Ambrose of Milan — as excerpted in the Catena Aurea on Luke 17:5-6

“Or this is said because faith keeps out the unclean spirit, especially since the nature of the tree falls in with this meaning. For the fruit of the mulberry is at first white in the blossom, and being formed from thence grows red, and blackens as it gets ripe. The devil also having by transgression fallen from the white flower of the angelic nature and the bright beams of his power, grows terrible in the black odour of sin.”