Boxwood Blight - Memorable Tips for Easy Identification

This weekend, the local Lexington newspaper, the Herald Leader, published a story on boxwood and boxwood blight.  Thus, I anticipate an influx of suspect samples and concerns.  Also, as weather becomes cooler and more rain is upon us, it is possible that the disease may appear in Kentucky this fall.  Below is a refresher on this devastating disease:

Boxwood blight (Cylindrocladium buxicola) was reported in southern Ohio this spring, but has yet to be found in Kentucky.  Nursery growers in the northern counties are especially concerned about movement of the disease across state lines. 



Figure 1 & 2.  Boxwood blight is most easily recognized by leaf drop.  Photos by Kelly Ivors, NC State.

 

Symptoms of boxwood blight are different from some of the most commonly observed boxwood problems.  For example, stem blight and drought damage result in foliage turning bright bronze or straw-colored while remaining intact.  Boxwood blight, in contrast, results in rapid defoliation of plants (Figure 1& 2).  Another distinguishing symptom of boxwood blight is brown stem lesions that are easily recognized after leaf drop (Fig 3).  Earliest symptoms include leaf spots, but these spots often go unnoticed unless a persistent scouting program is in place (Figure 4).  Roots are not affected.

Figure 3.  Brown stem lesions, a distinguishing characteristic of boxwood blight, are often noticed after leaf drop.  Photo permission by Kelly Ivors, NC State.

 

Avoid unhealthy plants at all costs.  Homeowners should examine plants carefully before purchase, avoiding plants with leaf or stem lesions or an unhealthy appearance.  Growers should carefully inspect incoming plants and liners before introducing them into production areas. 

Cultural practices can help prevent conditions that are conducive for the fungal pathogen.  Space plants for air circulation and rapid drying of foliage.  Overhead irrigation should be avoided.  Fungicides are not available for management of boxwood blight.  Infected plants must be destroyed by burning or burying.

Figure 4.  Leaf spots, the earliest symptoms of boxwood blight, can be detected by scouting.  Photo permission by Kelly Ivors, NC State.

 

Report suspected cases of boxwood blight immediately to your local Extension agent or specialist or to the UK Plant Disease Diagnostic Lab. 

Figure 5.  Comparison of boxwood blight to Volutella stem blight and Macrophoma blight.

 

Chrysanthemum Dieback – Rhizoctonia is Likely the Culprit

Dense canopies and frequent overhead irrigation of fall mums create ideal conditions for many plant pathogens, especially web blight and stem rot.  In greenhouses, plants grow under tight spacing and high humidity.  Furthermore, plant compactness creates microclimates within canopies.  These extreme conditions are conducive for growth of the web blight and stem rot pathogen, Rhizoctonia solani.  Unfortunately, some of these diseased plants make their way into retail centers and our front porches.

Figure 1– Stems and crowns infected by Rhizoctonia solani result in dieback in the upper canopy.  Photo by NC State.

 

Symptoms:  Stem or leaf dieback is often the first recognizable symptom.  This dieback is caused by infections of stems at the base of shoots and branches or by infection of roots and lower stems (Figure 1).  Opening up the canopy will often reveal stem lesions and/or webs or strands of fungal tissue (mycelia) (Figure 2).  Infected leaves appear water-soaked or necrotic, often becoming matted together with the web-like mycelia.

Figure 2– Under wet conditions, Rhizoctonia may spread to upper plant parts, inducing web blight symptoms.

 

Disease Management:  Both cultural practices and fungicides are required for proper disease management.  Keep foliage dry by avoiding overhead watering, by increasing air circulation (wider spacing, increased sunlight), and by practicing strict sanitation (remove diseased tissue and clean up fallen leaves).  Homeowners may use propiconazole (Green Light Systemic Fungicide), mancozeb (Mancozeb or Dithane), or captan (captan) fungicides.  See the Homeowner’s Guide to Fungicides.  Commercial growers and landscape professionals may take advantage of a wider array of fungicides, such as the active ingredients azoxystrobin, fludioxonil, iprodione, PCNB, pyraclostrobin, thiophanate-methyl, trifloxystrobin, and triflumizole.  Refer to the following publications for specific trade names:  Fungicides for Management of Diseases in Commercial Greenhouse Ornamentals and Fungicides for Management of Landscape Woody Ornamental Diseases.  Consult labels for specific information.