Decide what plum does in your recipe (chunks for texture, or jammy fruit for filling).
Choose the closest match from the table: prunes/cherry plums for plum-like sweetness; rhubarb for tartness; apple butter for jam layers; blueberries for a sturdy bake fruit.
Use the listed ratio. If your substitute is thicker (apple butter), thin it with 1-2 tablespoon water per cup fruit; if it’s wetter (blueberries), reduce added liquid.
Taste-test the mixture for balance (sweet vs tart). Adjust with a small amount of sugar or lemon juice before baking.
Bake as directed, but check 5-8 minutes early for fillings—some substitutes set faster than fresh plums.