Q: We have some dry basswood squares that have a blue or gray stain that goes right through the pieces. This lumber has been accumulating for many months before we dried it. Can you identify the stain?

A: The stain is blue stain (also called sap stain). It is caused by a blue colored fungus. This fungus can get into the log if the log is stored too long or can get into lumber if the lumber is stored in humid, slow drying conditions.

In order for the fungus to grow, it requires warm temperatures, oxygen, water, and food. The food is the sap in the wood. The water and oxygen are abundant in logs or freshly sawn lumber. The favorable growth temperatures are 70 to 100 degrees F.

The best control option is to dry the lumber promptly, eliminating the water before the fungus gets going. Otherwise, we can use a fungicide to poison the food supply.