MOLECULAR CHARACTERIZATION OF WHEAT STRIPE RUST PATHOTYPES ( Puccinia striiformis f. sp. tritici ) AND THEIR VIRULENC CEPATTERN, , COLLECTED FROM PAKISTAN AND U.S.A

Abstract

Stripe rust or yellow rust caused by Puccinia striiformis f. sp. tritici is an economically important disease of wheat in Pakistan and U.S.A. Present study was carried out to evaluate the diversity of virulences in pathogen populations of both countries. Additionally molecular markers were employed to study genetic diversity of stripe rust fungus. To control the disease, wheat genetic resources were tested for resistance in Pakistan and U.S.A. to enhance cultivar improvement efforts. To observe virulence pattern of stripe rust under field conditions, trap nursery consisting of wheat stripe rust differentials, near isogenic lines and commercial varieties were planted at four locations; Pirsabak, Islamabad, Faisalabad and Sakrand in Pakistan. Results revealed that genes Yr3, Yr5, Yr10, Yr15, Yr26, YrSP and YrCV were effective under field conditions. The adult plant resistance (APR) gene Yr18 showed moderate resistance. This early warning will serve as guideline to incorporate diverse sources of resistance in future wheat cultivars. The virulence testing of 55 Puccinia striiformis f. sp. tritici (PST) isolates (46 from Pakistan and 9 from United States) was carried out. Avirulence to the genes Yr5 and YrSp was common among all collected isolates from both countries. Additionally isolates collected within Pakistan showed avirulence to differential genotypes carrying gene(s) Yr10, Yr15,YrCV(Yr32) and (Yr10, YrMor). The pathogen races from United States were avirulent to genes Yr5, Yr15 and YrSP whereas exhibited lower frequency to differential genotypes containing gene(s) Yr1,Yr25, (Yr3a, YrD, YrDru), YrTye and Yr17+.Virulence profile implied that the pathogen populations from both countries have moderately common virulence pattern. Clustering based on virulence data grouped contemporary isolates together and revealed high genetic diversity among pathotypes of both countries. Molecular studies of same set of 55 isolates (46 from Pakistan and 9 from United States) were also carried out. Simple sequence repeat (SSR) and sequence tagged site (STS) markers revealed higher diversity based on Polymorphic Information Content (PIC) value and Marker Index (MI). PIC and MI was higher for SSRs (0.78 and 139.51, respectively) than STS marker system (0.04 & 0.29, respectively). Dendrogram based on molecular marker data grouped together contemporary pathotypes showing their genetic similarity. Appearance of PST races from Pakistan and USA in same cluster indicated the common ancestory among pathogen population. Furthermore, lower correlation (r=0.08) between virulence and molecular diversity was observed showing independence of both trends of diversity Resistance testing of 115 wheat land races originally collected from Pakistan was carried out at seedling stage with total 7 pathotypes of the pathogen, 4 from the U.S. and 3 from Pakistan. The germplasm showed lack of all-stage resistance. Only two genotypes were resistant against all pathotypes. Adult-plants of the wheat genotypes were also tested at high-temperatures under green house and field conditions in at Pullman and Mount Vernon, Washington in the U.S. The greenhouse and field tests revealed that the majority of the wheat landraces possess high temperature adult plant (HTAP) resistance against stripe rust. These landraces should be useful genetic resources for the U.S. and Pakistan where climatic conditions are suitable for the expression of the durable type of resistance. Under field conditions and greenhouse studies for stripe rust resistance in Pakistan revealed that Synthetic Hexaploids have seedling resistance with few possible genes; Yr3, Yr5, Yr10, Yr15, YrSP and YrCV. Whereas testing of advanced lines and Chines Cultivars showed adult plant resistance under field conditions, and majority was susceptible at seedling stage. Both type of resistance sources; seedling and adult plant durable resistance studied in wheat germplasm of Pakistan offer diverse sources of resistance and promising genetic stock for accumulating seedling and adult plant resistance to acquire durable and long lasting resistance against stripe rust pathogen.