The relationship between blue-stain and bark beetles
The attack of bark beetles on standing or in newly-felled stems provides special growth conditions to wood-inhabiting fungi. In the wood attacked by bark beetles, a specific and rich fungus flora is found, and from these fungi the economically important group of blueing fungi has been more thoroughly investigated. These fungi live on nutritive substances present in the cells, especially in the medullary rays and other parenchymatous cells. They attack lignified cell walls only to a limited extent but in the ray cells they may cause considerable destruction. Some of these fungi may attack the secondary cell walls where they develop cavities. The blueing fungi attack standing trees when their moisture content is low as well as timber at different stages of storage before it is completely seasoned. They spread very quickly both radially and longitudinally and thus they may cause rapid discolouration and considerable financial losses. The greatest losses are caused by blueing fungi which attack newly felled timber in the forest simultaneously with the infestation of bark beetles. Von Schrenk (1903) has already pointed out the relationship between the attack of the bark beetles and the blueing of the wood which at this time was thought to be caused by one fungus Ceratocystis pilifera. Later.the number of known Ceratocystis species which are, over the whole world, the most common fungi associated with the attack of bark beetles, have amounted to 80-90 at the present time, and they have been studied most thoroughly by R W Davidson (numerous papers between 1935 and 1970) in the USA. The greatest number of species has been found in North America, where the variability in host trees and in climatic conditions is greater than in North Europe.