Coreopsis (Coreopsis spp.), commonly called tickseeds, are sun-loving perennial flowers that grow in U.S. Department of Agriculture plant hardiness zones 4 through 9. Their cheery, daisylike blooms come in bright hues of yellow, orange and red. Although they suffer few problems, tickseeds are susceptible to mildews that can result in poor growth and negatively affect the appearance of the plant.
Powdery mildew is easily recognized by the white powdery spores that form on leaves, flowers and shoots. Infected tickseeds may experience leaf drop and distorted or stunted new growth. Powdery mildew doesn't need moisture to thrive like most fungi -- water will kill the powdery mildew spores, as will extreme heat and strong light. This damaging disease is most prevalent in shady conditions when temperatures fall between 60 and 80 degrees Fahrenheit.
Tickseeds infected with downy mildew will develop purple, white, lavender or black spots on the undersides of the leaves while the top surfaces have pale yellow areas. Leaf death and stunted or disfigured growth will occur on plants infected with downy mildew. Unlike powdery mildew, this fungal disease requires lots of moisture and humidity levels above 90 percent to grow and spread. Cool and moist conditions with temperatures between 40 and 60 F create the ideal environment for downy mildew growth.
Both powdery mildew and downy mildew can be controlled with a ready-to-use liquid copper fungicide. Apply the fungicide as a preventive spray two weeks before the fungal diseases typically appear or as a treatment after the mildew has infected the plants. If possible, apply the copper fungicide when there won’t be rain for at least 12 hours after spraying the tickseed. Cover the foliage liberally with the solution every seven days until the mildew is no longer a problem. Always wear protective clothing -- such as rubber gloves -- when working with chemicals. After use, thoroughly wash your hands with soap and water. Never apply copper fungicide to water as it is toxic to fish and aquatic wildlife. Follow the directions on the product you are using.
Provide your plants with proper care to prevent mildew problems in the first place. This includes planting tickseeds in full sun and keeping the area where they are growing free from debris and fallen plant matter. Increasing air circulation through proper spacing and pruning will help prevent downy mildew. Don't water plants from above because wet leaves invite disease.