By Clive005
United Kingdom 
Our silver birch tree has a very healthy canopy but hollow areas have developed at the base of the trunk. There is rusty coloured crumbly debris in the hollow areas. Is this honey fungus? Could it be an infestation? Really don't want to cut down the tree if it can be fixed but also don't want it to fall down onto the house.
11 May, 2024
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Would be better with a picture of the tree. Betula can suffer from canker. The disease starts off with goo and then dries out to give a powdery look and a bare patch. I'm afraid that the only true way to settle your concern is to contact a reputable tree surgeon.
12 May, 2024