EFFECTS OF SEASON AND UREA TREATMENT ON INFECTION OF STUMPS OF PICEA-ABIES BY HETEROBASIDION-ANNOSUM IN STANDS ON FORMER ARABLE LAND

Between 1986 and 1990, a series of thinnings were made in previously u nthinned first-rotation stands on former arable land located in the so uthern half of Sweden. The aim was to evaluate the effects of season a nd urea treatment on the frequency of infection of stumps of Norway sp ruce (Picea abies (L.) Karst.) by the root-rot fungus Heterobasidion a nnosum (Fr.) Bref. Untreated stumps, resulting from 60 thinnings (22-1 00 stumps each, altogether ca 3000 stumps) made at different times of year, were investigated 3-24 months after cutting to determine whether they were infected with H. annosum. On average only 2% of the stumps from thinnings made in November-February were infected, whereas the in cidence of infection among stumps thinned in June-July was 34%. Two me thods of treating stumps with urea to prevent stump infection by H. an nosum after thinning were evaluated in terms of effectiveness. The fre shly cut stumps were treated with a 20% urea solution, transformed to a gel by adding 0.2% carboxymethyl cellulose, or with a 30% urea solut ion. On average, the reduction in infection rate obtained was 62% with the first method and 85% with the latter. In a separate study involvi ng a concentration series of urea, there was a considerable drop in pr otection efficiency, from 89% to 58%, when the concentration was decre ased from 30% to 15%.