Infection of advance regeneration of Norway spruce by Heterobasidion parviporum

Abstract

The incidence of Heterobasidion root rot in the advance regeneration of Norway spruce (Picea abies (L.) Karst.) was studied in nine stands at four different localities in southern Finland. The mean age of the unthinned advance regeneration on the 17 sample plots ranged from 14 to 44 years. On infested 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% (mean 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%), respectively. Of the 138 Heterobasidion genets identified, 98.5% belonged to Heterobasidion parviporum Niemelä & Korhonen and 1.5% to Heterobasidion annosum (Fr.) Bref. s.s. The incidence of Heterobasidion root rot in advance regeneration was positively correlated with the mean size and age of the advance regeneration and the proportion of infected trees in the overstory and negatively correlated with the regeneration density. Vegetative spread through root contacts from overstory trees to the surrounding regeneration accounted for at least 53% of the Heterobasidion infections in the advance regeneration. The origin of the rest of the infections in advance regeneration remained unclear, but at least part of them may have started from spore infection on injured or dead roots. Our results suggest that, on sites infected by H. parviporum, advance growth of Norway spruce should not be used for regeneration even though the spruces look healthy and show no external signs of infection.