Although thyme grows readily for most gardeners, even in poor soil, thyme does present some difficulties. Thyme is particularly sensitive to root rot. Root rot is a fungus that spreads through moist soils, especially during the cooler months in the spring and autumn. If you notice your thyme plants lacking vigor and looking yellow and sickly, root rot may be the culprit. Yellow leaves and decaying roots are the main symptoms of root rot. If you detect this fungus, you must remove the plants and destroy them to prevent the fungus from spreading throughout your herb garden.
Determine whether your thyme plants have root rot. If the thyme leaves are droopy and are dropping from the plant, suspect root rot. If the roots at the base of the plant are exposed, appear unhealthy and are withered, suspect root rot. Often the soil around the roots will be too moist for too long, which should lead you to suspect root rot.
Use the shovel to dig up the thyme plants that appear to be dying of root rot. It is imperative that you dig up every part of the thyme plants to remove them from the planting area completely.
Place the thyme plants into the garbage bag as you remove them. Do not compost these plants because you risk contaminating your compost bin with fungi spores. You must discard or burn root rot plants.
Consider spraying surrounding plants with copper fungicide if you are concerned about root rot fungi spreading throughout the soil. Consult the package recommendations on the copper fungicide spray for the proper spraying amounts for the size of your planting area.