscapes of Buxus sempervirens (American), which have medium resistance to Boxwood Blight, which can survive and even thrive if certain steps are taken. Remove the diseased sections, prune to increase air movement within the plant, remove lower limbs, and mulch to lessen splashing of spores from the ground. In addition, the plants should be carefully monitored when infection is likely. A few well-timed sprays would also increase the chances of success.