Coreopsis (Coreopsis spp.) is an annual or perennial plant with cheerful daisy-like flowers that brighten the garden from late spring through the summer. The single or double flowers bloom in shades of yellow, gold and bronze, with the exception of Coreopsis rosea, which has delicate pink flowers. While a mass planting of coreopsis is spectacular, the flowers also add interest to mixed flower borders and wildflower gardens. The plants grow from 1 to 4 feet tall, depending on the variety. They are drought tolerant, grow well in poor soil, and are usually not affected by diseases.
1. Aster Yellows
A bacteria-like pathogen causes aster yellows on coreopsis. Infected plants have stunted growth and yellowish-green foliage and flowers. As the infection worsens, the plants turn brown and die. The pathogen survives the winter in insects, especially leafhoppers. They infect the plants in the spring when they begin feeding. To control the spread of aster yellows, destroy infected plants by burning or aerobic composting and remove areas of tall grass and brush where leafhoppers live during the winter.
2. Powdery Mildew
A fungus causes powdery mildew, which occurs when times of warm, dry days and cool, damp -- not wet -- nights. White powdery patches form on the leaves, young stems, buds and flowers of coreopsis. The fungal growth turns gray or tan as it ages. Powdery mildew causes deformed flower buds and leaves that may drop prematurely. Provide good air circulation to help prevent powdery mildew on coreo, psis. Do not apply excessive water or fertilizer since they encourage tender new growth. Remove and destroy infected leaves. A fungicide applied on small infestations at the beginning of the season may be effective. Use a fungicide approved for use on coreopsis and apply according to the manufacturer’s directions.
3. Verticillium Wilt
Verticillium spp., the fungi that causes verticillium wilt on coreopsis, can live in the soil for years. It infects the roots of stressed plants, such as when the plants are flowering or seeding. Initially, infected plants wilt and turn yellow. As the disease progresses, the plants turn brown and die. Maintain the proper moisture and drainage of coreopsis plants to avoid verticillium wilt. Do not cultivate deeply around the plants to prevent damage to the roots. Avoid using herbicides near the plants that can cause accidental damage to the foliage. Do not apply a fertilizer with high nitrogen content since it induces a flush of tender new growth and may stress the plants. Destroy infected plants by burning or aerobic composting.
4. Other Diseases
Bacterial leaf spots on coreopsis appear as irregular brown splotches or spots on the foliage, while leaf spots caused by fungi are purple to tan. Botrytis blight begins on the lower leaves of infected plants with fluffy white or gray growth, followed by gray spores. Downy mildew forms a felt like growth on the underside of infected leaves and black spots on the upper side of the leaves. A mass of orange-yellow spores on the underside of the leaves is a sign of rust and gray or brown spores on the leaves is a sign of scab. Plants infected with stem rot turn tan or brown and collapse, which results in the plant’s death. A white fungal growth at the base of coreopsis plants is a sign of crown rot, which causes the plants to wilt, turn brown and die. Root rot causes infected plants to turn yellow and wilt followed by the collapse and death of the plant. To prevent these diseases, do not over water, get the leaves wet when watering, or allow mulch to touch the stems of the plants.