Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2014
Publication Date: 3/1/2015
Citation: Guo, Y., Olsen, R.T., Kramer, M.H., Pooler, M.R. 2015. Effective bioassays for evaluating boxwood blight (Calonectria pseudonaviculata) susceptibility using detached stem inoculations. HortScience. 50(2):268-271.
Interpretive Summary: Boxwood (Buxus spp.) are slow-growing evergreen shrubs and small trees commonly grown as hedges and for topiary. Over 13 million boxwood plants are sold in the U.S. each year, with an annual market value of over $100 million. Boxwood blight, caused by the fungus Calonectria pseudonaviculata, is a destructive leaf-drop and stem-lesion disease, and is of significant concern throughout Europe, the U.S., and Canada. Although fungicidal management of the pathogen has been effective, the best long-term strategy for disease management is the development of blight resistant cultivars. The genus Buxus contains approximately 90 species, with more than 350 cultivars representing diverse forms, sizes, and foliage characteristics. Identifying which of this diverse material is most tolerant to boxwood blight is necessary for beginning a breeding program to develop blight resistant cultivars, and to screen the potentially thousands of seedlings from controlled hybridizations. We developed a simple and rapid bioassay for evaluating boxwood blight susceptibility using detached stem inoculations. We are continuing this work to correlate symptom expression on detached stems with whole-plant reactions, and also to determine the effect of plant physiological state on symptom expression.
Technical 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. 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 to quantify susceptibility but requires 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.