Introduction to Fungi, Third Edition

626 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
Fig 22.12 Diagrammatic representation of the distribution in
Canada of eight physiological races of wheat stem rust during
the period 1919 1951 (after Johnson,1953).
changes in the virulence of the rust population,
and this led to the use of supplementary
differentials to differentiate between further
physiological races. Gradually the nomenclature
for numbering rust races became quite
complicated. In the wake of the acceptance of
the gene-for-gene concept (see Flor, 1971), a
more rational way of identifying the
physiological races was to base their classification
on the resistance genes of the host plant. Using
the wheat cultivar Marquis, several ‘single gene
lines’, i.e. wheat cultivars carrying single resistance
genes, have been generated. The resistance
genes were given the symbols Sr1, Sr2, etc.
(Sr for ‘stem rust’). Some 40 resistance genes
are known (Roelfs, 1985). Efforts at characterizing
pathogen races are now standardized by
means of an international nomenclatural system
(Roelfs & Martens, 1988). An authoritative
account of the history and importance of rust