Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2008
Publication Date: 4/1/2009
Publication URL: http://handle.nal.usda.gov.d2.nal.usda.gov/10113/29186
Citation: Twizeyimana, M., Ojiambo, P.S., Haudenshield, J.S., Caetano-Anolles, G., Pedley, K.F., Bandyopadhyay, R., Hartman, G.L. 2009. Pathogenic variation of Phakopsora pachyrhizi infecting soybean in Nigeria. Phytopathology. 99(4):353-361. Interpretive Summary: Soybean rust is a disease caused by a fungus, which has developed into a serious threat in most major soybean producing countries in the world. Its recent spread to Africa, South America and North America has led to increased concerns about the control of the disease and the genetic structure of the pathogen. In this study, molecular DNA markers were used to determine that a high degree of genetic variation occurred in the sampled populations of the fungus. This information is important to plant pathologists, population biologists, and microbial geneticists that are interested in knowing about genetic diversity in this important soybean pathogen.
Technical Abstract: Soybean rust, caused by Phakopsora pachyrhizi, is a major disease in many soybean-producing areas in Nigeria. To determine the virulence and the genetic structure of Nigerian field populations of the soybean rust pathogen, a total of 116 purified isolates established from infected leaves randomly collected from soybean fields in four agroecological zones in 2005 were used. The virulence variability of the isolates was determined using a set of four soybean accessions with Rpp1, Rpp2, Rpp3, and Rpp4 resistance genes, two highly resistant and two highly susceptible genotypes. Principal component and cluster analyses on the number of uredinia per cm2 of leaf tissue separated the rust isolates into seven pathotype clusters. Isolates in cluster III were the most virulent, while those in cluster IV were the least virulent. In a follow-up study, 18 simple sequence repeat markers were used to study the genetic diversity using the same 116 isolates and an additional 146 isolates collected from infected plants in two fields (73 isolates in each field) located 292 km apart. There was a high genetic variation in Nigerian P. pachyrhizi populations. Eighty-four distinct genotypes were identified among isolates from the three agroecological zones, while 48 distinct genotypes were identified from 146 isolates analyzed from both fields. Nei’s average genetic diversity across geographical regions was 0.22 while both fields had 0.09. Hierarchical analysis of molecular variance revealed significant (P < 0.05) and low genetic differentiation among all populations of P. pachyrhizi. However, the majority (> 90%) of the genetic diversity was distributed within a soybean field, while almost 6% was distributed among fields within geographic regions. The phylogenetic analysis showed three groups in Nigerian rust populations with one major group comprising more than 90% of the isolates. However there was a poor correlation between virulence and genetic variation. This work will be useful in breeding and management of soybean rust by facilitating the deployment of rust-resistant cultivars.