First Report of Soybean Rust Caused by Phakopsora pachyrhizi in Ghana

Authors: BANDYOPADHYAY, RANAJIT - INT'L INSTITUTE - NIGERIA; OJIAMBO, PETER - INT'L INSTITUTE - NIGERIA; TWIZEYIMANA, MATHIS - INT'L INSTITUTE - NIGERIA; ASAFO-ADJEI, BAFFOUR - CROP RES INSTIT - GHANA; Frederick, Reid; Pedley, Kerry; Stone, Christine; Hartman, Glen

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2007
Publication Date: 8/1/2007
Citation: Bandyopadhyay, R., Ojiambo, P.S., Twizeyimana, M., Asafo-Adjei, B., Frederick, R.D., Pedley, K.F., Stone, C.L., Hartman, G.L. 2007. First Report of Soybean Rust Caused by Phakopsora pachyrhizi in Ghana. Plant Disease. 91(8):1057.

Interpretive Summary: Nigeria is the only country in West Africa where soybean rust, caused by Phakopsora pachyrhizi, has been officially reported. During a disease survey in Ghana in October 2006, soybean leaves with rust symptoms were observed in 11 fields. Disease incidence in these fields ranged from 50 to 100%, and disease severity ranged between 3 to 40% of the leaf area on infected plants. Leaf samples were examined at the USDA-ARS Foreign Disease-Weed Science Research Unit for verification of pathogen identity. All the 11 field samples were positive based on molecular typing. Infected leaves from three fields were separately washed in sterile water to collect urediniospores and were used to separately inoculate three detached leaves of a susceptible cultivar. Lesions on inoculated leaves developed 5 to 6 days after inoculation, and pustules formed 7 to 8 days later. This is the first report of P. pachyrhizi in Ghana. This information about the spread of soybean rust to other countries is important to soybean pathologists and epidemiologist who are studying the movement of soybean rust throughout the world.

Technical Abstract: Nigeria is the only country in West Africa where soybean rust, caused by Phakopsora pachyrhizi has been officially reported (1). During a disease survey in Ghana in October 2006, soybean (Glycine max) leaves with rust symptoms (tan, angular lesions with erumpent sori exuding urediniospores) were observed in 11 fields in the following districts: Kassena Nankana in the Upper East Region; East Gonja, Central Gonja and Tolon-Kumbungu in the Northern Region; and Ejisu-Juabeng in the Ashanti Region. Disease incidence in these fields ranged from 50 to 100%, and disease severity ranged between 3 to 40% of the leaf area on infected plants. Urediniospores were hyaline, minutely echinulate, 23 to 31 × 14 to 18 µm in size. Within a week of collection, leaf samples were couriered to USDA-ARS Foreign Disease-Weed Science Research Unit for verification of pathogen identity. DNA was extracted from leaf pieces containing sori using the Qiagen DNeasy Plant Mini kit (Valencia, CA), and all the 11 field samples amplified in a real-time fluorescent polymerase chain reaction with the P. pachyrhizi-specific primers Ppm1 and Ppa2 (2). Sequence alignment of the internal transcribed spacer (ITS) region 2 further confirmed the identification as P. pachyrhizi (2). Infected leaves from three fields were separately washed in sterile water to collect urediniospores that were used to separately inoculate three detached leaves of susceptible cultivar TGx 1485-1D (3). Lesions on inoculated leaves developed 5 to 6 days after inoculation (DAI), and pustules (105 to 120 µm) formed 7 to 8 DAI and erupted 3 days later exuding columns of urediniospores similar to size of initially collected isolates. Inoculating another set of detached leaves with a spore suspension (1×106 spores per ml) from the first set of detached leaves resulted in typical rust symptoms. The PCR assay, alignment of ITS region 2, morphological characters of the isolates and pathogenicity tests demonstrate that P. pachyrhizi occurs in Ghana. To our knowledge, this is the first report of P. pachyrhizi in Ghana.