A field of boxwood affected by Boxwood Blight.
Since its introduction to the United States in 2011, boxwood blight has been found in 26 states. Its affects have been seen from the Mid-Atlantic to the west coast in varying degrees. One of the biggest contributing factors to the prevalence and spread of this disease is climate. Boxwood blight requires specific environmental conditions which vary year to year. This fungal disease spreads by spores that move by water splashing, physical contact, or on debris. Infections occur during humid, warm periods, and can spread quickly when conditions are ideal.
The map above shows states with reported cases of Boxwood Blight by year.
Characteristic symptoms of Boxwood Bight include dark streaking on the branches, leaf spots, and defoliation.
As many growers recall, 2018 was a devastating year, especially in the Mid-Atlantic. With record breaking rainfall and extended periods of warm, wet weather, many growers, landscapers, and homeowners saw more cases of the disease than previous years. Mary Ann Hansen, who oversees the Virginia Tech Plant Disease Clinic reflects, “In 2018 when we had so much rain across the state, the VT Plant Disease Clinic received 188 boxwood samples that were positive for boxwood blight. This amounted to over 11% of our sample total.”
Bennett Saunders next to the Tye River at record high levels in September 2018 after another heavy rain storm.
In Piney River, VA we saw over 90 inches of rain throughout the year, compared to the annual average of about 45 inches. Reports of flooding and extended rain events throughout the spring, summer, and fall were followed by reports of infection.
Juxtaposed to the previous year, 2019 has been uncharacteristically dry, leading to fewer cases. Mary Ann from Virginia Tech reports, “In 2019, we only received 44 boxwood samples that were positive for boxwood blight, and this amounted to less than 3% of our total samples. The number of samples received by the Clinic doesn't necessarily represent the amount or severity of the disease across the state, but in this case, I think it probably did. The weather in 2018, especially in the fall, when boxwood blight tends to be a problem, was very conducive to boxwood blight, whereas the long drought and high temperatures in many areas of the state in 2019 were not conducive to boxwood blight. The fungal pathogen, Calonectria pseudonaviculata, does best at moderate temperatures with extended periods of leaf wetness.”
It is important to keep in mind the effect the environment can have on the spread of plant diseases. For more information on Boxwood Blight and how to prevent it, check out the Virginia Tech Boxwood Blight Task Force website or read more about it here.