Assessment of Greenhouse and Laboratory Screening Methods for Evaluating Potato Foliage for Resistance to Late Blight.

Greenhouse and laboratory screening methods for assessing potato foliage for resistance to late blight were compared using 15 cultivars and advanced breeding selections with known field response to late blight. Screening methods included greenhouse inoculation of plants in several age classes, and laboratory assays of detached leaflets, leaf disks, and stem cuttings. Greenhouse inoculation of plants 7 to 11 weeks after planting, near the time of flowering, corresponded best to results obtained in field evaluations, but there were significant differences in disease severity between separate greenhouse tests. This is consistent with variation in late blight severity on a year-to-year basis when cultivars are compared in the field. The greenhouse inoculation method allowed for testing of several components of partial resistance, such as infection efficiency and lesion growth rate, which may exist for each cultivar. Laboratory assays proved less reliable than greenhouse assays for overall ratings of partial resistance, but could be useful for measuring specific components of resistance. Screening evaluations for late blight resistance should include standard cultivars with known reaction to Phytophthora infestans to reference the disease potential within the screening evaluation.