Incidence of root and butt rot in consecutive rotations of Picea abies

The incidence of butt rot in two consecutive rotations of Norway spruce [Pi cea abies (L.) Karst.] in 28 permanent sample plots at four different sites in Denmark was evaluated. Incidence of butt rot was estimated by visual ex amination of stumps at final felling of the previous rotation and by examin ation of bore cores taken at the butt from a random sample of trees before first thinning of the subsequent rotation. There was no correlation between the incidence of butt rot at final felling of the previous rotation of Nor way spruce and the incidence of butt rot at first thinning of the subsequen t rotation of Norway spruce. The incidence of butt rot at final felling was between 19 and 100%, and at first thinning between 0 and 20%. The S-form o f Heterobasidion annosum (Fr.) Bref. was the most commonly found decay-caus ing organism at ail sites. Root systems of 28 trees without decay at stump height in the present rotation were excavated to estimate the incidence of root rot. Heterobasidion annosum was found in only one root. Resinicium bic olor (Alb. gr Schw. ex Fr.) Parm. was found in 25% of the excavated root sy stems. The result of the study shows that the incidence of butt rot at firs t thinning of Norway spruce is not necessarily higher on sites where the pr evious rotation was heavily infected than on sites where infection in the p revious rotation was low.