White Tree Fruit is an edible item that is acquired by picking the White tree located in the Varrock palace garden, after the Garden of Tranquillity quest. The tree grows 1 fruit approximately every 5 minutes, and can hold up to 5 fruit at a time. Picking one grants 12 experience in Farming.
When eaten, it restores 5-10% run energy and heals 3 health. It is a slightly more accessible alternative to Strange fruit if players do not have access to Great Kourend.
Note that due to being non-tradeable unlike other fruit, it cannot be turned to banknotes neither by Tool Leprechaun nor in bank.
For an exhaustive list of all known sources for this item, see here