Good management practices can help prevent this disease from infecting your stock and spreading further in the industry. Sanitation is key!
- Train your staff to recognize symptoms of box blight and scout frequently during conducive weather in spring, summer, and fall
- Purchase incoming plants from certified, reputable growers and inspect new liners and plants upon delivery
- Never introduce suspicious looking or unhealthy plants into a commercial production area or landscape
- Keep new stock separated from existing plants
- Always practice good sanitation measures when working with boxwood
- Apply fungicides preventively in commercial nurseries in counties suspected to have box blight during conducive weather in spring, summer & fall
- Get an accurate diagnosis quickly if you start to notice anything odd or unhealthy about your boxwood plants
Whatever you call it – boxwood blight, box blight, Cylindrocladium box blight, blight disease of boxwood, boxwood leaf drop – this disease of a nursery industry staple is a challenge. First confirmed in the U.S. in October 2011, the fungus that causes this disease has been detected in Connecticut, Maryland, Massachusetts, New York, North Carolina, Ohio, Oregon, Pennsylvania, Rhode Island and Virginia, as well as the Canadian provinces of British Columbia, Ontario and Quebec.
The fungus that causes box blight is widely known as Cylindrocladium buxicola, although it’s sometimes referred to as Cylindrocladium pseudonaviculatum or Calonectria pseudonaviculata. Different names; same fungus. And same dramatic effects.
Significant progress has been made in the past year, however, to help stem the tide of boxwood blight. We’ve identified cultivars that show resistance, and our study has proved the importance of keeping things clean. Practicing good sanitation – always – is critical.
Symptoms of blight
Box blight can significantly impact the appearance and aesthetics of boxwood, because the foliage typically becomes blighted and drops from the plant. Symptoms of the blight include dark- or light-brown, circular leaf spots often with darker margins; dark stem cankers or black “streaks” on stems; straw- to bronze-colored, blighted foliage; and leaf drop. Leaf spots may grow together to eventually cover the entire leaf. In container boxwood, sometimes only the lower foliage and stems become infected, leaving the tops green and making the plant appear top-heavy. On large field-grown or landscape plants, typically sections of the plant closest to the ground on the shaded side will become blighted. Blighting and defoliation can occur quickly with complete leaf loss under warm (64 to 80°F) and humid conditions. Shady conditions favor disease development.
Dark brown to black stem lesions or streaks are typical symptoms of box blight.
Under high humidity, white fuzzy masses consisting of numerous clumps of spores on infected stems and leaves may be observed with a hand lens. However, these fuzzy masses are often not observed if environmental conditions are not just right – that is, if relative humidity or temperatures are too low. Most boxwood cuttings are propagated in humidity chambers or shaded structures, and liners are often grown in shade; these are conditions that promote disease development. Therefore, young boxwood plants are especially at risk if the pathogen is unintentionally introduced into the growing area.
Circular leaf spots appear before leaf blighting.
Photo courtesy of Kelly Ivors and Landis Lacey unless otherwise noted.
Mature field-produced American boxwood appear infected with boxwood blight. Note the large amount of leaf drop, which is common when large plants get infected. This usually is more pronounced on the shadier side of the plant near the bottom (where leaves stay the wettest).
Photo courtesy of Kelly Ivors
Even after defoliation, root systems of box blight-infected plants remain healthy and intact, unlike roots infected with Phytophthora. The stems of infected boxwood typically remain green under the outer bark until a secondary invader or opportunistic pathogen attacks this tissue and eventually kills the e box blight pathogen is often confused or associated with the secondary fungus Volutella buxi, known to cause Volutella blight. However, Volutella alone will not cause box blight.
Pruning of lower branches helps reduce reinfection from fallen debris and runoff water.
Plant species within the genera Buxus, Pachysandra and Sarcococca have been reported as hosts to this fungus. There is limited information, however, about the role Pachysandra (spurge) and Sarcococca (sweetbox) species play as possible vectors for initiating the disease in boxwood. Although the full host range of this fungus has not been finalized yet, it is believed that only plants within the family Buxaceae can be infected by the pathogen.
Research conducted at North Carolina State University in 2012 indicate a wide range in susceptibility of boxwood cultivars to box blight. The good news is that a number of B. microphylla cultivars demonstrated resistance to this disease. B. sempervirens types were more susceptible in general, with B. sempervirens ‘Suffruticosa’ (English boxwood) and B. sempervirens ‘American’ (common or American boxwood) appearing especially susceptible. Differences in susceptibility are possibly due to plant genetics, as well as physical features of the plant, such as a dense and compact leaf canopy.
Under high humidity, white fuzzy masses consisting of numerous clumps of spores on infected stems and leaves may be observed with a hand lens.
Container plants may appear top heavy from loss of lower leaves due to boxwood blight. Roots, however, are healthy.
Photo courtesy of Kelly Ivors
Keep it clean!
The best way to sanitize tools is to dip them for 10 seconds into any of the following products and then allowing the tools to dry:
- ethyl or isopropyl alcohol at 70 to 100 percent (most Lysol formulations, grain/rubbing alcohol)
- sodium hypochlorite (10 percent Clorox or other brands of household bleach – the same as 1 part bleach to 9 parts clean water made fresh each day)
- phenolics at 0.4 to 5 pecent (trade name Pheno-Cen)
- quaternary ammonium at 0.5 to 1.5 percent (trade names Greenshield, Consan Triple Action 20, Physan 20).
Follow label precautions for all products.
How box blight gets around
The primary way this disease spreads is the movement of infected plants and cuttings. Another significant way this disease spreads locally in nurseries and landscapes is through contaminated tools and workers as well as by movement of boxwood debris – especially fallen leaves.
Activities such as pruning also contribute to spread. The sticky fungal spores are readily moved on contaminated tools and equipment, and on workers and animals that come into contact with infected foliage. Water also plays a key role in transmission – the fungus may be spread by splashing rain or overhead irrigation, flood water, runoff water or in droplets carried by the wind. Spores are unlikely to travel long distances by wind alone. The greatest potential for long-distance transport of box blight is the movement of infected plants, cuttings, people and tools.
Help is on the way! Identifying measures for preventing and managing box blight in commercial nursery and field settings is a work in progress, and researchers currently are evaluating fungicides and sanitizers, as well as identifying resistant boxwood cultivars.
Leaf spots often have a darker margin, ranging from brown to purple.
It’s important to remember that once this fungus has been introduced, limiting movement of this sticky, contagious fungus is very difficult. Prevention can only be accomplished by following good sanitation practices – always. These include:
- disinfect pruners and other tools frequently within and between different blocks of plants, especially between different field locations or landscapes in areas suspected to have box blight;
- never work in fields when the plants are wet;
- wear clean, disposable booties or wash off debris and dirt entirely from soles of shoes between different boxwood fields or landscapes, especially in areas suspected to have box blight;
- wear clean Tyveks or launder clothes between different field locations in areas suspected to have box blight;
- burn or bury box-blight infected plants on-site (composting is not recommended); and
- never discard boxwood waste material where it could contaminate other boxwood plants.
Boxwood cultivar susceptibility trials were conducted in summer 2012 at North Carolina State University’s Mills River facility.
It’s best to dispose of plants with box blight, as the chance of further spreading this fungus is highly probable. However, strict adherence to good management practices, with special attention to sanitation, can help stem boxwood blight and lessen its impact on this beloved plant.
Kelly Ivors is an extension plant pathologist in the Department of Plant Pathology at North Carolina State University in Mills River, N.C. She can be reached at email@example.com.