Reasons for Petunias Dying

Powdery Mildew

• Powdery mildew is a fungal disease that causes a white or gray dusty coating on infected areas such as leaves, causing death in extreme cases. It occurs in wet conditions. Protect the flowers by irrigating them early in the morning so the leaves are dry by evening. According to Cornell University, apply water at soil level--the area around the base of the plant--rather than from above because wet foliage increases susceptibility to mildew. In extreme cases, apply a fungicide containing active products such as sulfur or bicarbonate. Follow label directions for application rates.

Improper Nutrition

• Lack of proper nutrients in the soil causes the petunias to start declining. Initial signs include wilting or yellowing of foliage. According to the University of Rhode Island, apply 10-10-10 dry fertilizer or 5-10-5 dry fertilizer at the time of planting, in amounts specified on package directions. After that, feed potted petunias every two weeks with a 20-20-20 fertilizer diluted in water, and feed those in the ground an 8-8-8 or 10-10-10 liquid fertilizer once every three weeks. Make sure the plants are hydrated before fertilizing. Proper fertilization provides essential nutrients that strengthen the plant.

Gray Mold

• Gray Mold or Botrytis blight is a fungal disease that affects annuals such as petunias. According to Cornell University, the species of this disease petunias are susceptible to is the Botrytis cinerea, which causes gray, silver-gray or black molds on petals and foliage. Inspect the petunia for parts covered in such film and immediately snip them off. Cut all infected parts or dig out the entire plant if infestation is large, and place in a garbage bag. Knot immediately and discard to prevent the fungal spores from spreading the infection. Fungicides formulated with fixed copper or chlorothalonil may keep the disease under control.