Relative abundance and potential dispersal range of intersterility groups of Heterobasidion annosum in pure and mixed forests

In Europe the forest pathogen Heterobasidion annosum (Fr.) Bref. includes t he S, P, and F intersterility groups (ISGs), each displaying a preferential specialization on Norway spruce (Picea abies (L.) Karst.), pine, and silve r fir (Abies alba Mill.), respectively. In this paper, we present data abou t (i) H. annosum ISGs frequency in different forest types, (ii) the degree of host specificity of each ISG, (iii) the significance of the potential mo vement of airborne spores among forests, and (iv) the occurrence of S-P chi meras in the northwestern Alps. Using woody spore traps, we sampled natural pure spruce and fir forests and a mixed spruce-fir forest. The ISG of 582 spores was determined by ISG-diagnostic taxon-specific competitive priming (TSCP) polymerase chain reaction (PCR) combined with PCR-mediated detection of ISG-specific introns in the ML5-ML6 DNA region of the mitochondrial lar ge ribosomal RNA (mt LrRNA). All three ISGs were found, and a strong correl ation was observed between the F ISG and fir and the S ISG and spruce. In t he mixed forest, no clear relationship between tree host species and host-s pecialized ISGs was found. In spite of a relative dominance of fir in the o verstory of the mixed stand, the fir-associated F ISG represented only 11% of the total number of spores collected. This discrepancy was explained by the recent establishment of firs at this site. No S-P nuclear-mitochondrial chimeras were found. This suggests limited gene flow between these ISGs.