Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

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Pathotypes and human-guided evolution of Puccinia
striiformis West. f.sp tritici in Pakistan
Munawar Hussain, 1 M.A.S. Kirmani 1 and Ehsan-ul-Haque 2
1. Crop Disease Research Programme, NARC, Islamabad, Pakistan;
2. Crop Disease Research Programme, Murree, Pakistan
Analysis of yellow [stripe] rust collections during the 1969–1995 crop seasons
yielded 46 pathotypes (race groups). Amongst these race groups, 16E16, 66EO,
38E16, 64EO, 66EO, 67EO, and 66E(16) were predominant. These races carry
virulence for genes Yr1, Yr7 and Suwon 92×Omar. The local tall genotypes,
like C-271, C-273, C-591 and cv. Local White, were susceptible to these races,
while extensively cultivated semi-dwarf wheats, e.g. Mexipak 65, Chenab 70,
Khushal 69, Barani 70 and SA 42, were resistant. The reduction in acreage
under rust-susceptible tall cvs and large-scale cultivation of resistant varieties
bred by wheat breeders increased pathogen selection pressure. As a result cvs
Mexipak 65 and Chenab 70 succumbed to yellow rust pathotype 66EO in 1973.
The pathotype remained most prevalent during 1973–76, Wheat cvs SA 42,
Blue Silver, Lyallpur 73, Yecora, Pari 73 and Sandal 73 showed resistance to
this pathotype. Later, with changing varieties during 1977–80, the race pattern
also changed, and race groups 6E16, 38E16 and 6(38)E16 were predominant.
These pathotypes were responsible for the stripe rust epidemic of 1978. These
pathotypes were virulent on YR host genes Yr2, Yr6, Yr7, Yr8, YrA and Strobes
Dickopf. The pathotypes attacked cvs possessing YrA, (ArZ, Bahawalpur 79,
Chenab 79, Nuri 70), YrA, Yr6 or YrA+Yr6 (cvs LU 26, Lyallpur 73, Pari 73,
Sandal 73, Yecora, etc.), and cvs carrying Yr2 (Blue Silver, Sonalika and
WL711).
After 1980, a number of wheats with IB/IR translocation carrying Yr9 singly
or in combination with Yr27 were released. At that time the pathogen
population did not carry matching virulence for Yr9. By the early 1990s these
cvs occupied very large areas in the yellow rust-prone area of Pakistan. As a
result of selection pressure exerted by cultivation and release of rust resistant
cvs by the breeders, progressive increase in virulence on these cultivars was
observed during 1997 and 1998. The pathotype responsible for these epidemics
was 134E150. Wheat cvs Pak 81, Kohinoor, Faisalabad 85, Pirsabak 85,
Punjab 85, Sutlej 86, Khyber 87, Rawal 87, Rohtas 90, Pasban, Pirsabak 91,
Bakhtawar 93 and Kaghan 93 were found susceptible.
It is evident from these studies that the course of evolution of yellow rust is
guided by breeders, and races with low virulence have developed into
pathotypes with greater virulence.