Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2008
Publication Date: 3/1/2009
Citation: Twizeyimana, M., Ojiambo, P.S., Sonder, K., Ikotun, T., Hartman, G.L., Bandyopadhyay, R. 2009. Pathogenic Variation of Phakopsora pachyrhizi Infecting Soybean in Nigeria. Phytopathology. 99:353-361.
Interpretive Summary: Soybean rust, caused by the fungus Phakopsora pachyrhizi, is an important disease in many soybean-producing countries. It was first reported in the continental United States in 2004. Widespread epidemics have occurred since, but the intensity and time of infection has been late in the season and has not caused significant yield losses. In this study, the distribution of soybean rust and variation of isolates were tested in Nigeria, a country where rust has caused soybean yield losses. To determine the geographical distribution of soybean rust in Nigeria, soybean fields were surveyed in three agroecological zones in Nigeria between 2004 and 2006. Disease incidence was significantly different between geographical zones due to weather conditions. During the 2005 survey, 116 purified isolates were established in culture on detached soybean leaves. Based on data of fungal reproduction on selected soybean lines, the isolates were separated into seven groups. This research is important in that it is the first report to establish grouping of isolates based on fungal reproduction. This research is useful to soybean researchers that are interested in studying the occurrence and distribution of rust based on disease surveys and for those working on variability of the pathogen as it relates to host resistance.
Technical Abstract: Soybean rust is an important disease in Nigeria and many other soybean-producing countries world-wide. To determine the geographical distribution of soybean rust in Nigeria, soybean fields were surveyed in the Derived Savanna, Northern Guinea Savanna, and Southern Guinea Savanna agroecological zones in Nigeria between 2004 and 2006. Disease severity in each geographical zone was determined and analyzed using geostatistics and geographical information systems tools. Incidence of fields with infected plants was significantly different between geographical zones ('2 = 10.9, df = 4, P = 0.03). Across years, disease severity was significantly (P < 0.05) higher in the Derived Savanna than the other two agroecological zones. Semivariogram analysis revealed that rust severity was spatially correlated between fields, suggesting significant field-to-field spread or influence of inoculum in one field on other fields. During the 2005 survey, 116 purified isolates were established in culture on detached soybean leaves. To establish the nature of pathogenic variation in P. pachyrhizi, a set of four soybean accessions with Rpp1, Rpp2, Rpp3, and Rpp4 resistance genes, two highly resistant and two highly susceptible genotypes were inoculated with single uredinial isolates. Principal component analysis on the number of uredinia per cm2 of leaf tissue for 116 isolates indicated that an adequate summary of pathogenic variation could be obtained using only four genotypes. Of these four, PI 459025B (with Rpp4 resistance gene) and TGx 1485-1D had the lowest and highest number of uredinia per cm2, respectively. Based on cluster analysis of the number of uredinia per cm2, seven pathotype clusters were determined. Isolates in pathotype cluster III were the most virulent, while those in pathotype cluster IV were the least virulent. Normalized Shannon’s index of diversity (Sh) indicated phenotypic variation between geographical areas with Sh = 0.25, 0.32 and 0.33 in the Derived Savanna, Northern Guinea Savanna, and Southern Guinea Savanna zones, respectively.