Allele PaLAR3B in root rot resistance locus does not influence the infection pressure by Heterobasidion parviporum through root contacts

The most destructive root rot pathogen of Norway spruce (Picea abies) in Finland is Heterobasidion parviporum. After primary infection, this pathogen spreads from tree to tree through root contacts and eventually causes high levels of root and stem rot. Locus PaLAR3 has been associated with root rot resistance of Norway spruce, and higher resistance in Norway spruce against H. parviporum has been noted when trees have allele PaLAR3B instead of the more common PaLAR3A. In this study we tested if trees with homo- or heterozygous PaLAR3B allele can better resist the infection through root contacts. The study was established in 2018 in a site where spruce trees had grown at a dense spacing for 10 years. The 72 trees were cut down and a mixture of four H. parviporum strains was inoculated on the stumps. After 3 years, 459 trees surrounding the inoculated stumps were harvested and analysed. Based on the presence of conidiospores, forty percent (40%) of the trees were infected by H. parviporum. The homozygous PaLAR3B genotype was found only in 1% of the trees. There were no differences in infection rates between the genotypes AA and AB (or BB). Similarly, the distance from inoculated stump or the diameter of the tree did not influence the infection rate.