Yes, BayouPenturner, is correct, some woods when affected by fungi and to which we call "spalting", not all woods spalt the same way or have the same type of fungi developing on the wood.
Those lighter colour patches are indeed spalting and, may be much older than you and other may think. The black lines and marks from the common spalting are not present here and I'm afraid, they will never be for the reasons explained above.
Here in Australia, many of the "Gum/Eucalyptus" species develop that sort of spalting, and that is how far it will go, if anything, it may develop to a next stage (further deterioration) where the wood cells start to develop a very white colour "dots" that are quite pretty looking but at that stage, the wood will be maybe too soft to the work on without some sort of stabilisation/hardening.
So, there is nothing you can do to remove or disguise it, it will be part of the piece character for now and ever...!:wink:
In no way, that can be considered a "defect" but a result of Nature's work...!
I like it...!
Cheers
George