What caused the yellowish discoloration of the wood on the photo?

I have picked up this English walnut board at the local lumberyard recently, but it wasn't until I started planing I noticed it has yellowish spots. It appears the discoloration does not go very deep and I may be able to plane it away. Still, I am not sure if it is a natural fault of the wood or maybe it came in contact with impregnating chemicals, thus affecting its safety.

English walnut board with discoloration

  • Would you say the yellow colour is following the grain?
    – Matt
    Aug 22, 2015 at 18:52
  • @Matt: Yes, it visibly follows the grain, at least on the side face.
    – PeterK
    Aug 22, 2015 at 19:00

1 Answer 1


It is possible that this is evidence of a previous fungal infection. Drying the wood can trigger the colorations. Specific fungi can change woods so specific colours can later appear. I don't see the rest of the board in the picture but would you say that the discoloration follows the grain?

I found a pdf called Discolouration of timber in connection with drying. A section described a yellow stain that follows the grain which was caused by

Metabolization of tannic acids by the fungus: Paecilomyces variotii.

In trying to find another resource I found a Google book Wood and Tree Fungi: Biology, Damage, Protection, and Use which stated:

Paecilomyces variotii produces a yellow discoloration of oak wood during drying times through its pH-change, which causes chemical reactions of the hydrolyzable gallotannins.

While that quote states "oak wood" this fungus has been found on many wood species so it is not a stretch to think that is what you have here.

What does this mean?

Assuming I am right there is nothing wrong with the wood. This colour should not penetrate too far and you should be able to plane it off.

This is related to the process of spalting where molds and fungi are used to naturally colour wood.

I don't think there is a safety risk here if the above proves to be true. Not sure what else could have caused that colour.

  • Thank you, @Matt, this is exactly what I see. The first PDF specifically lists Juglans regia (English walnut) among the affected species. I guess now I know why this particular board was priced next to nothing, too bad the rough saw marks and dirt hid the fault.
    – PeterK
    Aug 23, 2015 at 4:47
  • 1
    A good lumberyard may let you lightly plane an area of a rough board to get a better view of the grain and color. Carry a block plane with you when shopping for raw wood... but always ask first. (Tip from one of the magazines. I still need to find a good source of unsurfaced wood within reasonable radius from Boston.)
    – keshlam
    Aug 23, 2015 at 16:33
  • @keshlam Good tip. They are definitely not overly concerned over what customers do... or need. Like they had 6 lots of black walnut (1 lot = 1 log sawn) on stock but been unable to tell where I find them. Every visit I pay to them is an adventure, why not spice it up with concealed planing :) Good luck with finding your Boston area supplier.
    – PeterK
    Aug 23, 2015 at 19:11

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