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

The forest pathogen Heterobasidion annosum (Fr.) Bref. includes in Europe the S, P, and F intersterility groups (ISGs), each displaying a preferential specialization on Norway spruce, pine and silver fir, respectively. In this paper, we present data about i) H. annosum ISGs frequency in different forest types, ii) the degree of host specificity of each ISG, iii) the significance of potential movement of airborne spores among forests, and iv) the occurence of S-P chimeres 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 the ISG-diagnostic Taxon Specific Competitive Priming (TSCP)-PCR combined with a PCR-mediated detection of ISG-specific introns in the ML5-ML6 DNA region of the mt LrRNA. All the three ISGs were found, and a strong correlation was observed between the F ISG and fir and the S ISG and spruce. In the mixed forest, no clear relationship between tree host species and host-specialized ISGs was found. In spite of a relative dominance of fir in the overstory 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 chimeres were found. This suggests limited gene flow between these ISGs.