Long-Term Efficacy of High-Volume Application of Triazole Fungicides (FRAC 3) Against Boxwood Blight

Despite the availability of cultural and chemical tactics, boxwood blight (BWB; caused by Calonectria pseudonaviculata, CPS) remains a major concern for boxwood growers. Surveys indicate that BWB is prevalent in the boxwood canopy, where fungicide spray coverage is difficult. The efficacy of five triazole fungicides (FRAC group 3) was evaluated over time using detached leaf assays. The triazole fungicides flutriafol, myclobutanil, propiconazole, prothioconazole, and tebuconazole were evaluated in containerized Buxus ‘Green Velvet’ using foliar spray and soil drench treatments. In the field, Buxus ‘Winter Gem’ was treated with drenched flutriafol or sprenched flutriafol or propiconazole. Leaves from multiple canopy locations were challenged with CPS in a time series of detached leaf assays. All fungicide applications resulted in lower disease severity compared with no-fungicide control treatments. In containerized boxwood, high efficacy occurred throughout the canopy, which lasted up to 12 weeks regardless of application method. The most efficacious treatments were the tebuconazole spray or flutriafol drench. In field-grown boxwood, flutriafol treatments resulted in only sporadic efficacy, and propiconazole efficacy was seen only in the first few weeks. Leaf position had significant effects on disease severity; however, efficacious treatments were able to lower disease severity regardless of the leaf location or application method. These results demonstrate the potential for triazole fungicide treatments to lower disease severity in boxwood canopies that are difficult to spray and resulted in long-term protection. [Formula: see text]