A new disease of Cosmos sulphureus Cav. causing external and internal stem discoloration, premature death, and wilting was observed in 27.8% of plants with an average disease severity rating of 4.4 in Gazipur, Bangladesh.Morphological, pathological, and molecular analyses identified the isolated fungus as Sclerotinia sclerotiorum (Lib) de Bary, the causative agent of white rot disease. The optimum growth and sclerotium formation of S. sclerotiorum occurred at 20°C and pH 5.0, while glucose, peptone, yeast extract, casein, and ascorbic acid were the appropriate nutrient sources. Furthermore, mycelial growth and sclerotial development were favored in media containing potassium, magnesium, calcium, and sodium. As many as 20 plant species of 10 families;