Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

The data presented in Table 3 show that during 1969–70 pathotypes 64E0,
66E0, 67E0 and 66E(16) were predominant. These pathotypes carry virulence
on genes Yr1, Yr7, Yr8 and YrSU. The local tall varieties like C 271, C 273, C 591
and Local White were susceptible to these pathotypes, but the extensively
cultivated semi–dwarf varieties Mexipak, Mangla 68, Khushal 69, Barani 70
and Chenab 70 were resistant. The reduction in the acreage of rust-susceptible
varieties coupled with large-scale cultivation of resistant cultivars from wheat
breeders subjected the pathogen to selection pressure. As a result, cvs
Mexipak, Chenab 70, Barani 70, Khushal and Mangla succumbed to an
aggressive form of pathotype 66E0 and 70E16 during 1973. These pathotypes
remained most prevalent during 1973–76. The wheat cvs SA 42, Blue Silver and
SA 75 possessing YrA, and cvs Pari 73, Sandal, Yecora and Lyallpur 73
possessing YrA and Yr6, remained resistant to these pathotypes as the
pathogen did not have matching virulence. During 1977–80, more cvs having
YrA, such as Arz, Sonalika, Chenab 79 and Jauhar 78, were released for
cultivation. As a result of the large acreage of wheat having YrA, the
pathotypes acquired matching virulence for YrA. Due to selection, the
frequency of races changed and races 6E16, 6(38)E16 and 38E16 appeared.
These pathotypes were able to infect Yr6+2, Yr7, Yr8, YrSD and YrA. The
cultivars Arz, Blue Silver, Sonalika, Bahawalpur 79, Chenab 79 and Nuri, all
possessing YrA, and cultivars possessing YrA or Yr6, or both, such as LU 26,
Lyallpur 73, Pari, Sandal, and Yecora, became susceptible (Hussain et al., 1988;
Kirmani, 1986; Pervaiz and Johnson, 1986). These pathotypes were mostly
responsible for the yellow rust epidemic of 1978.
During 1980–93, a number of wheats with 1B/1R translocation carrying host
gene Yr9 singly or in combination with Yr7 were released. The pathogen
population prevalent during this period, such as 2(70)E16, 7E150, 7(15)E150
and 66E17, were avirulent on Yr9. By the late 1980s and early 1990s these
genotypically similar cultivars occupied the major area in the yellow rustprone
area of Pakistan. Sporadic progressive increase in virulence was
observed on some of the cultivars. Finally there was a yellow rust epidemic in
1995 that caused an estimated Rs 270 million loss. The 1997 and 1998 epidemics
caused a loss of Rs 2 billion. The pathotype 134E150 was responsible for these
epidemics. Wheat cultivars Pak 81, Kohinoor, Faisalabad 85, Pirsabak 85,
Pirsabak 91, Punjab 85, Sutlej, Khyber 87, Rawal, Rohtas, Pasban, Bakhtawar 93
and Kaghan 93 were amongst those found susceptible.
It is evident from these studies that during the course of evolution of yellow
rust influenced by humans, races with low virulence (64E0) have developed
into pathotypes with higher virulence (Table 4).
The data suggest that the distribution of the factors of virulence of yellow
rust is closely related to the factors of resistance of the cultivated host varieties,
and the evolution of yellow rust develops in the same direction as the humanguided
evolution of host varieties.
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