Morpho-pathological characterization of Sarocladium oryzae causing sheath rot in paddy

Sheath rot caused by Sarocladium oryzae (Sawada) W. Gams & D. Hawksw is an important seed-borne fungal disease of paddy, which is slowly becoming a major concern to the rice-growing farmers in India. Very limited information is available about this pathogen from India as well outside. Keeping this into consideration, a comprehensive study was carried out at ICAR-IARI New Delhi from 2017 to 2019. Diseased samples from 55 locations were collected and isolations were made. It was very interesting to observe that many isolates did not show any sporulation in case they were isolated from sheath and very little in few. While those isolates obtained from seeds were profusely sporulating. Amongst media, Czapek Dox Agar was found to be giving better growth of colonies in comparison to the Potato Dextrose Agar medium. A higher temperature range of 28–30 °C was found to give better growth and sporulation of cultures which is different behaviour than many fungal species which thrive best at an optimum temperature of 25 °C. Based on colony diameter and sporulation, thirty isolates from different geographical regions have been grouped into three broad categories of fast, medium and slow sporulating. Based on the size of conidia at 40× compound microscope, the maximum size of conidia observed was 8.50 µm × 2.05 µm in isolate SO1 while, the minimum size was 5.50 µm × 1.45 µm in isolate SO7. There was no correlation of conidial size with the virulence of the isolates. Based on colony, these thirty isolates have been grouped into six categories representing different geographical regions of India.