Blue stain fungi can infest the outer layer of the trunk. Although they do not destroy the tree as fast as wood rot, they absorb sugar and interfere with water transport.
Blue stain fungi only infest sapwood, the youngest and active wood layer in the trunk. In this layer, the tree transports water and important nutrients. Blue stain fungi interfere with this transport and also feed on sugar, proteins and starch in the wood.
The inner part of the trunk - the harder heartwood - is not affected.