Certified GKH Gardening Expert
The taproot will constantly draw water from deep in the soil. Therefore, these lovely plants are drought-resistant. Remember to water very deeply, occasionally. Plants located in beds or in the yard or garden should be watered when the top couple of inches of the soil feel dry. Use a soaker hose or a slow trickle of water to water over an extended period to deliver deep, intensive watering.
I suspect this is root rot. Make sure the soil is well draining.
If the roots can recover the plant may survive.