Determining potential sources of race non-specific resistance to the late blight pathogen, Phytophthora infestans (Mont.) de Bary, in advanced breeding materials of cultivated potato, Solanum tuberosum L.

Abstract

Phytophthora infestans (Mont.) de Bary, the causal agent of late blight disease of potato, is widely considered to be the most important plant disease of potato. The presence of major genes (R-genes) in breeding materials under improvement for resistance to this oomycete plant pathogen has created difficulties in the identification of minor genes (polygenes), and has hindered breeding efforts focused on concentrating sources of race-nonspecific resistance into the cultivated potato (Solanum tuberosum L.). The International Potato Center (CIP) responded to this challenge in the 1990's by developing a methodology for R-gene extraction by use of detached leaf assay experimentation. The present study analyzes 31 advanced clones of the LBHT population, under improvement at CIP for race-nonspecific resistance to late blight disease and performance under elevated temperatures (>18 degrees Celcius), to determine potential sources of race-nonspecific resistance in the absence of known, Solanum demissum Lindl., derived R-genes. As a second objective we compared the disease response of the 31 advanced clones challenged with isolates; PE8006, Pox109, Pox155, and Pox110, in detached leaf assay experiments, with disease responses observed in previous field trials, where materials were challenged with the prevalent field inoculum found in Oxapampa, Peru, in order to validate methodology.

The nine advanced clones; 398180.292, 398190.89, 398208.219, 398208.704, 302531.43, 302533.49, 302534.43, 302542.62, and 302551.26, are reported to be either free of Solanum demissum derived R-genes or containing partially incompatible R-genes. From within these nine clones, a single clone (302551.26) demonstrated a response to inoculation by both a race 0 (Isolate PE8006) as well as the three complex races tested, which exceeded 50% mean lesion area in detached leaf assays. This response was confirmed by field evaluations. These results indicate that clone 302551.26 has the greatest potential to be free of known S. demissum derived R-genes according to established methodology. Of the nine advanced clones of interest, three clones (398180.292, 398190.89, and 398208.704) have demonstrated levels of resistance lower than 50% across both lab and field evaluations. These materials may contain either, partially defeated R-genes or high levels of polygenic resistance to P. infestans. The horizontal character of their resistance could be used to the benefit of breeding programs working towards the objectives of improving race-nonspecific resistance to P. infestans.

Using the Pearson correlation coefficient it was determined that a correlation at P less than or equal to 0.001 exists between the evaluation factors of detached leaf assay sporulation intensity (Lab PI) and detached leaf assay lesion area (Lab LA) within each individual isolate of P. infestans. However, a correlation between the factors of evaluation, (Lab LA) and field evaluation of lesion area (Field LA) were found in only a single complex isolate, Pox109 at P less than or equal to 0.05. These results validate the use of Lab PI and Lab LA together to increase certainty in resistance responses of materials tested, as well as, demonstrate that the use of known lab isolates in screening resistant materials is largely unrepresentative of virulence found in the seasonal field inoculum in Oxapampa, Peru.