Coreopsis turning brown
10 coreopsis Zagreb plants planted in May, 2017 did well until two weeks ago started to turn brown. They get ample sun with late afternoon shade. Watered with irrigation system three times a week. They are in a mulched bed with good drainage. Any thoughts or recommendations?
Baltimore County Maryland
We viewed your photos. This looks like Rhizoctonia web blight, a fungal disease. This is a dieback of foliage during hot humid conditions. It can also cause root rots and stem cankers. Provide plenty of spacing for good air circulation. Keep mulch no thicker than several inches and away from the base of the plants. If there is overhead irrigation, plants will be more susceptible. Also watering three times a week sounds excessive. Irrigation during the first year is important. 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. Dig into the soil to make sure that
the soil is wet to a depth of six inches. No chemical control is recommended.
At this point you can prune out the blighted areas. The root system should be okay.