Two species of cosmos are popular among gardeners: Sulphur cosmos (Cosmos sulphureus), and garden cosmos (Cosmos bipinnatus). Both plants react adversely to too much water or fertilizer. They are vulnerable to aphids and susceptible to gray mold, powdery mildew and stem canker.
Stem canker produces brown lesions along the stems of cosmos plants. Rot destroys leaf tissue as it moves up the plant. There is no known treatment.
Powdery mildew appears as a white flour-like dust on the upper parts of older leaves that turn yellow and dry up. The white powder may be rubbed off, infected plant parts may be removed and destroyed, or a fungicide spray containing sulfur or neem oil may be applied.
Overwatering creates excessive moisture, which—in turn—may cause gray mold. Symptoms include water-filled spots on the leaves and the wilting that follows. Plants require adequate space to ensure air circulation and sufficient time to dry out between waterings.
Aphids are tiny insects that suck the juices out of leaves. They are usually found on the undersides of leaves and they often leave a sticky substance called honeydew, which alone is not harmful, but it may cause fungal growth and attract other pests. Spraying the leaves with water helps dislodge the aphids, as does the application of an insecticidal soap.