Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

32
Disease assessment and scoring of host resistance
Observations on response and disease severity were recorded at all locations.
Disease severity was recorded as percentage infection of plants according to
the modified Cobb’s Scale (Peterson, Campbell and Hannah, 1948), where 0 =
no visible infection; R = Resistant (necrotic areas with or without small
pustules); MR = Moderately Resistant (small pustules surrounded by necrotic
areas); MS = Moderately Susceptible (medium-sized pustules, no necrosis, but
some chlorosis possible); and S = Susceptible (large pustules, no necrosis or
chlorosis). Subsequently the scale was converted to a coefficient of infection by
using R = 0.2, MR = 0.4, MS = 0.8, M = 0.6, MS = 0.8 and S = 1. Lines with up to
3 Average Coefficient of Infection (ACI) were considered resistant, lines falling
between 4 and 10 were rated as moderately susceptible, while those scoring
more than 10 were regarded as susceptible.
Results and discussion
Status of Bakhtawar 92 resistance in Pakistan
Bakhtawar 92 has been an important bread wheat cultivar, released in 1993 for
commercial cultivation in the North West Frontier Province (NWFP) by the
NWFP Seed Council. Its parentage is Kauz ‘S’ and has the pedigree CM 67458-
4Y-1M-3Y-1M-5Y-0B. During the pre-release disease-testing period (1988–1992)
the performance of this cultivar against yellow rust was satisfactory for all
prevalent pathogen virulences. Keeping in view its popularity and large
acreage occupied, it was included in the yellow rust monitoring studies.
Table 1 shows results of the observation of the post-release monitoring for
yellow rust resistance carried out for three growing seasons (1999/00, 2001/02
and 2002/03). Rust severity was high in 2002/03 but had been comparatively
low during the preceding years at all three test locations. Bakhtawar 92
consistently displayed a high level of resistance or immunity (as demonstrated
in the pre-release stage) over sites and seasons in comparison with the
susceptible check cv. Local White.
Table 1. Post-release yellow rust resistance status of Bakhtawar 92 at three locations
in Pakistan
Years
Peshawar Nowshara Islamabad
Bakhtawar 92 Local White Bakhtawar 92 Local White Bakhtawar 92 Local White
1999/00 0 20 S 0 40 S Tr-MR Tr-S
2001/02 0 10 S 0 10 S 0 20 S
2002/03 Tr-R 60 S Tr-R 90 S 5 MR-MS 90 S