Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

16
Pathotypes
Race analysis of yellow rust samples from Nepal started in 1979 with the help
of IPO in Wageningen, and continued with support from DWR (Shimla) after
1993, following Johnson et al. (1972) in the Netherlands and Nagarajan et al.
(1986) in India. As many as 16 pathotypes (0E0, 0E16, 2E0, 4E0, 6E0, 4E16, 7E0,
7E150, 7E158, 15E0, 15E150, 15E158, 66E18, 68E16, 70E0 and 70E16) were
identified at IPO (Karki, 1980; Louwers, van Silfhout and Stubbs, 1992). In
1979/80, 4E16 was dominant; 7E150 frequency increased in 1986–1992. No
pathotype could attack differential lines Triticum spelta var. album (Yr5),
Clement or Riebesel (Yr9), and Moro (Yr10) until the mid-1990s. Pathotypes
46S103 (P), 47S102 (K) and 47S103 (T) were identified during 1993–1997 using
the Indian differential sets (Nayar et al., 1992; Karki, 1994, 1998; Mahato, 1996;
Sharma, 1997). Pathotype 46S119 virulent against Yr9 was detected among
many samples during 1996–1998 (Sharma, 1997; Karki, 1998).
Virulence change and reactions on near-isogenic lines
After RR 21 became susceptible, infection was observed on Annapurna 1
(Veery #5) in Kabre and Kathmandu in the late 1980s. After 1997, Annapurna 1
and several other wheat cultivars possessing the Yr9 gene showed
susceptibility to yellow rust at many locations. Similarly, significant changes
were observed in the western hills, where recommended cultivars
Annapurna 4 and Kanti became severely infected, in addition to varieties
BL 1022 and BL 1066 (Sharma, 1997; Karki, 1998). Limited studies showed that
the virulence spectrum of yellow rust gradually increased over years. Only Yr6
and Yr8 were vulnerable in 1979/80, while resistance genes Yr1, Yr2, Yr6, Yr7
and Yr8 were infected in 1986/87. Yellow rust started attacking Yr3 and Yr4 in
the early 1990s (Louwers, van Silfhout and Stubbs, 1992), and since 1997 cv.
Riebesel, possessing Yr9, was found infected due to presence of pathotype
46S119 (Sharma, 1997).
Near-isogenic lines (NILs) developed in the cv. Avocet background in
Australia were tested under field conditions in Nepal. Data from Lumle
(western hills) during the 2001–2003 wheat seasons are presented in Table 2.
Resistance genes Yr5, Yr10 and Yr15 that were not infected earlier showed
mixed results, with some plants presenting susceptible reactions. Genes Yr24,
Yr26 and YrSP that were resistant are gradually becoming susceptible. All
other genes were no longer effective in Lumle, and check varieties
Annapurna 1, BL 1022 and RR 21 were susceptible. Severity and reaction
increased in 2003 compared with 2001 and 2002.