Ceratobasidium oryzicola, a new fungal species causing sheath blight on rice in Brazil

Rice (Oryza sativa) is among the most cultivated and consumed cereals in the world, and is of vital importance in ensuring global food security. Diseases are among the many factors limiting the increase in rice productivity. Currently, three species of Rhizoctonia (R. solani AG-1 IA, R. oryzae and R. oryzae-sativae) have been reported as causing sheath blight in rice, being one of the most important diseases in rice crops in several countries. However, in the 2021/22 harvest, a new pathogen was found in Rio Grande do Sul, Brazil, causing symptoms of sheath blight in a rice planting area. The fungus was isolated from symptomatic rice plants and morphologically and molecularly characterized. The initial identification of the fungal isolate was carried out by analyzing its morphological characteristics and contrasting the ITS sequences (nuc rDNA ITS1-5.8S-ITS4) and rpb2 (RPB2-980F and fRPB2-7cR) with the sequence database available on the GenBank platform, using the BLASTn algorithm, as a probable new species of Ceratobasidium. Therefore, phylogenetic analyses were carried out to confirm our hypothesis. The isolate obtained was allocated to distincts, well-supported clades with Posterior Probability (PP) and Bootstrap values (BS) above 70 in all trees generated. Moreover, we also compared these sequences of C. oryzicola to those of Ceratobasidium sp. C484, another rice-derived isolate, demonstrating that their similarity is greater than that observed among strains of Ceratobasidium ramicola species. With these results we propose a new Ceratobasidium species named Ceratobasidium oryzicola. The pathogenicity test confirmed that this new species also causes sheath blight in rice crops. This is the first description of this new species of Ceratobasidium that had not yet been identified in the world. The identification of this new pathogen, causing symptoms identical to those of other species of Rhizoctonia, is extremely important for implementing correct management measures and for rice genetic improvement programs.