The tree, thought to be around 350 years old, which shaded a young crime-writing Agatha Christie while she kept score at Barton Cricket Club, is under threat.
The oak is suffering from fungus, shedding bark and old age.
Torbay’s senior arboricultural officer told thisissouthdevon: “It is known as Agatha Christie’s tree, but it is in decline.
“That does not mean it has reached the end of its life. These trees can be in decline for around 50 years.
“At the moment it is 300 to 350 years old. It could live for around 500 years.
“We are not going to remove it. We have been looking after it for at least 11 years. But we do need to reduce its mass.
“If we reduce it down there is less chance of it falling over.”
To celebrate 120th anniversary of Agatha Christie, 120 red herrings will be hidden throughout Torquay, Paignton and Brixham, and is just one of more than 50 Christie themed events during the festival, which takes place from September 12 to 19, including a special fireworks display on September 15 – what would have been Agatha’s 120th birthday
Lydia Stone, tourism officer at the English Riviera Tourist Board, said: “Many of the fish will be hidden in places that featured in the life and works of Torquay’s most famous literary daughter, providing the perfect hook for visitors to explore Agatha Christie’s Riviera.”