Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

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detected. The spread of this virulence reduced the large-scale cultivation of
these cultivars. Subsequently, PBW 343 (Attila ) was registered as resistant to
yellow rust for cultivation in 1996. This cultivar not only carried the additional
gene Yr27 (Prashar et al., 2004) that protected it against yellow rust but also
had a yield advantage that made it popular amongst farmers of this region.
Due to these advantages, it spread very quickly, leading to a very strong
selection pressure that led to the emergence of a virulent pathotype designated
as 78S84 in 2005 in Punjab (NWPZ). Gradually, this pathotype started to
increase in frequency. At present, only two Yr9-virulent pathotypes are
prevalent in the country. The present study analyses the data of prevalence
(percentage frequency) of both these pathotypes over the yellow rust-prone
zones and their impact on wheat production in the country. Further studies on
the evaluation of the popular cultivars of this region were also conducted to
identify sources of resistance.
Materials and methods
Rust-infected leaves from experimental and commercial fields were collected,
shade-dried and transferred to the laboratory. These leaves were then plated
on 2% water agar in Petri dishes and kept at 10°C overnight. Later, the leaves
were scraped using a sterile lancet needle and the scraping was applied on
one-week-old seedlings of Agra Local (a local tall wheat that is being used as a
susceptible control in India) to increase the inoculum. For each sample, a single
pot (11 cm diameter) of Agra Local containing 6–7 leaves was used. These
inoculated seedlings were then sprayed with a fine mist of water and kept in a
dew chamber overnight at 10°C. These pots were then placed in a greenhouse
maintained at 16–18°C. Uredia developed on leaves of Agra Local after 12–15
days. The multiplied single-leaf-origin uredospores were then collected by
shaking the infected leaves over butter paper (non-hygroscopic paper). The
uredospores thus collected were then used for further inoculation.
For identifying the pathotypes, a set of 21 lines was used, comprising old
European and World set differentials along with a few supplementary
differentials (Nagarajan, Nayar and Bahadur, 1983). This inoculated set was
subjected to same of incubation conditions as above. The infection types,
developed after 12–14 days, were recorded as per Stakman, Stewart and
Loegering (1962). Infection types of 3 or 3+ were classified as susceptible, with
0, 1 or 2 rated as resistant.
Prevalence of a pathotype was determined as percent frequency of total
number of isolates analysed during the year. Following the determination of
the most predominant pathotypes, these were then multiplied in bulk for use
in screening adult plants. A set of the most popular 43 wheat varieties
currently cultivated in different agro-climatic zones of the of the country was
evaluated against the most predominant pathotypes. For adult plant