Variation in rust virulence and host resistance ofMelampsoraon black cottonwood

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Abstract
Disease severity, as expressed by spore production rate, was compared in a test of 14 clones of black cottonwood (Populus trichocarpa) inoculated with 10 isolates of melampsora rust (Melampsora occidentalis); both the clones and the isolates were collected from their natural pathosystem. Spore production rate was measured by average daily production on leaf disks over a period 2 .times. the latent period. The overall average spore production during the time from inoculation through a period equal to 2 .times. the latent period was 747 spores/disk/day. The latent period ranged from 6 to 12 days with a median of 8 days. Clones as well as isolates differed significantly in their contributions to spore production, but there was no differential interaction between clones and isolates. Analysis of variance of total spore production, total pustules, and latent period gave the same results. The number and urediniospores per uredinium varied significantly between clones but not between isolates. The absence of qualitative resistance and virulence indicates that qualitative interactions do not play a major role in disease in this natural pathosystem. This suggests that the resistance of cultivated black cottonwood to melampsora rust is unlikely to be readily overcome in genetically uniform plantations.