Background resistance to bacterial blight (BB): lesion expansion [Philippines]

Lesion expansion in infected leaves as an indicator of background resistance was analyzed. Component measurements may show differences in resistance among IR cultivars possessing the Xa-4 gene. Six races of the BB [bacterial blights] pathogen and IR20, IR22, IR26, IR36, IR42, IR48, IR54, IR58, and IR62, all known to carry Xa-4, were examined. Plants were raised in pots in the greenhouse, with two replications/cultivar arranged in a split-split-plot design. Plantings were staggered at 20-d intervals to synchronize inoculation. Three races-PX086, PX079, PX099- virulent on cultivars carrying Xa-4, and three avirulent races - PX061, PX0112, and PX071- were used to inoculate plants at 20, 40, and 60 d after sowing (DAS). IR24, which is not known to carry Xa-4, was the susceptible check. Lesion areas were estimated at 9, 15, and 23 d after inoculation (DAI) and converted to a relative resistance index (RI) based on the lesion area of IR24. (The estimation of RI based on lesion area was similar to that based on hill or leaf infection.) Here, RI is an indication of lesion expansion across time after initial infection. The IR cultivars showed high relative resistance to the three avirulent races. To the virulent race that matched gene Xa-4, relative resistance was low. In all cases, the virulence X resistance index interaction was not significant, apparently indicating that backgrounds resistance does not interact with race. RI for PX061, avirulent to Xa-4 and PX086, virulent, were compared to determine the effect of Xa-4 and the corresponding background resistance on selected IR cultivars. By 20 DAS, Xa-4 had shown its effect, as indicated by high RI to PX061. Background resistance was indicated by RI unmatched by PX086. At 40 and 60 DAS, lesions ceased to expand. RI values decreased because of the susceptible check cultivar. Lesion expansion may be a better parameter to detect the effect of both Xa-4 and background resistance at an early stage. Such measurements may provide an accurate assessment of background resistance, as the variance seems to be small