Anything that is obviously dead should be cut off now. Dispose of prunings in household rubbish to prevent spread from infected wood to healthy wood.

Depends on how old the potting mix is. If the petunias were potted more than 3 months ago, I'd certainly use fresh mix now or at least a 50:50 mix of ...

Looks good, Adam!

As @MitchellMc has already said, drainage holes in the bases of pots are essential to keep plants healthy, otherwise the roots will rot. If you want ...

Have to agree with @MitchellMc that the most likely cause is a bacterial disease such as canker. There's a good NSW Department of Primary Industries F ...