He believes it is at first, but he realizes it is not when Atticus tells him the tree is perfectly healthy.
Mr. Nathan Radley told the kids that the tree was dying because he wanted to stop Boo Radley from contacting the outside world.
I think it was Mr. Radley who blocked the hole of the tree with cements, telling Scout that the tree is dying.
Nathan Radley tells Jem and Scout that he filled the knot-hole with cement because the tree was dying and he didn't want it to fall on someone. However, it is later revealed that he filled it to prevent gifts being left for the children by Boo Radley.
Nathan Radley fills the knothole in the tree with cement, preventing the children from receiving gifts left by Boo Radley.
Nathan Radley, Boo Radley's brother filled the knothole with cement. When Jem and Scout asked him why he was doing this he told them that his tree was dying and filling the knothole with cement would keep it from dying. Scout and Jem went and asked Atticus if he thought the tree looked like it was dying and he said it looked perfectly healthy.
The tree was cut down by the property owner, Nathan Radley, who believed that the gifts left by Boo were a threat to the children. This event shook Jem and Scout as they realized the gravity of the situation and how misunderstood Boo Radley was by the townspeople.
Nathan Radley, Boo Radley's brother, tells Jem that he cemented the knothole because the tree in the yard is dying. This information delivers a blow to Jem and Scout, who have been cherishing the gifts left in the knothole by Boo Radley.
Nathan Radley seals the knot-hole in the tree in front of the Radley place because he believes it is a bad influence on Boo Radley and wants to cut off communication between Boo and the children. He fills the hole with cement to prevent gifts being left for Boo.
He tells them the tree is dying.
Nathan Radley is likely trying to prevent Boo Radley from making contact with the outside world through the tree. By saying that the tree is sick and needs to be cut down, he can keep Boo isolated and prevent him from communicating with Jem and Scout. Nathan's actions contribute to Boo Radley's continued seclusion and mystery in the neighborhood.
Nathan Radley tells the children he sealed the knothole with cement in order to prevent Boo Radley from communicating with them or giving them gifts. He likely did this to further isolate Boo and maintain control over his actions.
Nathan Radley said that when a tree is dying you fill is with cement to save it.