First Report of Rust Caused by Phakopsora pachyrhizi on soybean in Democratic Republic of Congo

Authors: OJIAMBO, PETER - INT. INSTITUTE - NIGERIA; BANDYOPADHYAY, RANAJIT - INT. INSTITUTE - NIGERIA; TWIZEYIMANA, MATHIS - INT. INSTITUTE - NIGERIA; LEMA, ANDREW - ISIRO - COMMUNE CONGO; Frederick, Reid; Pedley, Kerry; Stone, Christine; Hartman, Glen

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/2007
Publication Date: 9/1/2007
Citation: Ojiambo, P.S., Bandyopadhyay, R., Twizeyimana, M., Lema, A., Frederick, R.D., Pedley, K.F., Stone, C.L., Hartman, G.L. 2007. First Report of Rust Caused by Phakopsora pachyrhizi on soybean in Democratic Republic of Congo. Plant Disease. 91(9):1204.

Interpretive Summary: Soybean rust caused by fungus Phakopsora pachyrhizi has not been reported to occur in the Democratic Republic of Congo in Central Africa. Soybean leaves with rust symptoms were observed in 10 fields. Rust incidence in these fields ranged from 85 to 100%, while severity ranged between 3 to 35% leaf area diseased. Urediniospores were hyaline, minutely echinulate, 23 to 31 × 16 to 20 µm in size. Leaf samples were examined at the USDA-ARS Foreign Disease-Weed Science Research Unit for verification of pathogen identity. All the 10 field samples were positive based on molecular typing. Infected leaves from three fields were separately washed in sterile water to collect fungal spores 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 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: Soybean rust caused by Phakopsora pachyrhizi has not been reported to occur in the Democratic Republic of Congo (DRC) in Central Africa. Nigeria (1) and Uganda (3) are the closest countries to DRC in the west and east, respectively, where the disease has been reported. In February 2007 during a disease survey in DRC, soybean (Glycine max) leaves with rust symptoms (tan, angular lesions with erumpent sori exuding urediniospores) were observed in 10 fields in the following areas in Bas Congo province: Bangu, Kimpese, Kolo-Fuma, Lukala, Mbanza-Ngungu, Mpalukide, Mvuazi and Ntemo. Rust incidence in these fields ranged from 85 to 100%, while severity ranged between 3 to 35% leaf area diseased. Urediniospores were hyaline, minutely echinulate, 23 to 31 × 16 to 20 µm in size. Within a week of collection, infected leave samples were sent to USDA-ARS Foreign Disease-Weed Science Research Unit for verification of pathogen identification. DNA was extracted from leaf pieces containing sori using the Qiagen DNeasy Plant Mini kit (Valencia, CA), and all the 10 field samples amplified in a real-time fluorescent polymerase chain reaction with the P. pachyrhizi-specific primers Ppm1 and Ppa2 (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 (4). Lesions on inoculated leaves developed 5 days after inoculation (DAI), and pustules (118 to 120 µm) formed 7 DAI and erupted 2 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, morphological characters of the isolates and pathogenicity tests demonstrate that P. pachyrhizi occurs in DRC. To our knowledge, this is the first report of P. pachyrhizi infecting soybean in DRC.