PATHOTYPE VARIATION OF PUCCINIA STRIIFORMIS POPULATION TRIGGERING STRIPE RUST OF WHEAT IN THE NORTHERN PUNJAB AND NWFP

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 value7 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, YrSu, YrSk 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.