Effective bioassays for evaluating boxwood blight susceptibility using detached stem inoculations

Abstract

Two simple and rapid in vitro bioassays using detached stems were developed for evaluating the susceptibility of boxwood genotypes to the blight disease caused by Calonectria pseudonaviculata (Crous et al.) L. Lombard et al. Individual leaves were inoculated on detached stems or entire detached stems were sprayed to assess susceptibility. Both assay systems were optimized for inoculum concentration and disease rating time. The assay methods described here require minimal plant material and inoculum, especially the leaf inoculation assay, which uses as few as six leaves per stem and 500 spores per leaf for inoculation. The stem spray inoculation produced less variable results and was easier for quantifying susceptibility but required more inoculum than the leaf inoculation assay. No differences between the assays were found for the cultivars tested. The leaf inoculation assay is best used when limited plant material or inoculum is available; the spray inoculation of detached stems is suitable when larger plants are available.