A nation-wide inventory was undertaken in Norway in the winter of 1992 in cooperation with forestry authorities and about 5000 forest owners participated. The inventory was based on ocular assessment of stumps and rot was subjectively assessed to be caused by various fungi. The material consisted of 4914 stands and totalled 271023 stumps. Butt rot was assessed subjectively to be caused by Heterobasidion annosum in 16.2%, by Armillaria spp. in 4.7%, combined Heterobasidion annosum and Armillaria spp. in 2.8% and by others in 2.9% of the stumps. The overall mean frequency of butt rot was 26.7%. It was great variations between districts and counties. Generally, most rot was found in the areas with natural spruce forest, lowest in areas afforested with spruce. Earlier findings based on boring at breast height by the National Forest Inventory, butt rot frequencies were found to be 7.9% and 9.3% in two successive inventories. However, these figures highly underestimate the total amount of root and butt rot. In regional stump inventories the rot frequences were shown to be much higher, averaging 25%. The present results confirm the high frequency. Measures to reduce the root and butt rot problem are highly needed.
