Biological and molecular variability of Sarocladium oryzae, the sheath rot pathogen of rice (Oryza sativa L.)

Sheath rot disease of rice caused by Sarocladium oryzae (Sawada) (=Acrocylindrium oryzae, Sawada) has become an important production constraint in all rice-growing countries. Pathogenicity, phytotoxic metabolites, and random amplified polymorphic DNA (RAPD) markers were used to assess the level of genetic variability of S. oryzae derived from rice cultivars, CR1018, IR36, and IR50, of different locations in North East and South India. Variability in pathogenicity, phytotoxic metabolite production, and DNA polymorphisms was detected among S. oryzae isolates. Results indicated that S. oryzae isolates produced both cerulenin and helvolic acid at concentrations 0.3-0.62 and 0.9-4.8 microg mL(-1) of culture filtrate, respectively. Isolates that produce higher concentration of helvolic acid induced a high percent incidence of sheath rot disease. Oligonucleotide primers, GF and MR, generated either a simple (up to 2 bands) or complex (up to 6 bands) RAPD pattern. According to their level of similarity, S. oryzae isolates from North East and South India were grouped separately into two major clusters and 13 genotypes. Molecular- and pathogenicity-based classifications were not correlated, but a high level of genetic variability within S. oryzae isolates was identified. The molecular variability of S. oryzae isolates will be an important consideration in breeding programs to develop durable resistance for sheath rot disease.