Boxwood blight, caused by the fungal pathogen Cylindrocladium pseudonaviculatum, was found for the first time in the United States in 2011. The origin of the pathogen is unknown. The first reported infestation in the U.S. was in a North Carolina nursery. The disease came to Virginia on plants from that nursery. It is not known how it was introduced to North Carolina, but it has been found in surrounding states since 2011, and now has shown up in Georgia.
According to UGA Extension Service plant pathologist Dr. Jean L. Williams-Woodward, boxwood blight has been confirmed in two residential landscapes in the Buckhead area of Atlanta. The source of the introduction to one of the landscapes is unknown as new boxwood plants were not introduced into the landscape. The spores of the pathogen are very sticky and it is possible that the disease was introduced on workers’ tools or clothing. Plants in the second landscape were newly introduced from North Carolina. Once introduced, the disease can be devastating to boxwoods. In June, it was also confirmed in a retail nursery in Tennessee on plants coming from a large commercial nursery in Oregon. It is likely that the infected plants were shipped to other states, including Georgia.
Initial symptoms of boxwood blight include circular, tan leaf spots with a dark purple or brown border and black stem lesions or blackening of the stems. In warm, humid conditions, the fungus produces clusters of white spores on the underside of leaves and on stems. Infected leaves become tan and readily drop from the plant, leaving bare stems. Sections or whole plants turn tan and eventually die. The disease can resemble Volutella blight, except that the leaves often remain attached to the stems with Volutella, as well as symptoms of root stress or Phytophthora root disease. Boxwood blight can move quickly through plants, gardens and nurseries under favorable environmental conditions. Dense shade, humid, warmer and wet conditions favor disease development. The pathogen requires extended periods of leaf wetness, 24 to 48 hours, to infect. Under these conditions, leaf spots can develop within days of infection.
Dr. Williams-Woodward said the best control is exclusion. Do not introduce the disease on infected plants or tools. Inspect all new boxwood plants for symptoms of the disease. Be sure to check the lower leaf canopy and interior stems. Keep new plants isolated and separate from existing boxwoods. Do not apply fungicides to plants in isolation that would mask symptom development. Monitor plants for at least four weeks before introducing them into existing plantings. If Boxwood blight is detected, the infected plants and all fallen leaf debris needs to be bagged on-site and removed from the area to be buried in a landfill. Transport plants in closed bags. Leaf litter blowing from open trucks could spread the disease to plantings along the roadway. Fallen leaf debris should be vacuumed and bagged, burned on-site or buried. Debris should not be composted. The fungus also produces microsclerotia (small clumps of fungal hyphae) within roots and leaf debris of infected plants that allows the fungus to survive for years. Removal of existing garden soil and replacing it with new soil is an option, but there is no guarantee that this will completely remove the pathogen.
Boxwood blight cannot be controlled with fungicides. Fungicides are effective only when applied preventively. Fungicide efficacy trials have shown that those containing chlorothalonil (Daconil, Spectro, Concert II) and fludioxonil (Medallion, Palladium) provide the best control. Fungicides containing azoxystrobin (Heritage), pyraclostrobin (Pageant), trifloxystrobin (Compass), and thiophanate methyl (Cleary 3336) provide fair to good preventative control. Spraying after the disease is present will not control it.