You can see in infected wood a purple stain. Every pine tree I cut down in Roosevelt National Forest in the late 70s (firewood) was stained purple. The need to develop GM pine trees.
I logged for a lot of years in the Sangre de Cristos. I never did see purple stain, but I saw a lot of blue stain which was caused by improper handling of the logs after cutting. IIRC, it is a fungus which grows in the wet wood because of heat and high moisture content when the logs are not debarked soon after cutting. The bark holds in the moisture and contributes to heat rise in the log promoting the blue-stain. That will also happen to trees on a south slope which are beetle killed and become standing dead with the bark on and exposed to the sunlight. The log temperature raises and the hot wet interior develops blue-stain.
The forests of the greater West/Southwest have had less than adequate rainfall for several years. This has weakened the trees and made them attractive to the insects. When a plant is weakened because of drought or other environmental factors, the plant attempts to heal itself by manufacturing higher levels of sugars in the sap which attracts insects. When the cycle runs its course and the rainfalls come up and the winter temps go down, then the insects will go away. Fire could play a good part in correcting that, as it will do in many area. However, the fire attracts other insects, so it is a never ceasing circle.