Why does Mr. Nathan Radley fill the in the tree with cement? Do you accept his reason as truth?

This happens in Chapter 7 of the book.  When Nathan Radley fills the knot hole in the tree with cement, he says that it is because the tree is sick.

I really do not believe that Mr. Radley is telling the truth when he says that he did it because the tree was sick.  Cement would not be very likely to help a tree stop being sick.  In addition, Atticus tells Jem that the tree was not sick.  Instead, I believe that Nathan does it because he does not want Boo to keep leaving gifts for the kids -- just out of meanness, really.