Screening potatoes for resistance to late blight (Phytophthora infestans) under field conditions

Summary Late blight incidence under natural epiphytotic conditions was recorded on 270 potato germplasm accessions for three years. Replication effect was non-significant, whereas genotype and year effects were significant for AUDPC (area under disease progress curve). Disease incidence on the second date of observation (when the control had almost completely blighted) accounted for 85 to 96 percent of the variation in AUDPC. Spearman’s Rank correlation coefficients between AUDPC and disease incidence grades scored on 1–9 and 1–5 scales (on second date of observation) based on the means of three years data were similar to those calculated based on the means of any two years. It is concluded that grading the accessions based on discrete scales at an appropriate single date for two years in a non-replicated trial would be sufficient robust to broadly group accessions into different levels of resistance to late blight. Ten accessions were found to be highly resistant.