Screening of Existing and New Boxwood Cultivars for Disease Resistance and In Vitro Fungicide Screening to Manage Boxwood Dieback Caused by Colletotrichum theobromicola

Abstract

Boxwood (Buxus spp. L.) is one of the most common and widely planted perennial ornamentals in both home gardens and commercial landscapes. Although considered robust, boxwood is susceptible to several plant pathogens. Boxwood dieback, a recently reported fungal disease caused by Colletotrichum theobromicola, has been spreading rapidly within the United States. Symptoms of boxwood dieback are difficult to distinguish visually in affected plants due to the symptom resemblance to other known biotic and abiotic diseases of boxwood. Previous studies have shown that it may take 2 to 3 months for the symptoms to appear under greenhouse conditions. Although a molecular method has been designed for early, rapid, and accurate pathogen detection, boxwood breeders, nursery growers, and landscape professionals are concerned about the lack of effective management practices. Therefore, the primary objectives of this study were to devise effective disease management strategies by screening cultivars to determine their susceptibility to boxwood dieback and screening various fungicides to determine their effectiveness in managing the disease. Host range studies were conducted by screening a wide variety of boxwood cultivars under greenhouse conditions. The boxwood cultivar ‘Little Missy’ showed much-delayed symptom expression compared with the rest of the cultivars. None of the 11 cultivars were found to be resistant to boxwood dieback. In vitro screening of nine fungicide treatments was conducted to determine inhibition of mycelial growth and spore germination for eight isolates of C. theobromicola collected from eight states in the United States. Of the nine fungicide treatments, difenoconazole + pydiflumetofen showed maximum inhibition of mycelial growth and spore germination at 1 ppm active ingredient, followed by fluxapyroxad + pyraclostrobin, and pyraclostrobin + boscalid at 5 ppm active ingredient. Azoxystrobin + benzovindiflupyr significantly inhibited mycelial growth at 1 ppm but reduced spore germination only at 10 ppm active ingredient. This study provides boxwood industry professionals with critical information pertaining to host susceptibility and fungicide efficacy that may be applied to effectively mitigate boxwood dieback and reduce its further spread.