Petunias may be hardy plants, but they are vulnerable to a few destructive pests and diseases. Fungi, bugs, blight and malnourisnment can all cause your plants to produce fewer or smaller blooms or even die.
Aphids are tiny insects that eat the leaves and blooms of a plant. It can be hard to get rid of aphids, but a spray pesticide might do the trick. Also, if you fertilize your petunias with a chitin additive, you reduce the likelihood that pests will eat them.
Caterpillars might grow into beautiful butterflies or ugly moths. Before they get there, they snarf down everything they can to move to the chrysalis stage. Again, a crab meal fertilizer can help cut down on the amount of caterpillars that devour your petunias, though some pests don’t respond to chitin at all.
Mildew, Mold and Fungus
In most cases, the cause for fungal infection is negligence. The best way to avoid it is to make sure you are watering your petunias properly, i.e. not too much. The soil should be damp, but not soaking wet as this can cause root rot.
You should also take care to remove dead matter from the hanging planter, even if indoors. Spores can latch onto the dead matter and spread to the rest of the plant. If fungus does spread, it can be treated with fungicide. For each type of petunia and each region, the fungicides may vary, so consult your local nursery for more information.