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The paucity of genetic variation for the resistance score may reflect limitations of visual ratings in discerning small differences in disease severity among highly resistant genotypes (Shaw et al., 2009), whereas significant variance for the percentage of pathogen-free petioles implies that colonization extent might be superior in resolving differences among resistant genotypes. However, the actual level of statistical significance for the two traits differed little in magnitude: P?=?0��02 and 0��10, respectively, and the component of variance attributed to genotypes was small in both cases (Table?2): 10��1 and 11��5% of the residual error variance for percentage pathogen-free petioles and resistance score, respectively. The genotypic relationship between resistance score and the percentage of pathogen-free petioles changed somewhat during the season, but genotype-by-date interaction variances were non-significant for both variables (Table?2), suggesting a correspondence between pathogen development in the plant and symptom expression over the growing season. Furthermore, plots of mean genotypic resistance scores vs. their genotypic mean for the percentage of pathogen-free petioles were similar when expressed for the sampling dates independently (Figs?2 and 3). The coefficients for regression of genotypic resistance score on the percentage of pathogen-free petioles differed slightly (b?=?0��026 and 0��018 for the May and July evaluations, respectively), but not significantly (t18?=?0��87, P?=?0��21). The extent of colonization was a somewhat better predictor of visual symptoms in July (r2?=?0��67) than in May (r2?=?0��52), but the experiment did not provide sufficient precision to establish the statistical significance of this difference. Individual plant resistance scores had a strong phenotypic correlation (r?=?0��57, P? importantly, 35��1% of the 366 plants with at least one infected petiole showed no disease symptoms (resistance score?=?5). The average percentage of pathogen-free petioles for this symptomless subset was 64��5% and ranged from 0�C94��5%, demonstrating that even extensive within-plant colonization need not lead to disease symptoms. The genotypic correlation between the percentage of pathogen-free petioles and the resistance score was large and highly significant (rg?=?0��77, P?