Survival of American Chestnut Trees: Evaluation of Blight Resistance and Virulence inEndothia parasitica

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Abstract
Large (22-114-cm diameter at breast height [DBH]), surviving American chestnut trees in the natural range were evaluated in 1978-1982 for blight resistance and/or the presence of E. parasitica strains with low virulence. Canker length measurements following inoculation of virulent E. parasitica on grafted scions, seedlings and excised stems indicated that some surviving trees are blight resistant. In situ inoculation trials on large, surviving trees with virulent E. parasitica indicated that superficial canker development is also an important characteristic associated with blight resistance in American chestnut. Strains of E. parasitica that produced short, superficial cankers on blight-susceptible American chestnut stump sprouts were associated with large, surviving trees, smaller trees (5-14 cm DBH [diameter, breast height]) growing near large trees, and small stump sprouts (4-19 cm DBH). Of 542 isolates tested, 113 (20.8%) had low or intermediate virulence. Eight of 8 low-virulence and one of 3 intermediate isolates tested contained ds[double-stranded]RNA. The incidences of low-virulence isolates in the E. parasitica populations from large, surviving trees and from small stump-sprout trees in Virginia-West Virginia [USA] were 14.9 and 4.3%, respectively, whereas the incidences of intermediate strains were 13.2 and 2.6%, respectively. Many American chestnut trees may survive because of the combined effects of blight resistance and low virulence of the pathogen, but others may survive because of a single factor.