Analysis of blast resistance gene composition in Malagasy rice (Oryza sativa L.) cultivars, landraces and exotic lines

Sixty seven varieties were selected for blast resistance gene identification based on blast nursery results from Madagascar. Isozyme analyses were performed to characterize the varieties followed by pathogenicity test using selected blast isolates and a PCR-based marker linked to the resistance gene Pi-z sup 5. The isozyme analyses yielded a high index of diversity (H = 0.29). Among the 67 varieties, 39 belonged to group 1 (indica), 10 to group 1*, 8 to group 6 (japonica), 4 to intermediate group and 6 were non classified (INC). Under controlled inoculation using isolates of known virulence spectrum, IR-lines displayed generally a reaction pattern comparable to the NILs carrying similar gene(s) while gene inference was very limited for the Malagasy traditional varieties. Resistance gene Pi-b, Pi-k sup m and/or Pi-kp are frequent in the exotic lines while they were found only in two Malagasy landraces. Pi-z sup 5 was not found in the whole sample. Pi-ta sup 2 was inferred to be present in three IR-lines and, Pi-1 and Pi-k together in SR-7466-18-1-2-3-4 and Minehikare, while Pi-k without Pi-1 might be present in Rojofotsy. Pi-ta with Pi-? were found to be common throughout the sample. Pi-ta is associated with one of Pi-b, Pi-kP or Pi-k sup m in IR-lines, while Pi-ta is with other unknown genes in Malagasy landraces. Thirteen varieties were resistant to all isolates tested. Nine of them originated from Madagascar. Six of them belong to japonica group. Tsipala (VG337) was susceptible to all isolates used. As indicated by Tche Kouai response, the virulence genes present in the different populations of blast nurseries of Madagascar are not present in the set of Philippine isolates used in the study. Results of gene inference in the present IR-lines are consistent with previous work done in IRRI [International Rice Research Inst., Los Banos, Laguna, Philippines]. Traditional varieties of Madagascar carry several unknown genes