Heterokaryosis is not required for virulence of Heterobasidion annosum.

Intensive sampling of the pathogenic fun- gus Heterobasidion annosum in white fir stands yielded a large percentage of homokaryotic isolates. To assess the stability and virulence of homokaryotic isolates, four homokaryons and four heterokaryons were in- oculated on 200 roots from 50 mature trees. Roots were sampled after 4 and 12 months. Eighty-three percent of the inoculated roots were infected, and 95% of the isolates retrieved were identified as the isolates originally inoculated. Homokaryons were not heterokaryotized and no significant colonization dif- ference was observed between homokaryons and het- erokaryons. One year after the removal of inoculated roots and the subsequent creation of large basal wounds, fungal isolates that had reached the root col- lar and colonized the main stem of the tree could still be isolated. Eight putative heterokaryons and six putative homokaryons were obtained from stems of a subsample of the inoculated trees and were grown in the laboratory. Six of eight heterokaryons re- mained unchanged, whereas all six homokaryons had been heterokaryotized.

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