White cottony rot on my mustard stems with hard black lumps inside, what is it?

Wilting plants with white cottony growth on the lower stem and hard black lumps like rat droppings inside a hollow stem is Sclerotinia stem rot. Those black bodies are sclerotia that survive in the soil for years and start new infections, so containment and rotation matter. Remove and destroy infected plants before the sclerotia drop into the soil, and do not leave residue in the field. Avoid a too dense, lush, over fertilised canopy that stays damp at the base, since the disease thrives in humid still air, so proper spacing and balanced nitrogen help. Follow a long rotation away from mustard and other susceptible hosts. A recommended fungicide at the early flowering stage can protect the crop where the disease is regular, following label dose and PHI and rotating groups. Confirm the disease and any fungicide with your KVK or PAU Ludhiana.