Certified GKH Gardening Expert
This can be caused by too much fertilizer, or too much water.
You will want to treat the area with a fungicide, just in case. This article will help:
Make sure that the soil has a chance to dry out thoroughly down to about 3 or 4 inches between watering, then give them another very deep watering. They can tolerate dry soil better than they can saturated soils.
Here is an article that will help you with their care: