James McNaught said his father Graeme, 54, a world-renowned concert pianist, had demanded the instrument was sent back to him and had hired lawyers to have it returned.
James McNaught, 22, is the only child of Janice Galloway, 58 - the author of titles such as The Trick is to Keep Breathing, Foreign Parts and Blood - and Graeme McNaught.
The 54-year-old father, of Mount Vernon, Glasgow, is on trial at Hamilton Sheriff Court and faces a total of 10 charges of placing Ms Galloway in a state of fear and alarm. He has denied all the claims against him.
The pair met in 1990 and had a six year on-off relationship during which they had son, James, a London-based graphic designer.
Yesterday, he told a jury he had been given the piano as a present for his fourth birthday and had refused to give it back.
He said: "My dad was taking legal action against me over a piano in 2009. I owned the piano and it was a gift to me when I was four. When I was seven or eight, I started taking lessons but I did not use the piano in 2009.
"He requested it back if it wasn't in use but I told him I owned it. There was then a personal attempt to recover it and then there was a lawyer's letter which I responded to saying it was my property and not his, so I would not be giving him a date to come and collect it.
"But the piano is now probably in landfill somewhere. We kept it in an outhouse which got damp and the roof fell in above the piano and there was irreparable damage done to it.
"These incidents were upsetting and distressful."
James McNaught also alleged he had been involved in a stand-off with his father, who had arrived uninvited at Ms Galloway's home in Uddingston, Lanarkshire. He said he went to speak to him after hearing Ms Galloway's husband Jonathan May asking him to go away.
He said: "There was an evening around 9pm when we were watching television, this would have been around the end of 2011, when there was knock on the door.
"All four of us who live in the house were all in the room together and Jonathan went up to answer the door. This was unusual because we have quite a long front garden leading to the front door so most people use the back door.
"I can remember hearing Jonathan speaking to Graeme and telling him to leave.
"I could hear Graeme telling Jonathan he wanted to see me and after he said that a couple of times I went to the door.
"It was quite forceful what was getting said. It certainly wasn't chit chat by any means.
"I had previously told him that if he wanted to contact me to do it by email or he could have text me because I had given him my mobile number.
"I went to the door in the hope of smoothing things over and cooling off the situation so I chatted to him at the door and asked him to leave.
"He said he wouldn't leave and I asked him again to go but he refused. He then started to use belittling language against me.
"He did not make a great deal of sense, he was saying 'I'm the man, I'm the man you can't tell me to what to do, and I'm the man.'"
The trial before Sheriff Ray Small continues.