Host-Pathogen Interactions as Measured by Bioassay of Metabolites as Produced byHypoxylon mammatumwith its HostPopulus tremuloides

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Abstract
Five single-ascospore isolates of H. mammatum [stem canker] were used to produce metabolites in a chemically defined broth medium and in grain cultures. After treatment to remove high-molecular-weight materials, the culture filtrates and grain culture extracts were concentrated and tested for toxic activity against nine clones of P. tremuloides by using a leaf-puncture bioassy. The intensity of the reaction was measured by testing serial dilutions of the samples, and sensitivity of the host was measured according to the size of lesion produced at each dilution. The response was dependent on time and concentration of the materials. Both metabolite production methods gave preparations that yielded similar results. The aspen clones could be classified into 3 sensitivity groups by the bioassays, high, medium, and low. Analysis of variance indicated that horizontal response (clone main effects) was most important, but significant (although quantitatively less important) vertical response mechanisms (clone by isolate interactions) were also present. The host reactions were also compared to canker development data and showed negative correlation to canker length and positive correlation of frequency of callus formation. These correlations suggest that the bioassay of the sensitivity to the metabolites produced in culture by these methods measures the ability of the aspen clones to respond to elicitors. Metabolite-producing abilities of the fungal isolates were not related to their canker-forming abilities when inoculated into the trees.