You must assume either that both tree and fruit are good, or that both tree and fruit are worthless; since it is by it's fruits that a tree is known.

And, noticing a solitary fig tree by the road-side, he went up to it, but found nothing on it but leaves. So he said to it: "Never again shall fruit be gathered off you." And suddenly the fruit tree withered up.

So, too, every sound tree bears good fruit, while a worthless tree bears bad fruit.

A sound tree cannot produce bad fruit, nor can a worthless tree bear good fruit.

Already the axe is lying at the root of the trees. Therefore every tree that fails to bear good fruit will be cut down and thrown into the fire.