19. A spontaneous mutant for bacterial blight resistance and its inheritance

M. N. SRIVASTAVA, R. K. SAHU, G. R. SAHU and V.N. SAHU

Indira Gandhi Agricultural University, Raipur, M. P. 492 012, India

A spontaneous mutant for resistance to bacterial blight caused by Xanthomonas campestris pv. oryzae, was found in a plot of a purple leaved, semi-dwarf susceptible line R262-2-2-2 during 1984 wet season. It's resistance was confirmed under artificially inoculated conditions during subsequent years (Table 1).

This mutant, designated as RM1 was crossed with two susceptible parents and resulting F1's were grown during 1987 dry season. In 1988 wet season, 55 days old seedlings of parents, F1 and F2 populations were inoculated following the procedure outlined by Kauffman et al. (1973). The individual plants were scored 14 days after inoculation following the Standard Evaluation Syslem (IRRI, 1980).

As shown in Table 2, the F1s were susceptible and the F2 populations segregated in a ratio of 1 resistant to 3 suseceptible. These results show that the mutant has a single recessive gene for resistance. To-date, 15 genes for resistance to this disease have been identified and three of them (xa-5, xa-8 and xa-13) are recessive (Ogawa et al. 1987). Of these three recessive genes, xa-5 does not convey resistance to local races of bacterial blight. The other two recessive genes however, confer resistance to local races and we are now testing the allelic relationships of the new recessive gene with xa-8 and xa-13.

Table 1. Disease reaction of RMI and check varieties during several years at Raipur, M. P., India

================================================================
                                 Disease Score1
Variety                  =======================================
                         1986      1987      1988      1989
================================================================
RM1                        3         3         3         3
R 262-2-2-2                7         7         9         9
IR 54 (R-check)            1         1         1         1
T(N) 1 (S-check)           9         9         9         9
================================================================

1 1=highly resistant; 9=susceptible

Table 2. Reaction to bacterial blight of F1 and F2 populations from the
crosses of RM1 with two susceptible varieties

================================================================
                                Reaction of F2 populations
                             ===================================
Cross         Reaction of F1 Resistant Susceptible X2
                               (no.)      (no.)    1:3  P Value
================================================================
TN1/RM1       Susceptible      117        415      2.56 0.1-0.2
IET 6286/RM1  Susceptible      104        365      2.00 0.2-0.5
================================================================
References

IRRI, 1980. The International Rice Research Institute, Standard Evaluation System for Rice. P. 0. Box 933, Manila, Philippines, p. 36-37.

Kauffman, H. E., A.P.K. Reddy, S.P.V. Hsieh and S. D. Merca, 1973. An improved technique for evaluating resistance of rice varieties to Xanthomonas oryzae, Plant Dis. Reptr. 57: 537-541.

Ogawa, T., 1987. Gene symbols for resistance to bacterial blight. RGN 4: 41-43.