Characterization of Sclerotium rolfsii Causing Root Rot of Sugar Beet in Bangladesh

Sugar beet plants in the experimental plots of Bangladesh Agricultural University showed wilting symptom along with white cottony mycelia and brown sclerotia on crowns and surrounding areas on the soil surface. We estimated 9.74% of plants in 144 sugar beet plots were infected with Sclerotium rolfsii that showed root rot symptom. Isolation of the fungus from infected tissues, and obtaining pure culture was carried out following standard protocol on potato dextrose agar (PDA) medium. Pure culture on PDA produced fluppy cottony mycelia with clamp connections and brown sclerotia ranging from 0.5 to 1.5 mm, typical of S. rolfsii within 3 weeks of incubation at ambient laboratory temperature (22 ± 2 °C). We further confirmed the identity of two isolates BTSB2 and BTSB6 by performing phylogenetic analysis of the internal transcribed spacer sequences along with morphological features. We also proved Koch’s postulate to confirm that isolate of S. rolfsii is the causal agent of sugar beet root rot, not a secondary fungal contaminant. Isolate BTSB2 grew well on PDA and OMA with an average radial growth rate of 2.3 and 2.1 mm/day, respectively. We found mannitol and glucose as the best carbon sources to support radial growth, whereas sucrose and starch supported maximum sclerotia production on synthetic medium. Highest radial growth was favored by media pH 5.0–7.0 at temperature 25 °C. This study characterized S. rolfsii as a causal agent of root rot of sugar beet in Bangladesh by detailed morphological and molecular investigations. Optimization of factors that support quick growth of the pathogen on synthetic media will help in easy production of inocula required for large-scale future inoculation studies.