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Asian soybean rust found in Ghana on soybeans for first time
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8/20/2007 10:30 a.m. CDT -- Asian soybean rust was found in 11 soybean fields in Ghana last October -- the first report of rust caused by Phakopsora pachyrhizi in West Africa outside of Nigeria.
According to August issue of The American Phytopathological Society journal Plant Disease, during a disease survey in Ghana during October 2006, soybean leaves with rust symptoms 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 percent, and disease severity ranged between 3 and 40 percent of
the leaf area on infected plants.
Urediniospores were hyaline (transparent), minutely echinulate (set
with small spines), and 23 to 31 by 14 to 18 micrometers. Within a
week of collection, leaf samples were sent to the USDA Agricultural Research Service Foreign Disease-Weed Science Research Unit for verification of pathogen
identity.
DNA was extracted from leaf pieces containing sori (clusters of sporangia), and all 11 field samples amplified in a real-time fluorescent polymerase
chain reaction (PCR) with P. pachyrhizi-specific primers. Sequence alignment further confirmed the identification as P. pachyrhizi.
Infected leaves from three fields were separately washed in sterile water
to collect urediniospores that were used to separately inoculate three
detached leaves (for each isolate) of a susceptible soybean cultivar. Lesions on inoculated leaves developed five to six days after inoculation, and pustules (105 to 120 micrometers) formed seven to eight days after inoculation and erupted three days later exuding columns of urediniospores similar in size to the initially collected isolates. Inoculating another set of detached leaves with a spore suspension from the first set of detached leaves resulted in typical rust symptoms.
The PCR assay, sequence alignment, morphological characters of the
isolates, and pathogenicity tests demonstrate that P. pachyrhizi
occurs in Ghana, the report said. The authors said: "To our knowledge, this is the first report of P. pachyrhizi in Ghana."
Authors of the article were:
R. Bandyopadhyay, P. S. Ojiambo, and M. Twizeyimana, International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria; B. Asafo-Adjei, Crop Research Institute, Kumasi, Ghana; R. D. Frederick, K. F. Pedley, and C. L. Stone, USDA-ARS Foreign Disease-Weed Science Research Unit, Fort Detrick, Md.; and G. L. Hartman, USDA-ARS and Department of Crop Sciences, University of Illinois, Urbana.
According to commentary accompanying the report in a ProMED-mail post from the International Society for Infectious Diseases, Phakopsora pachyrhizi was identified in Japan in 1902 as the cause of soybean rust. It was first confirmed on the African continent on soybean in central Uganda in 1996, although several
unsubstantiated reports suggest an earlier presence on other legumes.
The fungus can cause premature defoliation, with yield losses of up
to 80 percent reported in Asia. It is now widespread in Australasia
and parts of Africa and has recently been reported from a number of
countries in the Americas, but data are lacking concerning its
establishment in Europe.
Source: The American Phytopathological Society, Plant Disease 2007; 91(8): 1057.
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