Kenyan isolates of Puccinia graminis f. sp. tritici from 2008 to 2014: Virulence to SrTmp in the Ug99 race group and implications for breeding programs

Authors: NEWCOMB, MARIA - University Of Minnesota; OLIVERA, PABLO - University Of Minnesota; Rouse, Matthew; Szabo, Les; Johnson, Jerry; Gale, Sam; Luster, Douglas - Doug; WANYERA, RUTH - Kenya Agricultural And Livestock Research Organization; MACHARIA, GODWIN - Kenya Agricultural And Livestock Research Organization; BHAVANI, SRIDHAR - International Maize & Wheat Improvement Center (CIMMYT); HODSON, DAVID - International Maize & Wheat Improvement Center (CIMMYT); PATPOUR, MEHRAN - Aarhus University; HOVMOLLER, MOGENS - Aarhus University; FETCH, THOMAS - Washington State University; Jin, Yue

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2016
Publication Date: 7/1/2016
Citation: Newcomb, M., Olivera, P., Rouse, M.N., Szabo, L.J., Johnson, J.L., Gale, S.W., Luster, D.G., Wanyera, R., Macharia, G., Bhavani, S., Hodson, D., Patpour, M., Hovmoller, M., Fetch, T., Jin, Y. 2016. Kenyan isolates of Puccinia graminis f. sp. tritici from 2008 to 2014: Virulence to SrTmp in the Ug99 race group and implications for breeding programs. Phytopathology. 106(7):729-36. doi: 10.1094/PHYTO-12-15-0337-R.

Interpretive Summary: Emergence of new virulence combinations in the Ug99 race group of the wheat stem rust pathogen has incited severe disease epidemics in Kenya in the past decade. The popular cultivar Robin was reported to be resistant to stem rust until 2013 and 2014 when stem rust outbreaks were observed in farm fields planted to Robin. The stem rust pathogen variations and race dynamics in Kenya were investigated by an international team of researchers from USDA-ARS in St. Paul MN and Ft. Detrick MD, the University of Minnesota, the Kenya Agricultural and Livestock Research Organization in Njoro, Kenya, the International Maize and Wheat Improvement Centers in Nairobi, Kenya and Addis Ababa, Ethiopia, Aarhus University in Slagelse, Denmark and Agriculture and Agri-Food Canada, Manitoba, Canada. Six races were detected from 78 field samples collected between 2008 and 2014, three of which differed from races described prior to 2013. Genotypic analysis concluded that the three new races belong to the Ug99 race group. One of the new races, TTKTT with combined virulence to resistance genes Sr24, Sr31 and SrTmp, was used to evaluate advanced breeding germplasm and cultivars at the seedling stage. We found that a high percentage of US wheat lines resistant to previously known races in the Ug99 race group, were susceptible to race TTKTT. These findings indicate the increased virulence and variability in the Kenyan wheat stem rust population, and increased vulnerability of wheat to emerging races with added virulence.

Technical Abstract: Frequent emergence of new variants in the Puccinia graminis f. sp. tritici (Pgt) Ug99 race group in Kenya has made pathogen survey a priority. We analyzed 140 isolates from 78 Pgt samples collected in Kenya between 2008 and 2014 and identified six races, including three not detected prior to 2013. Genotypic analysis of 20 isolates from 2013 and 2014 collections showed that the new races TTHST, TTKTK, and TTKTT belong to the Ug99 race group. International advanced breeding lines were evaluated against an isolate of TTKTT (Sr31, Sr24, and SrTmp virulence) at the seedling stage. From 169 advanced lines from Kenya, 23% of lines with resistance to races TTKSK and TTKST were susceptible to TTKTT, and from two North American regional nurseries 44% and 91% of resistant lines were susceptible. Three lines with combined resistance genes were developed to facilitate pathogen monitoring and race identification. These results indicate the increasing virulence and variability in the Kenyan Pgt population, and reveal vulnerabilities of elite germplasm to new races.