High infection rate of residual conifer stumps by Heterobasidion species in an area with assumed low infection pressure

The study was made in order to assess the infection rate of conifer stumps by Heterobasidion species in transition zone between southern boreal and middle boreal areas. Majority of surveys of stump infection rate have been made in areas where the fungus is already a considerable problem; typically the effective temperature sum in these areas exceeds 1200 degree days (d.d.). In the study, the incidence of stump infection was surveyed by intensive sampling of cut stump surfaces in the Central Finland in area where the temperature sum is 1100 d.d. and sporadic observations of damages caused by H. annosum s.l. have been made. Residual stumps were surveyed in total of 31 sites where majority of stumps had been harvested. Of those, 24 (75%) were infected. The stump infection rate was less than 20% on most sites. On average, 13% of spruce stumps were infected. The frequency of infected stumps varied considerably between the studied stands. The infection percentage of residual stumps in stump harvesting areas was corresponding to that obtained from earlier studies. Our results demonstrate that although the stump harvesting effectively decreases the number of infected stumps, the stumps left after summer cutting on the clear-cut area may be an additional infection pathway. No clear linear trend between mean temperature and proportion of infected stumps could be found.