Genetic Analysis of Resistance to Blast Disease in Rice (Oryza Sativa L.)

Abstract

The inheritance of leaf and kernel resistance to rice blast disease was studied in five crosses involving six rice cultivars. Parents, F,» F2,and Ft families of these crosses were grown in the field under natural and artificial infection at the Faculty of Agriculture Experimental Farm, Alexandria University, Egypt, during the 1971-1974 seasons. These crosses were classified into three groups, namely; resistant x resistant (R x R), resistant x susceptible (R x S), and susceptible x susceptible (S x S) according to their reaction to blast. Such groups included one, two, and two crosses, respectively.
Dealing with the leaf reaction, two different duplicate dominant genes for resistance were detected in the R x R cross, G. 170 x Y. M.T. Each parent probably carried one of both genes. Obviously, two dominant gene pairs governed the segregation in the two R x S crosses. G. 170 x I.R.8 and G.I70 x Ir.376. One of these dominant genes was the -gene for resistance, present in the resistant parent (G.I70), while the other might be present in each of the susceptible parents, I.R.8 and lr.376, and conditioned the susceptibility to leaf reaction. Phenotypically, all genotypes carrying the dominant gene for resistance were resistant probably because this gene masked the expression of the dominant gene for susceptibility. Hence, the phenotypic F2 ratio was found to be 12R.4S in these two R x S crosses. In the two S x S crosses, T.(N.)I x Na. and T.(N.)1 x I.R.8, susceptibility was completely dominant and was conditioned by two dominant duplicate genes, each of which could be carried by each parent.
Regarding the kernel type of reaction, two different duplicate dominant genes for resistance were found in the R x R cross, G. 170 x Y. M.T. A single gene pair might be carried by each resistant parent. Two pairs of dominant genes controlled the segregation in the R x S cross, G. 170 x Ir.376; the gene for kernel resistance in the resistant parent (G.170) and another gene for susceptibility in the susceptible parent (lr.376), the t expression of which was masked by the former gene. The phenotypic ratio was I2R .4S.
In the second R x Scross,G.170 x I.R.8, the segregation was also conditioned by two dominant genes. However, the gene for susceptibility, which could be present in the susceptible parent (I.R.8), dominated and masked the expression of the gene for resistance in the resistant parent (G.170). So, the phenotypic ratio was 4R:12S. In the two S x S crosses, two dominant duplicate genes, with complete dominance of suscept-ibility to kernel reaction, were detected. Each susceptible parent could carry a single gene pair.
With respect to the genetic association between leaf and kernel reactions, highly significant values were obtained in the cases of the R x S cross, G.170 x Y.M.T., and S x S cross, T.(N.)1 x I.R.8, using the F2 data. In the R x S cross, G.170 x I.R.8, a highly significant value was detected on the basis of F3 data. Moreover, a highly significant genetic association was obtained in the case of the S x S cross, T.(N.)1 x Na., according to the F3 data.