Thanks for your question. The yellow leaves and stalks rotting near the soil make me wonder if the soil is too wet. Excess moisture supports growth of fungi that can weaken and kill the plant. Remove affected leaves, stalks and plants (do not compost or use this plant material as mulch, in order to avoid reintroducing the fungi), and be careful not to overwater. If the plants are in clay soil or if the soil isn't draining well, try amending it with finished organic compost or move the plants to another location with well draining soil. Good luck!