Toward Enhanced Chemical Management of Boxwood Blight: Spray Coverage on Boxwood Canopies and Detection of Flutriafol Following Drench Application

Boxwood blight ( Calonectria pseudonaviculata) has increased nursery production costs and changed marketing strategies since it was first identified in the United States in 2011. Cultural and chemical practices manage boxwood blight, but surveys still find low levels of disease in parts of the canopy that are hard to cover with fungicide sprays. Spray coverage trials were conducted on boxwood shrubs grown in experimental plantings and at two production nurseries. Four sprayer types were evaluated at various rates per area using water-sensitive cards. Coverage on boxwood bushes ranged from 0.03 to 47.2%, 0.2 to 7.1%, 0.8 to 3.0%, and 0.4 to 4.3% on cards facing toward the inside of the boxwood canopy and from 4.3 to 100%, 0.9 to 16.8%, 0.3 to 17.3%, and 1.6 to 21.6% on cards facing toward the sprayer when using an airblast sprayer, cannon sprayer, over-the-row boom sprayer, and air-assisted over-the-row boom sprayer, respectively. Thorough coverage of boxwood canopies with foliar sprays was difficult for tightly sheared plants or cultivars with a tight growth habit regardless of the sprayer or rate used. This could perpetuate spread of boxwood blight within and between nurseries and regions. Greenhouse experiments indicated that flutriafol was detectable in leaves 1 to 2 years after a single drench application onto small potted plants. Drench applications could help protect plants for extended periods as they travel from wholesale nursery to retail nursery to planting out in a landscape. When relying on foliar fungicide applications for boxwood blight management, growers should use as high a volume as practical with their spray system. [Formula: see text]