Identification of blast resistance genes in rice germplasm is one of the most important activities of rice breeding programs in worldwide. The objective of this research was to characterize two major blast resistance genes, Pi-z and Pi-b, in selected rice germplasm. A simple sequence repeat DNA marker, AP5659-1 linked to the Pi-z gene, and a Pib dominant marker derived from the Pi-b gene were first used to screen a rice core collection consisting of 1700 accessions estimated to represent 70% of the genetic diversity of rice in the US National Small Grains Collection. There were 131 rice germplasm accessions with marker profiles indicating the presence of Pi-z, and178 indicating the presence of Pi-b. This research assessed accessions using the tightly linked SSR markers - AP5659-1, AP5659-5, AP4791 and RM527 - for the Pi-z gene; and a Pi-b dominant marker plus DNA markers RM 208 and RM 166 for the Pi-b gene respectively. Finally, isolates of Magnaporthe oryzae representing differential races, including IB33, IB49 and IE1k for Pi-z; and IB54 and IE1k for Pi-b; were used to evaluate disease reactions of the inoculated rice accessions. Experimental findings indicated that the total number of lines containing Pi-z and Pi-b were reduced to 117 for Pi-z and 164 for Pi-b as a result of seed mix. Using the combination of DNA marker and pathogenicity assays, 81 germplasm accessions with Pi-z and 130 germplasm accessions with Pi-b were identified. In addition, 54 germplasm accessions with different resistance genes were also noted. These accessions carrying Pi-z and Pi-b and other unknown blast resistance genes were once elite commercial cultivars in different rice production regions and may represent valuable genetic resources for breeders to use for improving blast resistance using marker assisted and conventional breeding.
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