The demand for soybean in Africa has been increasing steadily driven by the growing feed industry for poultry and aquaculture as well as for home consumption in the form of processed milk, baked beans, and for blending with maize and wheat flour. This in turn has spurred an increase in soybean production to respond to the growing demand―the crop’s production in sub-Saharan Africa has doubled over the last fifteen years.
However, the demand for soybean in Africa still outweighs the supply and hence a lot of soybean and soybean products are imported mostly from India, Argentina, and Brazil. In 2011, soybean imports were estimated at nearly 1.6 million tons, valued at US$1.22 billion with South Africa, Nigeria, and Kenya being the top importers.
The production of soybean in the continent is low and is greatly threatened by several biotic and abiotic stresses such as declining soil fertility, diseases, insect pests, and weeds. Among the diseases, Soybean rust disease, caused by the fungus Phakopsora pachyrhizi, is one of the major threats to soybean production in Africa due to its rapid spread. The fungus’ spores are easily blown by the wind, spreading over long distances.
According to a review paper by Harun Murithi, a plant pathologist at the International Institute of Tropical Agriculture (IITA) soybean rust is known to cause massive yield losses of between 10 and 90%. The paper “Soybean production in eastern and southern Africa and threat of yield loss due to soybean rust caused by Phakopsora pachyrhizi” was published recently in Plant Pathology. (http://onlinelibrary.wiley.com/doi/10.1111/ppa.12457/abstract).
“With its current rapid spread, soybean rust is an important disease that cannot be ignored,” Harun says. “Plants affected by the disease have leaves that have tan to dark brown, or reddish brown lesions. Soybean rust reduces yields mainly by decreasing the photosynthetic activity of the infected leaves.
The disease was first confirmed in Uganda on experimental plots and thereafter on farmers’ fields throughout the country in 1996 and all the soybean grown in the country was found to be susceptible. In 1998, the disease was reported in the major soybean growing regions in Kenya, Rwanda, Zimbabwe, and Zambia. Other countries where the disease has been detected include Nigeria in 1999, Mozambique in 2000, South Africa and Cameroon in 2001, Ghana and the Democratic Republic of Congo (DRC) in Central Africa in 2007, and Tanzania and Malawi in 2014.
The best method to control the disease is through the use of resistant soybean varieties. However, this has been difficult due to the presence of different populations of the fungus across the globe and at the country level. Nevertheless, a lot of research is ongoing in East and Southern Africa to understand the soybean pathogen. In Zimbabwe, resistant varieties have been developed and in other countries tolerant varieties have been identified and deployed.
“In the breeding program at IITA, soybean rust-resistant lines have been tested and released across Africa. We also monitor annually the spread of soybean rust in major soybean growing areas in the region and collect spore samples for analysis. We are also evaluating the effectiveness of simple methods to monitor the spore spread such as small-scale monitoring plots (sentinel plots) and use of spore traps in Tanzania and Zambia. These methods will be optimized for adaptation to the conditions in the region to contribute to controlling soybean rust,” Harun adds.
According to the study, due to the huge potential of soybean to improve the diet and the incomes and livelihoods of smallholder farmers in eastern and southern Africa, efforts to protect the crop from abiotic and biotic stresses and especially soybean rust disease, should be enhanced to ensure sustainable increase in the crop’s production.