Your easy-to-maintain annual petunias that offer color and interest to your garden are under the threat of fungus problems that may attack and cause disease. Maintain vigorous petunias because healthy plants have a much greater capacity for avoiding and recovering from disease than weakened plants. Petunias require full sunlight for optimal growth as well as light, well-drained soil with a pH of 6.0 to 7.0.
Verticillium wilt is a fungal disease that attacks petunias among approximately 300 other host plants. Caused by the pathogens Verticillium albo-atrum and Verticilliumdahliae, this fungus problem can be extremely damaging to your home garden petunias. A soil-borne disease, symptoms include discoloration on stems as well as areas of dying plant tissue. Wilting, yellowing and leaf death is also common. As plant death can occur, do not replant with another petunia. Soil-borne pathogens continue to inhabit the soil after the plant dies. Though fungicides applied directly to your petunias are not recommended as a control measure, licensed professionals may be able to assist with a fumigation process. For home management, remove and destroy affected plant parts or entire plants in severe cases. Maintain vigorous petunias and keep weeds under control with hand removal or the application of herbicides because weeds attract pathogens and often act as initial host plants.
Powdery mildew fungi attack petunias with a display of easy-to-recognize symptoms. When fungus attacks, powder-like spots of a mold-like substance forms on plant surfaces in addition to the formation of white fruiting bodies, or growths, that become black. Symptoms occur on leaves, stems and flowers. Appearing most prevalently in drier climates and warm temperatures as well as in damp, shaded conditions, powdery mildew is controllable through cultural management by avoiding overhead irrigation and removing and destroying affected plant parts to prevent further spread of the fungus. For chemical control, apply a sulfur-based fungicide or a fungicide with the active ingredient triforine. Employ the use of a botanical fungicide like neem oil for lower toxicity. Make applications once every one to two weeks during the growing season, according to the Colorado State University Extension.
Phytophthora Root and Crown Rot
Phytophthora root and crown rot of petunias is caused by the fungus Phytophthora parasitica. This soil-borne pathogen continues to live within soil without a definite life expectancy. The use of resistant plants like snapdragons and marigolds is ideal because they are much less likely to become infected. If you are removing a petunia plant that has died from root rot, replace it with a resistant option instead of with another petunia. Symptoms include decay near the soil line including the crown, or base, of the petunia plant as well as the roots. Visible symptoms are hard to come by but discoloration and dying plant tissue may occur. Additionally, your petunias may stop growing and producing flowers or the entire plant will die. For control, contact a licensed professional to set up a preventive fungicidal soil drench program. Maintain extremely well-drained soil and apply a fungicide with the active ingredient metalaxyl. Only apply a fungicide if you are providing the optimal maintenance to your petunia plants, according to the Utah State University Cooperative Extension.