Detached Leaf Assays Reveal Long-Term Efficacy of the Systemic Fungicide Flutriafol Against Boxwood Blight

Intensive fungicide regimes are commonly used on <i>Buxus</i> spp. in the United States to manage boxwood blight (BWB), a plant disease caused by <i>Calonectria pseudonaviculata</i> (CPS). BWB causes losses in both nursery production and landscapes. The disease is often found within boxwood canopies, where achieving adequate spray coverage and penetration is difficult. The efficacy of the systemic triazole fungicide flutriafol (Fungicide Resistance Action Committee group 3) was evaluated over time by detached leaf assays. The efficacy of several concentrations of flutriafol against three CPS isolates and at several spore concentrations was assessed in the lab. Flutriafol was also applied by drench or sprench (portmanteau of spray and drench, a high-volume foliar application with substantial soil wetting) to container- or field-grown boxwood plants. The triazole fungicide propiconazole, which was previously shown to be efficacious against BWB, and no-fungicide water application were included as control treatments. Leaves of boxwood 'Common', 'Winter Gem', 'Green Velvet', or 'Suffruticosa' were collected periodically, challenged with CPS conidia, and rated for percent symptomatic area over 14-day incubations. In general, lower disease incidence and severity developed on leaves collected from fungicide-treated plants than on leaves from water-treated control plants regardless of the application method. Flutriafol provided protection from CPS infection on leaves from drench-treated boxwood for at least 20 weeks. However, there was a delay of 1 to 4 weeks observed between fungicide application and maximum efficacy depending on treatment and trial. This delay was shorter among leaves from plants treated with higher flutriafol doses or via sprench applications.