Screening of Supa Aromatic Rice (Oryza Sativa L.) F2 Population for Blast (Magnaporthe Oryzae) Resistance at Irri-Esa Burundi
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Blast disease (Pyricularia oryzae or Magnaporte oryzae) is a disease of the economic importance in rice, causing up to 100% yield loss. The aroma trait of rice grain encoded by the badh2 gene significantly increases its market value. Unfortunately, the aromatic IR97012-27-3-1-1-B (Supa234) line developed at IRRI is susceptible to Pyricularia grisea pathotypes. Improvement of IR97012-27-3-1-1-B (Supa234) rice has been carried out through introgression of R genes (Pi9 and Pita) at IRRI-ESA, Burundi. However, studies confirming if the genes have been introgressed into the Supa234 F2 generation have not been carried out. Therefore, this study aims to; determine blast disease severity of the segregating (F2 rice plants) population at vegetative and reproductive stages, to assess the introgression of Pita and Pi9 genes for blast resistance and to assess the presence of badh2 gene for aroma in F2 plants. Blast resistance of F2 IR97012-27-3-1-1-B plants was evaluated in IRRI-ESA breeding HUB from August 2017 to April 2018. One hundred and thirty-five plants selected from the F2 plants grown in trays were inoculated in vitro with two blast isolates (POBDI A11-01 and POBDI A11-02) on detached leaves. Disease severity was determined by analyzing the transformed scores of symptoms recorded on the inoculated leaflets. The DNA was extracted and genotyped from leaves of 103 selected rice plants which had grown to maturity. The genotyping was carried out using KASP genotyping method targeting Pita, Pi9 genes for blast resistance and badh2 gene for aromatic fragrance. Variations of disease severity mean score among the rice plants were analyzed using Two-way Analysis of Variance (ANOVA). There was a significant difference (p < 0.001) in infection scores between the rice genotypes and disease severity scores between Pyricularia oryzae Burundi A11-001 (POBDI A11-001) and Pyricularia oryzae Burundi A11-002 (POBDI A11-002) isolates. There were 31.85% resistant plants at vegetative stage while at reproductive stage, 15.56% of the plants were high resistant and 46.67% of the plants were resistant. The plants with high resistant (HR) type of infection response had the mean of transformed scores ranging from 0 to 0.075 while plants with resistant (R) type of infection had mean of transformed scores varying from 0.151 to 0.367. Disease severity decreased from vegetative stage to reproductive stage. At vegetative stage, 25.63% of the screened plants were moderately susceptible while at reproductive stage 2.22% were moderately susceptible. The genotyping results show that 38 F2 plants had Pita gene represented on both alleles, 31 F2 plants with Pita gene on one allele and only one plant (3B1) was found with Pi9 gene on one allele. The badh2 gene for aroma was detected in 27 F2 plants on both alleles and in 57 F2 plants on one allele. There were thirteen plants which had both Pita gene and badh2 gene for aroma and only one plant (3B1) had a combination of three genes (Pita, Pi9 and badh2). By comparing phenotypical and molecular results, 7 resistant plants (2H2, 2H4, 1G2, 1C12, 1E13, 1B12 and 1C5) with Pita and badh2 genes were found and only one resistant plant (3B1) had a combination of three genes Pi9, Pita and badh2 which is recommended to be bulked and evaluated for grain quality (Supa type) for the development of Supa aromatic variety resistant to blast disease. The resistant Supa aromatic variety will be used to enhance rice quality in Eastern and Southern Africa.