Petunias are a popular perennial that enjoy warmth. In climes cooler than those petunias prefer, they die after the growing season, to be replanted the following season as for an annual.
One of the reasons for the petunia plant's popularity among home gardeners is that the flower is easy to grow. It has few problems with pests or disease and many of those problems can be prevented. Others, like aphids, are not particular to petunias and many home gardeners already have experience controlling the problem.
Aphids are quick colonizers, reproducing quickly to infest plants. Damage shows as curled, distorted and yellow leaves. The bugs are commonly green, though some are other colors like yellow. One way to start to identify an aphid infestation is to try to startle them. They won't scatter when disturbed.
Aphids produce a sweet excretion beloved of ants called honeydew. Indeed, ants protect aphids in order to have a honeydew source, so if you see a lot of ants herding another bug, you've got aphids.
Control aphids by introducing aphid predators such as ladybugs. If you don't know where to obtain ladybugs, contact a nursery or your local county extension office for advice.
Black sooty mold can follow in the wake of an aphid invasion. The mold grows in the honeydew excretions, and can turn a petunia black. You can try washing the mold off--this might dislodge some bugs, too--but that won't solve the problem. Instead, control the aphids.
A fungal disease, gray mold attacks many kinds of flowers and fruit. The mold causes a blight on flower buds, flowers and foliage, turning the affected area brown. In high enough humidity, a web of gray mold develops. The most important way to control gray mold is to prevent its development by cleaning up plant debris regularly and not allowing the area to stay wet.
Slugs and Snails
Petunias have tender vegetation that slugs love to eat. If you have holes in leaves and flowers along with slime trails, you can bet that slugs or snails are the problem. They aren't hard to control. You can pick slugs off yourself at night when they feed, buy bait or set out saucers of beer.
One prime way to prevent slugs is to avoid moist, cluttered conditions in your flowerbed. Get rid of debris, allow for air circulation and don't water in the evening.
Tobacco Mosaic Virus
If your petunias contract a virus, it is likely to be TMV or tobacco mosaic virus. This is a bad virus to have in your garden, for it can infect many plants, including tomatoes and peppers.
Luckily, the virus is spread by contact, so good handling and sanitation practices can prevent the virus's spread. Make sure not to reuse pruners before dipping them in a 1 part bleach/10 part water solution for at least a minute. Any suspect plant should be immediately isolated from others and destroyed if the virus is found.
Symptoms include stunted and deformed leaves, especially if they become spindly and narrow. This spindly growth is described commonly as "rat-tailed." Flowers "drop color"--that is, color won't be properly uniform across petals. Leaves can develop a mosaic pattern of discoloration.