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6

Pathogenic variation of Phakopsora pachyrhizi in Nigeria

Presenter: Mathias Twizeyimana(1)

Other authors and affiliations: P. S. Ojiambo(1), C. Paul(2), G. L. Hartman(2, 3), R. Bandyopadhyay(1). (1)International Institute for Tropical Agriculture, Ibadan, Nigeria; (2)University of Illinois, Department of Crop Sciences, Urbana, IL, U.S.A.; (3)USDA-ARS, Urbana, IL, U.S.A.

Soybean rust (Phakopsora pachyrhizi) is endemic to soybean growing areas in Nigeria. To determine the pathogenic variation, infected leaves were collected from 85 locations in the Derived Savanna, Southern Guinea Savanna, Northern Guinea Savanna, and Mid-Altitude agroecological zones. A total of 116 single-spore isolates was inoculated on four accessions containing Rpp1, Rpp2, Rpp3, and Rpp4 genes and on two highly resistant and two highly susceptible accessions. Based on principle component analysis and severity data, an adequate summary of pathogenic variation was obtained using only four of these differentials. Of the four differentials, the source of Rpp1 was most resistant and TGx 1485-1D was the most susceptible to all isolates. Using cluster analysis, seven races of P. pachyrhizi occurred with race clusters 1 and 2, accounting for 84.5% of all the isolates. All seven race clusters were present in the Derived Savanna, unlike the other agroecological zones where only some races occurred. Shannon diversity index indicated considerable variation in the population of P. pachyrhizi between agroecological zones, with diversity being highest in Derived Savanna and Southern Guinea Savanna zones (h = 1.17 and 0.907, respectively). Knowledge on the distribution of pathogenic races may help in deployment of rust-resistant cultivars.

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