Planet with a huge tree , people are going inside tree as food

I read this book a long time ago (+30 years)

The story is about a scientist that arrive on a planet where people (humans) worship a huge tree. There are priest and a worker class on the planet. The scientist find out that the workers are going inside the tree as food.

When the priest get a seed of the tree to an other planet, the scientist and others find a way to destroy the son of tree.

In the end they will try to destroy the huge tree on the other planet.

Looking for the title of the book and the writer.