D.J. Mackill1, J.M. Bonman1, H.S. Suh2 and R. Srilingam3
1)International Rice Research Institute, P.O. Box 933, Manila Philippines; 2) Yeungnam University, Gyeongsan, Korea; and 3) Department of Agriculure, Paranthan, Sri Lanka
Systematic studies of genes for resistance to rice blast (Pyricularia oryzae Cav.) have mostly been conducted in temperate countries using Japonica rices. Kiyosawa (1981) identified 13 R genes in Japan at 8 different loci. Five of these were introduced from Indica into Japonica rices. Systematic studies on Indica rices in the tropics have been difficult due to the diversity of the host and pathogen and the environmental variability encountered in culturing the fungus. Recent work at IRRI has shown that stable isolates can be maintained, and genetic studies have been initiated.
Results of Suh et al. (1983) and Srilingam (1984) have shown that the number of resistance genes is high (Table 1). For all cultivar/isolate combinations studied, two dominant duplicate genes were implicated in 59% and one dominant gene in 30% of all cases. Only cultivars with single R genes could be reliably tested for allelism. In many cases, linkage analysis was performed by inoculating the same F2 or F3 plants with more than one isolate. Resistance to different isolates was often inherited independently. At present we do not know the total number of genes in these cultivars that confer resistance to the seven isolates used. However, it would require a minimum of 18 different R genes in the nine cultivars studied to explain all of the results.
In order to fully characterize these R genes they must be individually transferred into a susceptible background. It has been difficult to identify a suitable Indica cultivar which is sufficiently susceptible to be used as a recurrent parent. The cultivar CO 39, a short duration Indica cultivar from India, has been used as a susceptible check in the International Rice Blast Nursery (IRBN). It is highly susceptible to all the isolates being used at IRRI. We are now transferring R genes from various donor parents to CO 39 by backcrossing so that more detailed genetic analaysis can be done.
Table 1. Number of blast R genes in some rice cultivars, summarized from data of Suh et al. (1983) and Srilingam (1984).
============================================================================== Cultivar Origin Typea Number of R genes for each isolate ------------------------------------------ IK81 IK81 PO3 PO6-6 PO3 PO3 43 -25 -3 -82-41 -82-51 -82-17 ============================================================================== Pai-kan-tao Taiwan,China J 1 1 1b 1 1 1 Dawn U.S.A. J 2 2 2 2 2 2 Moroberekan Guinea J 2 2 2 1 2 1 Pankhari 203 India 1 1 1 Ram Tulasi India 3 3 3 2c 2 3 Ta-poo-cho-z Taiwan, China I 2d 2d 2 2d 2 2 2d Carreon Philippines I 2e 2 2 2f 1 2 1 Tetep Vietnam I 1 2 1 (S)g 2 5173 Columbia I 2 2 2 1 2 2 ==============================================================================aJ=Japonica, I=Indica (based on isozyme analysis by Dr. J.C. Glaszmann, personal communication). Pankhari 203 and Ram Tulasi do not fit into either group.
Kiyosawa, S., 1981. Gene analysis for blast resistance. Oryza 18:196-203.
Srilingam, R., 1984. Inheritance of resistance to blast in some rice varieties. Master's Thesis, Post Graduate Institute of Agriculture, University of Sri Lanka, Peradeniya.
Suh, H.S., G.S. Khush, M.H. Heu, G.S. Chung, F. L. Nyque, and J. M. Bonman, 1983. Inheritance of resistance to blast disease in rice and breeding strategies. IRRI Saturday Seminar. January 8, 1983.