Phenotyping early blight resistance in potato cultivars and breeding clones

Thirty-two potato cultivars/clones were evaluated for resistance to early blight using different methods. The evaluations were performed under field and greenhouse conditions. In the field experiments, plants were evaluated for disease symptoms, and the relative area under the disease progress curve and percent defoliation were determined. In the greenhouse experiments, leaf lesion sizes were determined on either intact plants or detached leaves after drop inoculation with Alternaria solani. The effect of leaf position (lower, middle or upper part of the plant) on lesion size was investigated. There was no correlation between lesion sizes on lower leaves and upper leaves after inoculation. However, significant correlations between lesion sizes on lower and middle leaves and also between middle and upper leaves were found. Furthermore, we found significant correlation between the results of resistance studies in the field and in intact plant inoculation experiments in the greenhouse. In contrast, results from the detached-leaf experiment were not correlated with either greenhouse intact plant tests or field results. The results indicate that using detached-leaf assays for screening potato for early blight resistance is not accurate. We found significant differences in resistance to A. solani among cultivars/clones in both the field and in greenhouse experiments.