Integrated molecular and phenotypic screening of Pakistani wheat landraces for resistance to stem rust (Puccinia graminis f. sp. tritici)

Stem rust caused by Puccinia graminis f. sp. tritici remains a major threat to global wheat production, necessitating the identification and deployment of durable resistance genes. Among these, the adult plant resistance gene Sr2 in Triticum aestivum is widely recognized for conferring partial but durable resistance. This study aimed to determine the presence of Sr2 in 86 Pakistani wheat landraces using the linked molecular marker stm598tcac and to evaluate their phenotypic response to stem rust under field conditions at multiple disease assessments. PCR amplification with the Sr2-linked marker failed to produce the expected 56 bp diagnostic fragment in any accession, indicating the apparent absence of Sr2 in the evaluated germplasm. Field evaluations revealed limited resistance across observations. At the first assessment, only a few landraces exhibited resistant (R) reactions, while most were moderately susceptible (MS) or susceptible (S). Disease severity increased at the second observation, where only seven accessions remained resistant and the majority showed high susceptibility (≥80S). By the third assessment, nine accessions were classified as resistant (R/10MR), eleven as moderately resistant, four as moderately susceptible, and the remainder as susceptible (≥60-100S). Notably, accessions 210904, 210898-210903, and 220072 consistently expressed resistance across stages. In conclusion, the absence of the Sr2-linked marker and the predominance of susceptible reactions indicate that Sr2-mediated resistance is largely lacking in these Pakistani wheat landraces. However, a small subset of accessions demonstrated stable phenotypic resistance, suggesting the possible presence of alternative resistance sources valuable for future stem rust breeding programs.