Cosmos are a delight in gardens due to their bright blossoms. Yellow flowers, pink, magenta or white are the most prevalent shades. They are generally highly resistant to plant diseases but can fall victim to a few. Here are suggestions on how to recognize, treat and prevent plant diseases that may threaten your cosmos flowers.
If the stems and leaves of your cosmos show signs of sudden wilting, bacterial wilt is the culprit. The disease invades at the base of the stem, causing rot that softens and weakens it. This problem cannot be treated effectively, so you must dig up and discard infected plants in an isolated trash bag. You will not be able to replant cosmos in the affected area.
This disease can attack your cosmos once the flowers have bloomed. It causes dark brown scab-like formations on the flower stems where they emerge from supporting leaves. The scabs change color to dark gray as they penetrate the stems. They will merge and encircle the stem, killing the growth above. There is no useful treatment that can save the flowers. Eradicate infected plants immediately, and destroy all remaining plant parts still in the ground in the early autumn.
A disease caused by spores, powdery mildew forms on the cosmos leaves, first as distinct white spots, then spreads out to film over the underside of each leaf. The mildew can cause distortion of leaves, stems and flowers, with yellowing of leaves that will deprive the cosmos plant of nutrition. Many effective fungicides exist to treat this condition. Choose one that is appropriate for flowering annuals.
Cosmos, as part of the aster family, can be afflicted with plant diseases common to asters. Aster yellows occurs most often in cosmos in late spring as the buds are forming on the stems. The leaves and stems, and often the flowers as well, turn yellow-green, and do not reach full maturity. This disease originates as a phytoplasma, and is spread by leafhopper insects, which eat affected plants, then transmit the organism to the next plant they chew. Aster yellows is most often a problem when summer weather is cool and wet. Once your cosmos is infected with aster yellows, it must be eradicated, as there is no cure for this disease. You can prevent it by keeping soils slightly dry, and weeding such undesirables as dandelions promptly.
Insects such as aphids and borers can also attack cosmos plants. The signs of insect infestation in cosmos are broken stems (borers) and distorted growth of stems, leaves and the flower buds (aphids). Natural predators such as pirate bugs, lacewings and ladybugs can control the aphids. Spray with a borer-toxic insecticide such as BTK early in the season at the first sign of borers.
Prevention of Disease in Cosmos
Plant the cosmos in slightly dry soil with excellent drainage. Add sand or gravel to the planting bed to help reduce excess moisture. Water sparingly and do not use any type of root or flower-stimulating fertilizer.