• The leaves become yellow and die-back and when the plants are pulled up, roots are found to be rotten and the base of the bulb covered with a white or grey fungal growth.
• Later, numerous small black spherical sclerotia are produced. The bulb of the onion completely rots.

• Crop rotation and clean seed are the only effective control.
• Heavy manuring with organic manures reduces the disease in the crop. Seed dressing with Benomyl, Carbendazim or Thiophanate-methyl (100 to 150 g/kg seed) gives effective control

Source:

Images: https://www.rhs.org.uk/advice/profile?pid=226 , Plate: Mariaterasa, R., Francesa. S, Santo. P, Demetrio. S, Angela, R and E.A.Giovanni.2013. Essential oil chemical composition and antifungal effects of Sclerotium cepivorum of Thymus capitatus wild popilations from Calabria, Southern Italy. Rev. bras. farmacogn .,23 (2 ), pp-239-248.