Incidence of Heterobasidion spp. following different thinning regimes in Norway spruce in southern Sweden

The incidence of Heterobasidion spp. infections following different thinning regimes in Norway spruce was evaluated on 12 plots in a trial in southern Sweden. The plots were established in 1961 and followed until final felling in 2001. The treatments ranged from one light thinning at the experimental set-up to six thinnings. The average incidence of decay was 44% across all thinning regimes of which Heterobasidion spp. was identified in 85% of the cases. There was no difference in the incidence of infections, decayed volumes, and the size or number of Heterobasidion spp. genets among the thinning regimes. The largest genet was though found on the plot with just one light thinning and encompassed 25 trees. On average, 11.1% of the stem volume was decayed and the average height of the decay column was 4 m. Decay incidence was 3.7 times the decayed volume and the height of the decay columns was 19 times the diameter of the decay at stump height. Conclusively, early thinning appears to be more important for disease development than subsequent multiple thinning. To restrict the disease development in the stand later during the rotation period, it is of utmost importance that the earlier thinnings are made during winter conditions or otherwise properly protected against spore infection of Heterobasidion.

► Different thinning regimes and Heterobasidion in Norway spruce was investigated.
► There was no difference in development of Heterobasidion among different thinnings.
► Early thinning is more important for disease development than later multiple thinning.
► Benefit from stump treatment over a rotation relies on early prevention.