Soybean rust caused by the fungus Phakospora pachyrizi Sydow is the most limiting factor to soybean production in Uganda and the tropics. Its entry and rapid establishment in sub-Saharan Africa (SSA) have caused major yield losses.

The disease is believed to have originated from Asia thus the name Asian soybean rust (ASR). Soybean rust is the disease of economic importance in Uganda and the tropics. Symptoms of soybean rust include small-water soaked lesions on the underside of leaves, blister-like uredia with a central pore with extruding urediniospores on the lower side of the leaf (Plate 8).

Lesions gradually increase in size and later turn from gray to tan, reddish-brown or dark brown and assume a polygonal shape restricted by leaf veins.

The impact of soybean rust is thus linked to the high specialization and the significant genetic variation that exists in the population of this obligate pathogen, which reduces the effectiveness of specific resistance genes deployed against the pathogen.

Management of soybean rust disease

Fungicides have been proven effective in managing soybean rust, however, the additional costs of purchasing and application of fungicides have made chemical control impractical among many farmers in Uganda. However, to provide an optional management strategy, the soybean research team screened seven fungicides against soybean rust disease in Uganda and scored yield loss of up to 52.6%. Interestingly, the resistant control UG5 showed a significant reduction in yield when sprayed with the fungicide, suggesting that it is uneconomical to spray resistant varieties.

Therefore, under resource-limited farming systems, use of rust resistant varieties is the only sustainable management strategy. However, sustainable resistance is difficult to obtain at present, due to the high degree of genetic variability of the pathogen, that causes resistance breakdown in a short period after new resistant varieties are released. Therefore development of soybean varieties with resistance to soybean rust should be a continuous process.