Petrarch uses apostrophe in line 28 to address the onslaught of foreign fighters into the Italian countryside:
O deluge that was gathered from what strange wilderness to inundate all our sweet countryside! (28-30)
Though he's talking about a figurative deluge (i.e., a whole lot of foreign mercenaries crossing into Italy), there's something more literal at the back of it. It would be impossible not to think of The Deluge, the great flood spoken of in Genesis that spared only Noah, his family and an ark full of animals.
That flood allegedly happens because people were behaving "iniquitously"—in other words, they had abandoned their morals. We know that Petrarch is calling the Italian nobility out for their poor sense of civic duty, so it's no surprise to us that Petrarch would include this image.