Blast, caused by Pyricularia grisea, is a major constraint on rice production. To broaden genetic diversity for resistance to this disease, two rice cultivars, GA20 and GA25 from Yunnan Province, China, were analysed for the genetic basis of their high resistance to blast. GA20 was crossed with 10 Japanese differential cultivars, and GA25 was crossed with nine of them and with the susceptible Chinese cultivar Lijiangxintuanheigu (LTH). The resistance of GA20 was governed by two dominant genes allelic to genes at thePi-k and Pi-ta loci. The allele at the Pi-k locus was new, based on a reaction pattern different from known alleles at this locus. It could not be shown whether or not the allele at thePi-ta locus is new, because races with virulence forPi-ta were not tested. GA25 has one resistance gene, which is not allelic to genes at the loci Pi-a, Pi-k, Pi-z, Pi-ta, Pi-b, andPi-t, but is linked to the Pi-i gene on chromosome 9 with a recombination frequency of 15·1 ± 2·8%.
The new allele at the Pi-k locus in GA20 is designated asPi-kg(t), and the new resistance gene in GA25 asPi15(t).