Clean the soil around the base of the thyme plant. Remove any accumulated dead leaves.
Remove any dead or yellow leaves from the herb plant.
Spray the plant with a copper fungicide if the root rot is particularly widespread.
Plant thyme in raised beds to encourage proper drainage.
Avoid overwatering, and be particularly careful if you live in a humid climate. Thyme prefers poor soil but is almost sure to develop root rot if the soil is too moist.
Mulch with builder's sand or limestone gravel to improve drainage.
Ensure that your thyme is in full sun and that the soil pH is around 7.0.