Heterobasidion annosum (Fr.:Fr.) Bref. sensu lato, one of the most damaging root and butt rot agents on conifers, was recently segregated into three species in Europe based on morphology, intersterility grouping (ISGs), and host preferences (3). These species include: H. annosum (Fr.) Bref. sensu stricto (ISG P) on Pinus, other conifers and some hardwoods; H. parviporum Niemelä & Korhonen (ISG S), primarily on Picea; and H. abietinum Niemelä & Korhonen (ISG F) on Abies. In the summer of 1998, a Swiss stone pine (Pinus cembra L.), growing at 1,900 m in a mixed spruce (Picea) and larch (Larix) forest in the Aosta Valley (northwest Italian Alps), was found infected by H. parviporum and H. annosum sensu stricto. The pine (approximately 14 m tall and at least 75 years old) was without crown symptoms, but the stem, stump, and all the main roots showed internal decay. Disks, 3 to 4 cm thick, were cut consecutively from the roots, stump, and stem, incubated, and examined for conidiophore production. After 8 days, 63 isolates were obtained from all disks taken from the stump and roots, and from disks taken up to 4 m above the collar in the stem. Isolates were identified by polymerase chain reaction (PCR) of mitochondrial and nuclear markers (2) and by sexual compatibility with testers of each European Heterobasidion spp. The stem and one root were colonized by H. parviporum while the other roots and most of the stump was colonized by H. annosum sensu stricto. Somatic incompatibility tests among conspecific isolates suggested that there was only one genet of each species. The coexistence of different Heterobasidion spp. (ISGs) in the same tree has been reported only in Pinus ponderosa Dougl. ex Laws. in California (1) and in Picea abies (L.) Karst. in Europe (4). To our knowledge, this is the first report of H. annosum sensu stricto on P. cembra and of a European pine to be simultaneously infected by more than one species of Heterobasidion.
References: (1) M. Garbelotto et al. Phytopathology 86:543, 1996. (2) P. Gonthier et al. Can. J. Bot. 79:1057, 2001. (3) T. Niemelä and K. Korhonen. Taxonomy of the genus Heterobasidion. Pages 27--33 in Heterobasidion annosum, Biology, Ecology, Impact and Control. CAB International, Wallingford, UK, 1998. (4) R. Vasiliauskas and J. Stenlid. Can. J. Forest Res. 28:961, 1998.
