Infection of advance regeneration of Norway spruce by Heterobasidion parviporum

The incidence of Heterobasidion root rot in the advance regeneration of Nor way spruce (Picea abies (L.) Karst.) was studied in nine stands at four dif ferent localities in southern Finland. The mean age of the unthinned advanc e regeneration on the 17 sample plots ranged from 14 to 44 years. On infest ed plots, the proportion of Norway spruce infected by Heterobasidion varied from 22.2 to 75.0% (mean 52.5%) in the overstory and from 1.8 to 68.2% (me an 21.1%) in the advance regeneration. The corresponding values on healthy- looking control plots were 0-6.7% (mean 5.9%) and 1.3-3.9% (mean 2.4%), res pectively. Of the 138 Heterobasidion genets identified, 98.5% belonged to H eterobasidion parviporum Niemela & Korhonen and 1.5% to Heterobasidion anno sum (Fr.) Bref. s.s. The incidence of Heterobasidion root rot in advance re generation was positively correlated with the mean size and age of the adva nce regeneration and the proportion of infected trees in the overstory and negatively correlated with the regeneration density. Vegetative spread thro ugh root contacts from overstory trees to the surrounding regeneration acco unted for at least 53% of the Heterobasidion infections in the advance rege neration. The origin of the rest of the infections in advance regeneration remained unclear, but at least part of them may have started from spore inf ection on injured or dead roots. Our results suggest that, on sites infecte d by H. parviporum, advance growth of Norway spruce should not be used for regeneration even though the spruces look healthy and show no external sign s of infection.