The effect of winter thinning on the spread of Heterobasidion parviporum in Norway spruce stands

Genets of Heterobasidion were identified in eight 43- to 56-year-old Norway spruce (Picea abies (L.) Karst.) stands in southern Finland. Four of the stands had been thinned in winter ca. 10ayears prior to the study, and four had been left unthinned. It was hypothesized that the felling of decayed trees promotes the vegetative spread of the fungus from thinning stumps into the adjacent trees, and hence the Heterobasidion genets should be larger in thinned stands. Although the number of infected stumps per areal unit was lower and the genets were slightly smaller in the previous tree generation of thinned plots, the genets, particularly the old ones, in the present tree generation were larger on thinned plots. The genets originating from the previous rotation included, on average, 12.2 trees on thinned plots and 6.7 trees on unthinned plots. The mean number of trees infected by new genets (not found in previous generation) was also slightly higher (pa < 0.05) on thinned than on unthinned plots, 1.8 and 1.2 trees, respectively. The results indicate that the rapid expansion of old Heterobasidion infections after a thinning operation may significantly contribute to the occurrence of root rot in the residual stand.