Petunias are relatively pest- and disease-free, but from time to time problems do arise. One common complaint is that white spots have appeared on the flowers, foliage or both. These mystery white spots could be one of a few different problems. Thrips, botrytis and fertilizer burn are common causes of white spots on petunias. If you still aren't sure, consult a specialist at your local county extension office.
Thrips are tiny insects that feed on petunia flowers and foliage. They actually scrape away plant tissue and leave bleached spots or streaks where once there was bright, beautiful color. Foliage and flowers may also appear silvery or bronze due to feeding. Thrips hide inside petunia flowers so their control is very difficult to achieve. Add to that the fact that they are small enough to be blown in on the wind. Serious infestations may leave flowers deformed. Organic pest control agents are best -- such as neem, pyrethrins or insecticidal soap -- because they allow natural predators to thrive.
Botrytis is a common fungal infection that attacks so many ornamental and flowering plants and petunias are no exception. The good news is that if botrytis is the cause of the white spots, you've caught the disease early and have time to treat it. Eventually those white spots turn to a gray, powdery mess that completely destroys the petunia flowers. It is easily transmissible to other plants in the garden and thrives in wet, humid conditions. Petunias drop their flowers once they've died, and spent flowers on healthy foliage is one cause of the infection. Stay on top of petunias: deadhead the plants and clean up dead plant material. Remove infected plants and destroy them.
Another potential cause of white spots on petunias is fertilizer burn, according to Ron Smith of the North Dakota State University Extension. In an effort to grow the biggest, best plants, some gardeners may over-fertilize. Too much fertilizer results in too much salt in the soil, which burns petunia roots. Inaccurate applications of dry fertilizers may end up on foliage and flowers, causing burns and white spots. Water in dry fertilizers at the time of application and don't let them touch aboveground plant parts.
Healthy petunias have a better chance of withstanding infection and infestations than weak ones. Make sure your petunias are getting enough sun, water and fertilizer. Petunias prefer full sun but, in places with very hot afternoon sun, a little afternoon shade is best. Water to a depth of 6 to 8 inches, as the University of Rhode Island recommends, once the soil is dry. Drip irrigation or soaker hoses will keep the water at the roots and keep it off the foliage where it invites botrytis and other fungal infections. According to the University of Rhode Island Extension, 1 lb. of 10-10-10 dry fertilizer or 2 lbs. of 5-10-5 fertilizer applied at planting per 100 square feet will suffice.