Pm4b from spelt wheat 'Hubel' confers recessive powdery mildew resistance

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is a devastating disease affecting yield and quality. The spelt wheat cultivar 'Hubel' exhibits resistance at both seedling and adult plant stages. Although a previous study placed the resistance gene in Hubel on chromosome 2D, we re‑evaluated its location using bulked segregant RNA sequencing combined with high‑density genetic mapping and mapped the resistance locus to a 1.84cM interval on chromosome arm 2AL, flanked by markers 2A77898 and 2A78083. This interval corresponds to a 1.85Mb region in the spelt wheat PI190962 reference genome (v1.0) and is associated with a single recessive resistance locus, temporarily designated pmHubel. Sequence analysis revealed that the resistant parent 'Hubel' carries a sequence identical to the cloned Pm4b allele, whereas the susceptible parent 'Altgold' and the PI190962 reference genome lack the Pm4b allele. Gene‑specific marker assays, independent mutants, and virus‑induced gene silencing confirmed that the resistance is conferred by Pm4b and that pmHubel represents a recessive allele at the Pm4 locus. Our findings correct the previous 2D assignment of this resistance gene, highlighting the necessity of homoeology‑informed marker selection in polyploid wheat. Furthermore, we demonstrate that the powdery mildew resistance in spelt 'Hubel' is conferred by Pm4b, and its recessive behavior in this background is likely associated with reduced transcript abundance in the heterozygous state.