The use of potato tubers in pathogenicity studies of Armillaria isolates *

The pathogenicity to potato tubers of seven isolates, representing four species of Armillaria , was investigated to test the use of tubers for rapid pathogenicity screening. The pathogenicity of the isolates to young conifers was either known or was established in a preliminary trial. The tubers were presented to small woody inocula under conditions that allowed infection by rhizomorphs. In all experiments isolates classed as highly pathogenic to young conifers infected a higher proportion of tubers than did isolates of low pathogenicity. However, the magnitude of the differences between isolates varied between experiments and, in some instances, particularly with isolates of A. ostoyae , the failure to produce rhizomorphs led to unclear results. Although the technique described did not seem reliable enough for routine testing of field isolates, the experiments did reveal differences between isolates in the length of rhizomorph which became attached to the host surface. This was consistently greater in isolates of low pathogenicity than in those of high pathogenicity and may be of significance to field behaviour.