Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2016
Publication Date: 8/1/2016
Citation: Guo, Y.H., Olsen, R.T., Kramer, M.H., Pooler, M.R. 2016. Use of mycelium and detached leaves in bioassays for assessing resistance to boxwood blight. Plant Disease. 100:1622-2626.
Interpretive Summary: Boxwoods are an important landscape plant in the U.S., with an annual market value of over $100 million. A fungal disease, boxwood blight, was recently identified in the U.S. This disease causes leaf drop, stem lesions, and plant death, and threatens the boxwood industry in the U.S. and Europe. We developed a rapid and reliable laboratory assay that enables screening hundreds of boxwood cultivars and accessions and potentially thousands of hybrid seedlings for resistance to boxwood blight. This assay uses fungal mycelium rather than spores as inoculum, which is easier to prepare in large quantities for multiple assays. Identification of the least susceptible boxwood genotypes will be useful for informing best management practices in new boxwood plantings and for developing new blight-tolerant cultivars.
Technical Abstract: Boxwood blight caused by Calonectria pseudonaviculata is a newly emergent disease of boxwood (Buxus L.) in the United States that causes leaf drop, stem lesions, and plant death. A rapid and reliable laboratory assay that enables screening hundreds of boxwood genotypes for resistance to boxwood blight is needed to enable breeding and selection of resistant cultivars. Using eight boxwood cultivars with differing susceptibilities, we examined parameters for a screening assay comparing whole plant inoculation with detached leaf inoculation; use of mycelium vs. spores as the inoculum; comparison of times of the year for inoculation; and comparison of two leaf inoculation methods. Inoculation of detached leaves gave comparable results to inoculation of whole plants when compared across genotypes, although the detached leaf assay resulted in greater percentages of symptom expression. The time of year that we inoculated plants (spring, summer, or winter) did not affect the relative expression of symptoms among the most resistant and susceptible genotypes. Inoculating plants with mycelium was as effective as spore inoculation for causing disease symptoms that allowed us to distinguish the more resistant genotypes, yet mycelium inoculation was much easier to prepare in large quantities for multiple assays.