Screening for resistance to beech bark disease: Improvements and results from seedlings and grafted field selections

Beech bark disease (BBD) is an insect-disease complex that has been killing American beech (Fagus grandifolia Ehrh.) trees since the accidental introduction of the beech scale insect (Cryptococcus fagisuga) to Canada around 1890. Insect infestation is followed by infection with Neonectria ditissima or N. faginata. Mortality levels in the first wave of the disease can be as high as 50 percent, with consequent loss to stand health, merchantable timber, and many wildlife and ecosystem services. It is currently estimated that between 1 and 5 percent of the native American beech are resistant to beech bark disease, and resistance has been shown to be to the insect part of the complex. Recent work has shown that artificial infestation techniques can be used to screen seedlings for scale resistance. Here we present results from additional beech insect resistance screening experiments, including additional seedling families, grafted parental ramets of seedlings, and nonnative beech species. Results further confirm the utility of the screen to allow selection of better performing individuals, even within families that perform poorly overall, and to rank families for overall performance. When full-sibling families using parents of known scale phenotype were screened, an enriched proportion of resistant progeny were observed only in families with two resistant parents. Trees selected in the field can be grafted and the assay is useful to confirm the field-assessed scale resistant phenotype. We are currently identifying, grafting, and testing scale-resistant beech trees as part of a multi-state, multi-agency cooperative effort. Confirmed resistant genotypes will be used to establish seed orchards to supply regionally adapted disease-resistant beechnuts for use in restoration plantings and BBD management. The use of resistant parents is necessary to produce significant improvement over unselected seed lots. Key words: beech bark disease, beech scale, Cryptococcus fagisuga, Neonectria, scale resistance