Chestnut Leaf Inoculation Assay as a Rapid Predictor of Blight Susceptibility

Newhouse, A. E., Spitzer, J. E., Maynard, C. A., and Powell, W. A. 2014. Chestnut leaf inoculation assay as a rapid predictor of blight susceptibility. Plant Dis. 98:4-9. American chestnuts (Castanea dentata), effectively eliminated from eastern North America by chestnut blight in the twentieth century, are the subject of multiple restoration efforts. Screening individual trees (or tree types) for blight resistance is a critical step in all of these programs. Traditional screening involves inoculating stems of >3-year-old trees with the blight fungus (Cryphonectria parasitica), then measuring resulting cankers a few months later. A quicker, nondestructive, quantitative assay, usable on younger plants, would enhance restoration efforts by speeding the screening process. The assay presented here meets these requirements by inoculating excised leaves with the blight fungus and measuring resulting necrotic lesions. Leaves can be collected from few-month-old seedlings or fully mature trees, and results are measured after less than a week. Leaves from several lines of both American and Chinese chestnuts were inoculated, as well as the congener Allegheny chinquapin, and experimental leaf assay results correlate well with stem assay results from these species. Inoculations with virulent and hypovirulent blight fungi strains also showed relative patterns similar to traditional inoculations. Given the correlations to established stem assay results, this procedure could be a valuable tool to quickly evaluate blight resistance in American chestnut trees used for restoration. The American chestnut (Castanea dentata (Marshall) Borkh.) was once a widespread and integral part of the ecology and economy of the eastern United States. The timber was strong, straightgrained, and rot-resistant, and the nuts provided nutrition and income to humans, as well as a reliable mast crop for wildlife. However, starting around the turn of the twentieth century, chestnut blight eradicated nearly all mature American chestnut trees throughout the chestnut’s North American range. Chestnut blight is caused by the necrotrophic fungal pathogen Cryphonectria parasitica (Murr.) Barr., which colonizes bark wounds, causes cankers, and eventually girdles trunks of susceptible trees. No practical prevention or treatment methods have been found for landscapescale use, and various restoration programs have been underway for many years with the goal of producing blight-resistant trees. Success in these programs is determined by field inoculation trials, which typically involve inoculating trees that have been growing outdoors for at least 3 years (9,12). While American chestnut grows relatively quickly compared to other North American hardwoods, it still may be 5 years or more before a given tree can be screened for blight resistance.