Apply a fungicide to protect plants. They are in a mulched bed with good drainage. The like to run on the dry side. To prevent aster yellows, use diatomaceous earth or insecticidal soap to keep leafhoppers at bay. Coreopsis flowers may be annual or perennial and come in a variety of heights. It can also cause root rots and stem cankers. A bit of compost in spring is all the fertilizer they need. Provide plenty of spacing for good air circulation. I have another bed of Coreopsis, these plants (so far) have not shown this problem. The root system should be okay. We viewed your photos. They are in a mulched bed with good drainage. A bit of compost in spring is all the fertilizer they need. Coreopsis flowers may develop green or elongated petals. No chemical control is recommended. Watered with irrigation system three times a week. This looks like Rhizoctonia web blight, a fungal disease. They get ample sun with late afternoon shade. Coreopsis do best in fast draining soil that is not overly rich. Various leaf spots are also caused by fungi. This is a dieback of foliage during hot humid conditions. They get ample sun with late afternoon shade. Infrequent, deep irrigation is better than frequent shallow irrigation, which only encourages shallow roots and will encourage weed growth.Check the soil moisture at least once a week. Leaves and flowers turning brown can be a result of improper watering. 10 coreopsis Zagreb plants planted in May, 2017 did well until two weeks ago started to turn brown. Coreopsis can suffer from crown rot (Sclerotium fungus), root rot (Rhizoctonia fungus) and stem rot (Alternaria, Rhizoctonia or Sclerotinia fungi). Also watering three times a week sounds excessive. … Last year one of my Coreopsis turned brown and appeared dead. Dig into the soil to make sure thatthe soil is wet to a depth of six inches. Any thoughts or recommendations? If there is overhead irrigation, plants will be more susceptible. The Sclerotium rolfsii fungus causes crown rot in coreopsis plantings. Scab: Leaves have a coating of gray to brown spores. http://extension.umd.edu/hgic/rhizoctonia-web-blight-flowersgroundcoversAt this point you can prune out the blighted areas. Remove all the dead material. A member of the Asteraceae family, blooms of growing coreopsis are similar to those of the daisy. Ask an Expert is made up of groups and individual experts. Powdery mildew, root rot, blight, and rust can be treated with a fungicide. Remove all the dead material. Best And Happy Yardening, Nancy. mh. Sounds like stem rot. The one next to this one has started to turn brown also. Watered with irrigation system three times a week. It is now turning brown again. Best And Happy Yardening, Nancy. Colors of petals include red, pink, white and yellow, many with dark brown or maroon centers, which makes an interesting contrast to the petals. White fungal webbing forms around the base of the plant, and the leaves and blossoms turn brown as they wilt and die. Coreopsis do best in fast draining soil that is not overly rich. Question From: Mason, Ohio, United States. Bottom line, you may be killing it with kindness. Mildew rarely affects coreopsis until after the first flowering. Soil that is dry beneath the surface means the plant has been underwatered. Keep mulch no thicker than several inches and away from the base of the plants. The plant came back this spring and appreaed healthy. Sounds like stem rot. These thrive on moist leaf surfaces and cause transparent to brown or black spots that disfigure the leaves. If you suspect wilt — your plants will wilt, turn brown, and die — remove infected plants. http://extension.umd.edu/hgic/rhizoctonia-web-blight-flowersgroundcovers. Badly infected coreopsis leaves become discolored and distorted, then drop off. Bottom line, you may be killing it with kindness. Irrigation during the first year is important. 10 coreopsis Zagreb plants planted in May, 2017 did well until two weeks ago started to turn brown. The like to run on the dry side.