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 introduc tions), representing three Lycopersicon species, were evaluated for resista nce to early blight (EB) caused by the fungus Alternaria solani. Evaluation s were conducted in replicated trials in multiple years under field and gre enhouse conditions (with whole plants) and in growth chamber (with detached leaflets). In the field experiments, plants were evaluated for disease sym ptoms, and area under the disease progress curve (AUDPC) and final percent defoliation were determined. In the greenhouse experiments, plants were eva luated for percent defoliation following spray-inoculation with isolates of A. solani. In the growth chamber experiments, lesion radius, rate of lesio n expansion, and final disease severity were determined for individual deta ched leaflets inoculated with isolates of A. solani. There were significant differences among genotypes in their response to A. solani infection in th e field, greenhouse, and growth chamber experiments. In the field and green house 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 breed ing line NC84173. The previously developed EB-resistant breeding lines 88B2 31, 89B21, C1943, NCEBR-1, NCEBR-2, NCEBR-5, NCEBR-6, NC24E, and NC39E exhi bited more resistance than New Yorker and NC84173. Field and greenhouse res ults were comparable across replications and years, and there were great co rrespondences (r approximate to 0.71, P < 0.01) between field and greenhous e resistance across genotypes. In contrast, results from the detached-leafl et assays were inconsistent across experiments and not correlated with eith er greenhouse or field results. The overall results indicate the utility of greenhouse evaluation and the inadequacy of detached-leaflet assay for scr eening tomatoes for EB resistance.