Gregory the Dialogist

That the world ought to be trampled upon and despised, He proves by a wise comparison, adding, Behold the fig tree and all the trees, when they now put forth fruit, ye know that summer is near. As if He says, As from the fruit of the tree the summer is perceived to be near, so from the fall of the world the kingdom of God is known to be at hand. Hereby is it manifested that the world's fall is our fruit. For hereunto it puts forth buds, that whomsoever it has fostered in the bud it may consume in slaughter. …

Catena Aurea by Aquinas ↗

Bede

And He told them a parable: Look at the fig tree and all the trees, as soon as they put forth their fruit, you know that summer is near. So also, when you see these things happening, know that the kingdom of God is near. He therefore clearly teaches that just as the coming summer is known by the fruit of the trees, so also the kingdom of God is known to be near by the ruin of the world. By these words it is certainly shown that the fruit of the world is ruin. For it grows to fall. …

On the Gospel of Luke ↗