Differences in virulence of genets of Heterobasidion annosum and susceptibility of young plants of different conifer species and origins

Heterobasidion species are the most important pathogens causing root and stem rot on conifers in northern hemisphere forests. The host list of this complex is very wide and includes over 200 species of trees and shrubs. Among the members of this complex, Heterobasidion annosum s. s. has the largest host range. In this study, young plants of Pinus sylvestris, Picea orientalis, Abies nordmanniana, Cedrus libani and Pinus brutia (three different origins) were inoculated on the lower stem with known genets of Heterobasidion annosum s.s. collected from Pinus brutia stands in south-western Türkiye. Infection frequency, assessed as presence of the conidial stage in stem discs following incubation, in the inoculated seedlings was 100%. The Heterobasidion annosum s. s. isolates were re-isolated from all inoculated host species. Control seedlings showed no symptoms of disease. Mortality in inoculated plants was 11.5% of the 735 inoculated plants, which died over an 8-weeks incubation period. The isolates showed greater growth on Cedrus libani, Pinus sylvestris and Picea orientalis seedlings compared to other species tested. On the other hand, it was found that the least affected seedlings were Pinus brutia TB12 and Abies nordmanniana. This study proved that differences occur in aggressiveness of Heterobasidion annosum s. s. to host species. A striking point in the results is that, despite being the host species from which the isolates were obtained, Pinus brutia seedlings showed lower sensitivity to Heterobasidion annosum s. s. than the other conifer species tested. Inoculations of three different Pinus brutia provenances suggested there was no significant difference in mean lesion lengths and fungal growth values in Pinus brutia plants, except in Pinus brutia TB14, which was more susceptible to extension growth of the pathogen.