An Improved Leaf Disc Bioassay for Detecting Calonectria pseudonaviculata in Soil and Potting Media

Boxwood blight caused by Calonectria pseudonaviculata results in severe defoliation and dieback to boxwood (Buxus spp.). The pathogen was first described in the United Kingdom and New Zealand in the mid to late 1990s and has since spread throughout Europe and most recently to the United States and Canada. While many Calonectria spp. have an epidemiologically significant soil phase, little is known of the role of the soil phase of C. pseudonaviculata in the epidemiology of boxwood blight. We optimized a leaf disc bioassay for detecting and quantifying this pathogen in soil and compared this bioassay with a standard soil plating assay originally developed for quantifying Calonectria using a Suffolk sandy-loam soil. Additionally, the sensitivity of both assays was compared among three distinct soil types (sand-loam, silt-loam, and sand-peat potting media). The optimal incubation time for baiting C. pseudonaviculata from soil using the leaf disc bioassay was 96 h. The optimal soil moisture for the bioassay was 1,000% of field capacity (flooded with 3 to 5 mm water). The leaf disc bioassay was able to detect C. pseudonaviculata at levels as low as 1 microsclerotium/ g soil while the soil plating bioassay was unable to detect the pathogen below inoculum levels of 10 microsclerotia/g soil in the Suffolk sandy-loam soil. Soil type had a significant impact on the sensitivity of both assays.