Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

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Role of yellow rust-resistance
genes of wheat in Pakistan
A. Hakro and Aly Khan
Crop Diseases Research Institute, Pakistan Agricultural Research Council, Karachi
University Campus, Karachi-75270, Pakistan
Introduction
Yellow rust (Puccinia striiformis f.sp. tritici) is one of the most important
diseases of bread wheat (Triticum aestivum) in temperate areas of the world.
Yellow rust can seriously reduce grain yields, particularly when a severe attack
develops before ear emergence. The disease mainly affects the leaves but
glumes can also become infected. Doodson, Manners and Myers (1965)
reported that infected plants of susceptible varieties have a reduced number of
florets and grains per ear and sometimes lower weight of individual grains.
Leaves of infected plants are shorter and narrower than those of healthy plants
and the dry weight of roots of susceptible plants can be reduced by more than
75% following infection.
The control of yellow rust is almost exclusively by the use of resistant
varieties. It has been observed that breeding for resistance to yellow rust has
been a major objective in wheat breeding programmes. There are four types of
resistance to yellow rust: (1) seedling resistance, in which the host is resistant
both as a seedling and as a mature plant; (2) adult plant resistance, in which
the host is susceptible as a seedling but resistant as mature plant;
(3) environmentally determined resistance (commonly called field resistance),
in which resistance can be manifested in seedlings, in the adult plant, or in
both, but the expression of which is liable to changes with temperature; and
(4) tolerance, in which the host is susceptible but yields well despite infection.
Many workers have confirmed that the expression of resistance to yellow
rust can differ greatly at different growth stages of individual genotypes.
Stubbs (1968) has shown that even the first and second leaves of wheat plants
can show quite different responses to yellow rust. In Pakistan, we have tested
commercial varieties carrying YrA, Yr2, Yr4, Yr6, Yr7 and Yr9 genes alone or in
combination, and a few varieties with unknown Yr genes, in different agroclimatic
zones of the country.