Email: brent.mccallum@agr.gc.ca

The Thatcher-Lr1 near isogenic wheat line (RL6003) was noticed to have a mixture of resistant and susceptible plants when inoculated with a small number of Canadian leaf rust (Puccinia triticina Eriks). Genetic analysis of the resistant plants revealed the presence of a second unlinked resistance gene. This gene, mapped to 7AL and given the temporary designation LrCen, is characterized by an unusual mesothetic infection type.

A similar infection phenotype, in response to the same small set of P. triticina isolates, was found for a gene in RL6071 (Prelude/8*Marquis*2/3/Prelude//Prelude/8*Marquis), however this gene mapped to 7BL and was temporarily named LrMar. These two resistance genes appear to be homeologous and condition resistance to the same small group of isolates.

When Canadian wheat cultivars were tested phenotypically and genotypically for the presence of these genes LrCen was widely distributed, whereas LrMar was only found in Marquis, Red Fife and closely related cultivars. LrCen also appears to be widely distributed worldwide in wheat germplasm including otherwise susceptible cultivars like Chinese Spring. Given that LrCen only conditions resistance to a very small group of isolates, it is interesting that it seems to be so widespread in wheat cultivars throughout the world, which may indicate another function for this gene that is leading to selection for LrCen.