In midsummer mountain pine beetles emerge from lodgepole pine trees and fly to unattacked trees. While chewing vertical egg galleries in the inner bark of the tree, they inoculate into it a blue-stain fungus complex. Initially, the fungi are confined to the beetle frass of the egg gallery, but they soon grow into the sapwood. The fungi spread radially via the parenchyma of the xylem rays. Once established in the xylem rays, fungal hyphae move into the tracheids of the axial water-conducting system. Here they occlude bordered-pit pairs and occasionally the entire lumen of the cell. Fungal hyphae also attack and destroy resin-duct epithelial cells. This may result in release of resin into surrounding tissues. Destruction of storage and water-conducting tissues in the tree trunk is detrimental to renewed shoot tip expansion the following spring.
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