After plants are established, they are still not immune to disease attack. The major diseases of annuals can be grouped as those that attack the flowers and foliage and those that cause the plant to wilt and die. The diseases that attack the flowers and foliage can usually be reduced by planting in areas that get sun early in the morning so that the foliage will dry quickly and by providing adequate spacing to ensure air circulation around the plants. The most serious leaf diseases are various leaf spot diseases and powdery mildew. The leaf spots cause irregular dead blotches on leaves and are most commonly seen on petunias and zinnias.
Powdery mildew starts as a white coating on leaves and flowers.
This is usually followed by the death of the leaves. Powdery mildew is common on zinnias and black-eyed Susans. Consult your local county Extension agent for specific control recommendations if problems develop. Root rot diseases are those that kill the roots of plants, while the wilt diseases plug the vascular system of the plants. Wilt and root rot diseases are difficult to control.