Armillaria mellea
Initially, following Veikko Hintikka's discovery of a technique to distinguish between "Armillaria" species by growing them together as single spore isolates on petri dishes and observing changes in the morphology of the cultures, Korhonen showed in 1978 that the European "Armillaria mellea" species complex could be separated into five reproductively isolated species. Similarly he studied the sexual incompatibility of bipolar systems also in "Heterobasidion". As result of these studies the genus "Heterobasidion" was divided into three species while "Armillaria" has by now been divided into more than 40 species.