Growers in the Midwest should be monitoring their crops for boxwood blight, a serious fungal disease that has been confirmed in Illinois.
In late 2016, two boxwood samples were submitted to the University of Illinois Plant Clinic. Both samples came from northeastern Illinois and both were from recent landscape additions (one location was planted in 2016, the other was planted in 2015). While the characteristic leaf spots were not apparent on the samples, defoliation and stem cankers were noted.
The samples were quarantined and, after sufficient incubation, fungal spores consistent with Calonectria spp. were recovered. The Illinois Department of Agriculture was notified, and samples were sent to USDA’s Animal Plant Health Inspection Service Laboratory in Maryland, where the genus identification was confirmed. Species identification is ongoing.
According to University of Illinois Extension, boxwood blight is a potentially devastating disease affecting members of the Buxaceae family. It has been found on boxwood, pachysandra, and sarcococca. The disease is caused by the fungi Calonectria pseudonaviculata (syn. Cylindrocladium pseudonaviculatum and C. buxicola) and Calonectria henricotiae. To date, C. henricotiae has not been found in the U.S.
Boxwood blight was formerly federally regulated, but is now regulated at the state level. While it can cause widespread death of hosts in the environment, the spores of the pathogen do not appear to travel extensively, reducing its overall impact. However, in production facilities where equipment can be contaminated and expose hundreds or thousands of plants, the pathogen is a much larger concern.
Symptoms of boxwood blight include leaf spots, stem cankers, and defoliation. Leaf spots usually appear as light or dark-brown circular lesions, often surrounded by a large yellow halo. If the infection occurs near the margin of the leaf, the lesion may be semi-circular or V-shaped. Stem cankers are easiest to see on new, green stem tissue. The cankers are dark brown or black, and are often linear or diamond-shaped. Defoliation occurs as the final symptom.
Because these symptoms can be similar to other common fungal and environmental problems on boxwood, experts from the University of Illinois strongly suggest submitting samples to your local plant clinic for confirmation. You should also be scouting boxwood and pachysandra plants, especially those that were installed in the last few years or plants that are near host plants that were planted recently.