Physiologic specialization ofPuccinia reconditaf. sp.triticiin Canada in 1995

In 1995 leaf rust of wheat (Triticum aestivum L.) caused by Puccinia recondita f. sp. tritici was first observed on June 16 in winter wheat fields in southern Manitoba. Warm summer temperatures with frequent dew periods favoured spread of the disease in spring wheat fields throughout southern Manitoba in 1995. High levels of infection were found on wheat cultivars that had only resistance gene Lr13. Wheat cultivars with Lr13 and Lr34 had moderate levels of rust infection, while cultivars with Lr13 or Lr34 plus either Lr21 or Lr16 had very little leaf rust. Forty-six virulence phenotypes of P.r. tritici were identified in 1995 using 16 near-isogenic Thatcher differential lines. Phenotypes with virulence to Lr16 were detected in Manitoba and Saskatchewan for the first time since 1988. Selected isolates were also tested for virulence on adult-plants of isogenic lines with Lr13, Lr34, and Lr13,34. Most isolates were virulent to isolines with Lr13, while all isolates had lower infection type and fewer uredinia on isolines with Lr34 or Lr13,34. Continued increase of virulence to genes Lr13 and Lr16 may erode the leaf rust resistance of recently released western Canada spring wheats.