RESIDUAL EFFECTS OF THE XA-4 RESISTANCE GENE IN 3 RICE CULTIVARS WHEN EXPOSED TO A VIRULENT ISOLATE OF XANTHOMONAS-CAMPESTRIS PV ORYZAE

F2 plants of five, and F3 plants of three, crosses between genotypes carrying the race-specific resistance gene Xa-4 and genotypes not carrying this gene were inoculated with two isolates of Xanthomonas campestris pv. oryzae. Half the tillers of each plant received isolate PXO61, avirulent on the Xa-4 gene, the other half of the tillers received isolate PXO99, virulent for the Xa-4 gene. The F2 and F3 populations segregated for a single dominant resistance gene, Xa-4. The parental, F2 and F3 genotypes not carrying Xa-4 had mean lesion lengths between 28 and 29 cm for both isolates. The Xa-4 carrying parents showed a mean lesion length of 2.7 cm with the avirulent isolate and of 12.4 cm with the virulent isolate. The Xa-4 carrying F2 and F3 genotypes had mean lesion lengths of 5.2 and 20.1 cm for the two isolates, respectively. These observations strongly indicate that the Xa-4 gene, carried by the rice genotypes studied (IR28, Cisadane and BR51-282-8), had a considerable residual effect when exposed to virulent isolate PXO99.