Cotton plant (Gossypium hirsutum L.) belonging to the family Malvaceae is an important fiber yielding plant throughout the world. During the period of January and February 2013, severe boll rot symptom was noticed on cotton varieties CB-1 to CB-11 those were grown in the Botanic Garden, University of Dhaka. Rhizopus oryzae Went & Prins. Geerl. was diagonised as causal agent of boll rot of cotton. Among the cotton varieties examined CB 6 and CB 11 were found severely attacked by the fungus showing boll rot symptom. Disease severity of fruit recorded 0-6 scale. Prevalence of R. oryzae was 100%. Present investigation revealed that R. oryzae was pathogenic to cotton. The boll rot of cotton caused by R. oryzae is new record in Bangladesh.

Cotton plants suffer from more than 20 fungal diseases throughout the world (Wikipedia, 2013). In Bangladesh, the most common diseases of the crop are anthracnose, Alternnaria leaf spot, Cercospora leaf spot, Rust, Sclerotium rot, Wilt, and Boll rot (Lutfunnessa and Shamsi, 2011). Among the diseases, boll rot is considered as the most destructive one. In the USA, at least 170 microorganisms are capable of causing cotton boll rot. The majority of the damage is attributed to the species of Fusarium, Diplodia, Glomerella,