AbstractTwo populations from the crosses DOR 364 x XAN 176 and DOR 364 x WBB-20-1 were tested in the field in the F3 and F4 generations and under greenhouse conditions in the F5 generation to determine the effectiveness of selection for resistance to Xanthomonas campestris pv phaseoli (Xcp) in early generations and to estimate the heritability (h2) of foliage resistance to Xcp. The study was conducted at the Isabela Substation of the University of Puerto Rico in 1991. Significant differences among lines for Xcp reaction were observed in the three generations evaluated.The parents, XAN 176 and WBB-20-1, showed more resistance to Xcp than DOR 364. The h2 of the resistance to Xcp in the leaves was intermediate to high (> 0.60) for the DOR 364 x XAN 176 population. In the DOR 364 x WBB-20-1 population, however, the h2 was low (0.30) in the F3 generation, and intermediate to high (> 0.60) in the F4 and F5 generations. Evaluation in the F3 generation requires at least three replications to have adequate precision to detect differences among lines. Selection in the F4 generation was more effective because of higher heritabilities.The evaluation of the disease on the basis of a 1 to 9 scale was better than estimates of the percentage of leaf area infected because there was less variation among generations. In addition, variances among groups of lines were more homogeneous.
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