Heritability of early blight resistance in a Lycopersicon esculentum × Lycopersicon hirsutum cross estimated by correlation between parent and progeny

Sixteen-hundred BC₁ plants of a cross between an early blight (EB) susceptible tomato (Lycopersicon esculentum Mill.) breeding line ('NC84173'; maternal and recurrent parent) and a resistant accession ('PI126445') of the tomato wild species Lycopersicon hirsutum Humb. and Bonpl. were grown in a field in 1998. This population was segregating (among other traits) for growth habit, self-incompatibility and earliness in maturity. To eliminate confounding effects of these factors on disease evaluation and h² estimation, plants that were self-incompatible, indeterminate and/or late-maturing were eliminated. The remaining plants (146), which were self-compatible and determinate (sp./sp.) in growth habit, with early- to mid-season maturity, were evaluated for EB resistance and self-pollinated to produce BC₁S₁ seed. The 146 BC₁S₁ progeny families, consisting of 30 plants per family, were grown in a replicated field trial in 1999 and evaluated for EB resistance and plant maturity. For each of the 146 BC₁ plants and corresponding BC₁S₁ families, the area under the disease progress curve (AUDPC) and final disease severity (final percentage defoliation) were determined and used to measure disease resistance. The distributions of the AUDPC and final percentage defoliation values in the BC₁ and BC₁S₁ generations indicated that resistance from 'PI126445' was quantitative in nature. Estimates of h² for EB resistance, computed by correlation between BC₁S₁ progeny family means and BC₁ individual plant values, ranged from 0.69 to 0.70, indicating that EB resistance of 'PI126445' was heritable. Across BC₁S₁ families, a small, but significant, negative correlation (r = -0.26, P < 0.01) was observed between disease resistance and earliness in maturity. However, several BC₁S₁ families were identified with considerable EB resistance and reasonably early maturity. These families should be useful for the development of commercially acceptable EB-resistant tomato lines.