Genetic Variation in Resistance Against Heterobasidion annosum (Fr.) Bref. in Picea abies (L.) Karst. Expressed After Inoculation of Neighboring Stumps

Based on the well-established knowledge that important spread of H. annosum is performed through spore infection of stumps and subsequent spread of mycelium to neighboring trees at points of root contact, an inoculation experiment was established in three adjacent fully pedigreed 17-year old Norway spruce field trials. In June 1995, every third row was cut and stump-inoculated with conidia belonging to the two contrasting types, P and S. 5 years later the remaining trees were evaluated by clear-cutting and mapping of rot occurrence. 34% of the trees were attacked. For each host-tree, the most likely source of infestation (i.e. possible inoculated donor-stump 5 years earlier) was evaluated based on orientation of the rot on the stump surface supplemented with samples of re-isolation of H. annosum of the two types. According to these estimates, the tested S-type turned out to be more aggressive towards Norway spruce than the tested P-type. Genetic variation in resistance was most convincingly detected in the most informative trial F175B. The pattern of genetic variation in the two roles as donor and host seems to differ. However, genetic variation was also expressed as general resistance, which is defined as the combined effect of donor stump, living host and across the two tested types of H. annosum. No indication of host x pathogen interaction was detected. This evidence combined with the general experience that resistance against root rot pathogens usually is partial and based on the cumulative effects of several genes, suggests that sustainable genetic gains in relative resistance may be obtainable in breeding programs of Norway spruce. The developed experimental set-up represents a useful concept for screening existing genetic trials for field resistance within a time scale of 5-6 years.