Variation in incidence and genetic impact on natural infection of Heterobasidion annosum in Picea abies (L.) Karst. in genetic trials in south Sweden

The incidence of natural infection of Heterobasidion annosum was recorded in two genetic trials at different sites. Site 1: In a 15-year-old clone trial with Norway spruce (Picea abies) planted on a second-generation spruce site the presence of H. annosum at stump height was investigated in two steps. Step 1: The incidence of H. annosum was detected on the stump surface of 48 clones (8–9 ramets of each, totally 394 trees) on disks collected after felling the trees. Step 2: Sixty-four clones and 70 trees propagated from seeds were investigated using an increment-borer. Data from the two investigations were analysed jointly. Totally 112 clones, 6–10 ramets for each clone, and 70 trees propagated from seeds were examined.Step1: H. annosum was found on 29.5% of the trees. No significant difference in infection frequency was found among clones, but for 12% of the clones more than 50% of the ramets were attacked by H. annosum and for 21% of the clones less than 15% of the ramets were attacked. Step 2: H. annosum was found on 16.5% of the trees. There was no significant difference in infection frequency between cuttings and plants propagated from seeds or among clones. In 16% of the clones more than 33% of the ramets were attacked and in 20% of the clones there were no signs of decay. There was a significant difference in incidence of infection between the two investigations. The difference was supposed to be caused by the different assessment methods. No correlations were found between size of the trees and incidence of infection. Fruit bodies of H. annosum were found on stumps formed when the previous generation spruce was clear-cut. Site 2: A 44-year-old Norway spruce progeny trial with 72 full sib families from 37 parents was clear-cut and the frequency of visual decay on stump surfaces was estimated. In addition, the area of decayed wood was estimated for each stump. At age 41, the diameter and resin flow on stems had been measured. After clearcut, 66% of the stumps were showing visual decay and on average, 67% of the stump surface was decayed. No significant genetic variation in frequency of decay was found. Decayed trees showed more resin flow than healthy ones.