Rice Sheath Blight Disease Resistance Identified in Oryza spp. Accessions.

Oryza spp., wild relatives of cultivated rice (Oryza sativa), may contain novel resistance genes for sheath blight, caused by Rhizoctonia solani, that could be used to enhance resistance to this important disease in commercial rice. To identify resistant sources for sheath blight disease, 73 Oryza genotypes were evaluated with three different methods conducted in the greenhouse, growth chamber, or laboratory because there are significant limitations to screening wild Oryza spp. under field conditions. For the microchamber method, 4-week-old seedlings were inoculated with a potato dextrose agar plug containing mycelia, covered with a 2-liter soft drink bottle, and rated 1 week after inoculation. A detached-leaf method involved placing a potato dextrose agar plug containing mycelia on the abaxial surface of a leaf section that was cut from a 5-week-old plant and placed on moist filter paper in a petri dish under constant light, then evaluated after 72 h. For the toothpick inoculation method, toothpicks colonized with mycelia were placed in the leaf collar region of plants at the panicle initiation stage, plants were placed in a growth chamber, and disease symptoms were evaluated after 7 days. The microchamber method gave a more uniform, reproducible response, and was easier to use under greenhouse conditions. Seven Oryza spp. accessions were identified as moderately resistant with three accessions classified as O. nivara (IRGC104705, IRGC100898, and IRGC104443) and one each as O. barthii (IRGC100223), O. meridionalis (IRGC105306), O. nivara/O. sativa (IRGC100943), and O. officinalis (IRGC105979).

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.

Meiotic and isozymic characterization of plants regenerated from euploid and selfed monosomic tall fescue embryos.

Tissue culture of tall fescue (Festuca arundinacea Schreb., 2n=6x=42) would be enhanced by improving the callus induction and plant regeneration efficiency, and evaluating the meiotic and isozymic variation induced by culture. Mature embryos were cultured from four lines of Kenhy tall fescue and from the progeny of three selfed monosomics. Evaluation of six media-auxin combinations showed callus initiation was greatest on SH medium with 2.5 mg/l 2,4,5-T or 7.4 mg/l pCPA, while plant regeneration was greatest on SH medium with 0.5 mg/l 2,4-D. Cytological analyses of 27 plants derived from euploid parents showed a high frequency of aneuploidy (15/27). Chromosome numbers of aneuploids ranged from 36 to 41, with one plant having 80 chromosomes and two plants being asynaptic. Two of ten monosomic-derived plants were euploid, five were monosomic, one was monosomic with a fragment and two were double monosomic. Zymograms of the parents and regenerants were obtained for the enzymes ACPH, ADH, GOT, 6-PGD and PGI. Isozyme variation was observed for two groups of plants derived from the same Kenhy embryos. One group of four monosomic-derived plants differed for the enzymes GOT and ACPH, and all four plants had a PGI pattern. different from that of the parental monosomic plant. This indicated loss of a PGI allele, probably as a result of callus culture.