Incidence of Heterobasidion annosum infection in chemically thinned Norway spruce (Picea abies) stands in Northern Ireland.

This study compared the incidence of Heterobasidion annosum (Fr.) Bref. infection in stands of Norway spruce (Picea abies (L.) Karst.) that had been chemically thinned (by injecting a chemical herbicide into the stem), conventionally thinned (one tree in three) and unthinned. Results showed a high incidence of infection in the chemically treated stands with 95 per cent of the treated trees and 87 per cent of the untreated trees infected by H. annosum. By contrast, none of the trees in both the conventionally thinned and unthinned stands had the infection. Even the low rate of infection (8 per cent) found in the stumps in the conventionally thinned plots had not transferred to the standing trees. The reason why chemically treated trees became so susceptible to colonization by H. annosum is not clear. Research is ongoing to determine the mechanism of infection and whether modifications to this silvicultural procedure can be made.