Diversify Your Filler Options - Not With Boxwood

Boxwood (Buxus) provides versatile filler material for cut flower designs, and is especially popular around the winter holiday seasons.

In 2013 I compiled a summary of the best boxwoods for cut stems, based on evaluations we had conducted over several years. We were looking for species and cultivars that were least susceptible to boxwood leafminers and boxwood mites, and were good for cut stem use. Leafminers and boxwood mites are important considerations for those harvesting stems, as they make the foliage unattractive—if not completely unusable—for commercial use. So these data are still valid.

Times change, and sometime very rapidly. Boxwood may no longer be such a good choice for cut flower growers unless you already have established plants, and are not bringing in plants from outside sources. What changed? The answer is boxwood blight, caused by Cylindrocladium buxicola, an invasive fungus that hit the United States and hit it hard. This disease has been around in Europe, where it was described by a plant pathologist, and is now an American problem. It is not known how it got to Europe or how it entered the United States.

Boxwood blight was first confirmed in the U.S. in October of 2011 and has spread rapidly. Since it was found first in Connecticut and North Carolina, it has also cropped up in Ohio, Delaware, Oregon, Massachusetts, Maryland, New Jersey, New York, Rhode Island, Pennsylvania, and Virginia, as well as British Columbia, Ontario, and Quebec. The disease is introduced with new plant material or infested plant parts. If your plants are well established, I recommend you not bring in new boxwoods until we have a really good way of ensuring new material is clean. If you must bring in other plants I would make sure the supplying nursery is following the American Nursery and Landscape Association’s standards for monitoring and treating this disease.

Blight Symptoms

Boxwoods have always had lots of disease problems but this one is very serious. Severe infestations can result in complete defoliation. Some species of boxwood suffer from milder cases, which still result in significant leaf drop and stem lesions, creating bare and brown patches.

Initial symptoms appear as dark or light brown spots or lesions on the leaves, often with dark borders. The spots enlarge and then coalesce, often with a concentric pattern. Infected leaves then turn brown or straw colored. Defoliation often occurs very quickly after foliar symptoms. Stems can develop black to brown colored cankers with angular, diamond-shaped patterns.

Samples of boxwoods can be double bagged (plastic bags) and sent to your state university plant pathologist or the pathologist at the state department of agriculture for identification.

North Carolina State University published a list of boxwood varieties they felt were somewhat resistant to boxwood blight which gave us a glimmer of hope in 2012. Since then research showed this hope may have been premature.

Plant pathologists from NCSU recently shared pre-publication results regarding what they have called their “Boxwood Blight Trojan Horse Study.” In the study, graduate student Miranda Ganci and her professors Drs. Kelly Ivors and Mike Benson evaluated the ability of cultivars to be tolerant of boxwood blight.  According to their report, the tolerant varieties, which showed minimal symptoms of infection following inoculation, were still able to produce viable spores and efficiently spread the infection. These results suggest that boxwood plants considered tolerant are still capable of carrying and spreading the disease despite a general healthy appearance.

Not Just Boxwood is at Risk

The pathogen Cylindrocladium buxicola, (=Calonectria pseudonaviculata– old name) has shown an ability to infect every species of the Buxaceae. A recently published article includes the native Allegheny spurge (Pachysandra procumbens), which is found in isolated pockets from Florida to Pennsylvania, as a potential host.

Plant Health Progress published an article, by lead author Dr. Jim LaMondia of the Connecticut Agriculture Experimental Station (CAES), describing the ability of boxwood blight to infect native pachysandra under lab conditions. The same lab had conducted similar work last year showing the susceptibility of Japanese spurge to the pathogen. Subsequently, more than twenty infections of Japanese spurge have been found in landscapes in Connecticut.

Adding to the concern is the study’s conclusion that the infections on Allegheny spurge appear to be more aggressive than what was found on Japanese spurge. In the case of our native pachysandra, necrotic lesions occurred on the stems and leaves. Stems are eventually girdled, causing shoot and/or plant death. Furthermore, sporulation of boxwood blight was greater on Allegheny spurge than its Japanese counterpart.

If you use spurge foliage as filler, avoid planting it near boxwood.

Research scientists continue to pursue new identification and treatment tools while the Agriculture Research Service of the USDA has begun evaluating varieties for boxwood blight-resistant genes. The Boxwood Society of America is conducting a special seminar on boxwood blight on May 12 at the USDA facility in Beltsville, Maryland. The pathologist that original identified the disease in England will be presenting at the conference as well as Dr. Kelly Ivor, one of the NCSU researchers.

If you’re growing boxwoods for cut stems, check them regularly during the season for cankers on the stems, spotting of foliage, and defoliation.  Visit http://americanhort.theknowledgecenter.com to see pictures of symptoms. You do not want sell cut stems from infested plants to be used in arrangement and chance spreading this disease to new sites. If you are willing to spray fungicides you can look at the recommendations from North Carolina State University Extension at the Americanhort website.

It would not be a bad idea to start looking around for other plant material that can be used as fillers in arrangements until this whole boxwood blight problem is sorted out and a really good control or resistant varieties are developed. Just like the stock market: it is best to be diversified.

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]