During the last 15 years the intersterility group of c. 4 200 Finnish isolates of Heterobasidion annosum has been identified. The material includes isolates from diseased trees, from airborne infections in spruce and pine stumps, and from spore rain. 1 776 isolates are from diseased Norway spruce (89% S group, 11% P group), 499 from Scots pine (81% P, 19% S), 31 from birch (94% P, 6% S) and 97 from other native woody species (92% P, 8% S). Both the S and P groups are common in southern Finland, however, the S type is dominant in spruce-growing areas in the southwest and the P type in the southeast where pine forests are predominant. The known northern limit of the S type extend up to c. latitude 68 Grad and that of the P group to c. 64 Grad. The S type very seldom attacks birch and mature pine but may kill young pines on former spruce sites. Only the P type causes disease on old pine sites. Birch is sometimes attacked by P, but only when growing together with pine. Among 563 airborne Heterobasidion annosum infections in one-year-old spruce stumps investigated in spruce forests in South Finland, 97% were of the S type (of them, 30% homokaryons and the rest heterokaryons) and 3% P (47% homok.). The corresponding figures for 127 infections in pine stumps, mostly in SE Finland, were 8% S (80% homok.) and 92% P (29% homok.). The S type is predominant in spore rain along the southern coast, but even at the same locality the proportion of the S and P groups may vary greatly at different times.
