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Resistance

Canker Elongation, Branch Death, and Callus Formation as Resistance or Susceptibility Responses in Populus tremuloides and Virulence or Avirulence Characteristics of Hypoxylon mammatum. D. H. Griffin, Professor, Department of Environmental and Forest Biology, College of Environmental Science and Forestry, Syracuse, NY 13210; P. D. Manion(2), F. A. Valentine(3), and L. Gustavson(4). (2)(3)(4)Professors and graduate student, respectively, Department of Environmental and Forest Biology, College of Environmental Science and Forestry, Syracuse, NY 13210. Phytopathology 74:683-687. Accepted for publication 9 February 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-683.

Five single-ascospore isolates of Hypoxylon mammatum were tested for virulence against nine clones of Populus tremuloides planted in a randomized complete block experiment. Measurements of canker length, branch death frequency, and callus formation frequency were made over a 16-mo period encompassing two growing seasons. There were significant differences among clones and isolates for all three measurements. The fungal isolates accounted for much more of the variation in canker lengths than did the aspen clones, while the fungal isolates and aspen clones varied to about the same extent in branch death and callus formation. Branch death and callus formation were mutually exclusive on a given canker, but showed no significant correlations in frequencies among the clones and isolates. Canker length showed a moderately negative correlation with callus frequency and a moderately positive correlation with branch death. These results are consistent with the hypothesis that branch death and canker length are indicators of virulence in the fungus. The varying degrees of interrelationships among canker length, branch death, and callus formation indicate the complexity of the host-pathogen interaction. There was considerable independence of these characteristics among both the aspen clones and the fungal isolates.