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Screening Oryza Species Plants for Rice Sheath Blight Resistance

Rice wild relatives, Oryza species, are one possible source of sheath blight (Rhizoctonia solani) resistance genes. However, Oryza spp. cannot be screened in the field as is done for cultivated rice (O. sativa) because the plant canopy does not favor disease development and many plants drop mature seed. Thus, a growth chamber-greenhouse method of screening Oryza spp. and their early generation progeny is needed. Primary-secondary and ratoon tillers of rice cultivars-germplasm which ranged from moderately resistant to very susceptible were evaluated first for sheath blight susceptibility. Plants were inoculated by placing R. solani-colonized toothpicks at the leaf collar, then incubating plants in a growth chamber. After 7 days, plants were visually rated for sheath blight severity, and the lesion length of each leaf was measured. Ranking of cultivar-germplasm susceptibility by visual rating of primary-secondary tillers corresponded to the ranking from field ratings. Visual ratings correlated best with combined lesion length of the second and third leaves. For ratoon tillers, visual ratings correlated best with second-leaf lesion length. Next, this method was used with ratoon tillers to evaluate sheath blight susceptibility of 21 Oryza spp. accessions and F₁ progeny from crosses between 17 accessions and cultivated rice. This method proved useful on a limited scale for screening germplasm that could not be evaluated under field conditions.