One of my customers has a Japanese Maple that I found a problem with last year. At the base of the trunk I found some "rot". She had an extension agent from the local university come and have a look. He suggested that I remove the rotten wood and pack the cavity with an asphalt roof coating. I did as he suggested and all looked fine until last week when I inspected it agian, because the tree looks to be in poor health. I found that the coating did not harden anny except for the outer skin. Today I removed the "packing" and found that it was holding a fair amount of water, which concerns me because it may make things worse. I did not however find anymore rotten wood, in fact I found some new feeder roots. After doing a soil test I think I know why it looks in poor health. Now for my main question. Is there anything I can use to fill the cavity that will not hold water, or just leave it exposed? The rot was pretty severe, over half of the base is gone. I am just worried if I leave it open that it would be to inviting for further problems. I have a few pics, but they are not as detail as I would like. Any suggestions?

Thanks in advance.

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