|Keywords (Extracted from title, table of contents and
abstract of thesis)|
Pathotype, Variation, Striiformis,
Triggering, Rust, Wheat, Humid, Prevailing, Westend, Virulences,
Undesirable, Resistance, Genes, Exploitation
Wheat is one of the
leading grain crops of Pakistan and being staple diet of the
inhabitants, it grasps a key position in the agricultural policies.
Sustainable productivity of wheat is of paramount importance in the
context of many biotic and abiotic factors that limit its
production. Stripe or yellow rust is one of such biotic factors,
caused by an obligate parasite Puccinia striiformis Westend. f.sp.
tritici Eriks., that confines wheat production throughout the world.
Presence of several races of each and ever-changing nature of the
stripe rust pathogen cautions cultivation of susceptible wheat
cultivars in humid, high uplands and cooler regions of the country.
Monitoring of the stripe rust population is imperative to determine
pathotype variation so that new virulences with the potential to
overcome resistance genes currently deployed in the wheat cultivars
can be detected. Accordingly, the research was executed in the
Northern Punjab and NWFP to identify prevailing Pst virulence
pattern and pathotype variation and trap the stripe rust pathogen
through establishing “Trap Nurseries” at selected sites of the study
area; estimate wheat yield losses due to stripe rust in field under
disease stress conditions; and evaluate breeders material for its
disease reaction under field condition.
Analyses of yield loss data confirmed significant loss where disease
level was very high.The most severely diseased cultivars had the
lowest yields. Morocco depicted extremely low yields whereas
Inquilab-91 expressed a loss of only 38.73 kg ha-1 against the
disease severity of 36.25 percent in 2007 as compared to the loss of
143.3 kg ha-1 when the disease severity was just 8.12 percent during
2006. Stripe rust was also severe against
Bakhtawar and Wafaq-2001 during 2007 at the test sites in Rawalpindi
and showed a dramatic affect on yield. Considerable yield losses
were observed in 2007 as compared to the losses estimated during
2006 attributing to the conducive environmental factor that
persisted for more than four weeks and prolonged the infectious
period of the stripe rust pathogen.
Screening of 188 varieties / advanced breeding lines against stripe
rust was also carried out during 2005-06 and 2006-07. Cluster
analysis based on the RRI was performed, which indicated that the
entire cultivars could be distributed into six clusters at 20
percent linkage level. Average Coefficient of Infection (ACI) and
Relative Resistance Index (RRI) values of two year trial showed that
out of 188 cultivars 150 had RRI value>7 9 and were found in the
desirable range; 28 cultivars were included among the acceptable
range having RRI value 5 <7. However, only 10 cultivars showed RRI
value <5 and fell under undesirable range.
In the present study, pathotype variation of Pst population that
occur naturally in the major wheat growing areas of Pakistan were
analyzed. The outcomes were highlighted in the context of prevailing
virulences and identification of the Yr resistance genes that are
still effective. During 2005-06 and 2006-07, 12 previously
identified Pst races were confirmed while one new race was
identified. In Pakistan, virulence is present for the stripe rust
resistance genes Yr1, Y6, Yr7, Yr8, Yr9, Yr12, Yr17, Yr18, Yr24,
and YrA. Virulence for Yr resistance genes Yr2+, Yr3V, Yr3N, Yr5,
Yr6,2+, Yr7+,Yr9,2+, Yr10, Yr11, Yr15, YrSd, YrCv and YrSp was
neither observed during the glasshouse investigations nor prevalent
at any of the six WSRTN sites. Although resistance genes Yr4+, Yr8+,
Yr26 and Yr29 (Pavon 76) expressed partial virulence but still have
prospective for exploitation. To deploy the identified Yr resistance
genes either singly or in combination in the upcoming wheat breeding
program could play an effective role to lessen yield losses
inflicted by stripe rust.