Genetic analysis of, and selection for, factors affecting quantitative resistance to Xanthomonas campestris pv. oryzae in rice

Summary

Four cultivars moderately resistant to Philippine isolates of Xanthomonas campestris pv. oryzae races 2, 3, 5 and 6 and highly resistant to race 1 were crossed with two susceptible cultivars and with each other. The F₁ populations were as or more susceptible than the least resistant parent cultivar when assessed for lesion length (cm) by clip-inoculating booting plants with two race 2 and one race 6 isolates of X.c. oryzae. The F₂ populations showed continuous distributions when assessed with the race 6 isolate PX099, although populations from crosses between moderately resistant cultivars were positively skewed. Mean broad-sense heritability in the F₂ was 0.50. Selection for varying levels of resistance was carried out in the F₂ and F₃ generations. F₃ lines selected from the F₂ population modes had variances and ranges equal to those selected from the F₂ population extremes and larger than the variances of the parent cultivars.

Line selection in the F₃ generation was more effective than plant selection in the F₃ and in the F₂. Realized h² was 0.39 for line selection in the F₃ but only 0.24 for plant selection. A number of lines more resistant than both parents were recovered in crosses between moderately resistant cultivars. Lines more susceptible than both parents were also recovered in crosses between moderately resistant cultivars, but few of these lines were as susceptible as the susceptible cultivars. This indicates that the moderately resistant cultivars had some resistance factor(s) in common. All test cultivars, including the susceptible cultivars, carry few to several factors for quantitative resistance. A model based on nine minor resistance factors is proposed to explain the pattern of transgression found in crosses between the six cultivars.