Boxwood blight is a relatively recent disease that is devastating Buxus plants on the east coast, and which has made limited inroads in Oregon.
We recently received, from a landscape in Marion County, a boxwood sample that was very heavily infected with the boxwood blight fungus. Although symptoms must have been obvious earlier in the year, we did not receive a sample until extensive damage had been done.
Boxwood (or box) blight is caused by the fungus Calonectria pseudonaviculata, which produces spores abundantly, each of which can initiate a new infection. Infected leaves fall to the ground, and the fungus can remain there, capable of causing infection for as long as five years. Microsclerotia, tiny bits of fungal material that are resistant to environmental degradation, may be produced in the diseased tissue, which can contribute to the fungus’s survival.
Symptoms are relatively distinctive: black spots of irregular shape on leaves, twigs, and branches are characteristic. Leaves that turn orange only and do not have black spots do not have boxwood blight. If you see dark spots on your boxwood plants, take a sample to your local county Extension office for advice. Additional information on the disease and its management is available at in the PNW Plant Disease Management Handbook.