Boxwood blight, a serious fungal disease, has been confirmed in Illinois. According to a University Diagnostic Outreach Extension Specialist, two boxwood samples were submitted to the University of Illinois Plant Clinic in late 2016. The samples came from Lake and Cook Counties in northeastern Illinois. Both were from recent landscape additions.
“Although the characteristic leaf spots were not apparent on the samples, defoliation and stem cankers were noted,” says Diane Plewa.
The samples were quarantined and, after sufficient incubation, fungal spores consistent with the Calonectria spp. fungi were recovered. The Illinois Department of Agriculture was notified, and samples were sent to the United States Department of Agriculture Animal Plant Health Inspection Service Laboratory in Maryland, where the genus identification was confirmed. Species identification is ongoing.
“To our knowledge, the infected plants where not from Illinois production facilities,” Plewa adds.
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,” says Suzanne Bissonnette, director of the U of I Plant Clinic.
“Because these symptoms can be similar to other, common fungal and environmental problems on boxwood, we strongly suggest submitting samples to the U of I Plant Clinic for confirmation. We recommend 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.”
Boxwood blight is a potentially devastating disease affecting members of the Buxaceae family. The disease 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 United States.
Bissonnette adds that boxwood blight was formerly federally regulated, but is now regulated at the state level. “Although 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.”
The pathogen was identified for the first time in the United States in 2011, and has since been found in 18 states. Most are located in the eastern part of the country, though confirmations have been made in Missouri and Ohio.