Comparison of field, greenhouse, and detached-leaflet evaluations of tomato germ plasm for early blight resistance

Twenty-nine tomato genotypes (cultivars, breeding lines, and plant introductions), representing three Lycopersicon species, were evaluated for resistance to early blight (EB) caused by the fungus Alternaria solani. Evaluations were conducted in replicated trials in multiple years under field and greenhouse conditions (with whole plants) and in growth chamber (with detached leaflets). In the field experiments, plants were evaluated for disease symptoms, and area under the disease progress curve (AUDPC) and final percent defoliation were determined. In the greenhouse experiments, plants were evaluated for percent defoliation following spray-inoculation with isolates of A. solani. In the growth chamber experiments, lesion radius, rate of lesion expansion, and final disease severity were determined for individual detached leaflets inoculated with isolates of A. solani. There were significant differences among genotypes in their response to A. solani infection in the field, greenhouse, and growth chamber experiments. In the field and greenhouse experiments, disease response varied from near-complete resistance in some accessions of the wild tomato species L. hirsutum (e.g., PI126445 and LA2099) to complete susceptibility in tomato cultivar New Yorker and breeding line NC84173. The previously developed EB-resistant breeding lines 88B231, 89B21, C1943, NCEBR-1, NCEBR-2, NCEBR-5, NCEBR-6, NC24E, and NC39E exhibited more resistance than New Yorker and NC84173. Field and greenhouse results were comparable across replications and years, and there were great correspondences (r ≃ 0.71, P < 0.01) between field and greenhouse resistance across genotypes. In contrast, results from the detached-leaflet assays were inconsistent across experiments and not correlated with either greenhouse or field results. The overall results indicate the utility of greenhouse evaluation and the inadequacy of detached-leaflet assay for screening tomatoes for EB resistance.