New Soybean disease in South Africa

Dr Rikus Kloppers PANNAR, Greytown Soybean rust is one of the latest arrivals of serious plant diseases in South Africa and a potential threat to soybean production in South Africa. The disease was first observed in the Vryheid area by one of PANNAR's agronomists, Heinz Oellerman. The symptoms and causal organism were confirmed by Anthony Jarvie and Rikus Kloppers of the PANNAR research team and samples were also sent to rust experts at the University of the Free State. An awareness campaign was immediately set up in KZN to inform all soybean producers to be on the look out for the symptoms and to take the necessary control measures. The disease has since been reported from various areas in KZN including Karkloof, Cedara, Howick and Greytown and there are also some unconfirmed reports that the disease was found in the Highveld region.

Urediniospores of P. pachyrhizi

Soybean rust is caused by the fungus Phakopsora pachyrhizi.

The fungus is originally from Asia and Australia and has previously been reported in other African countries such as Rwanda, Uganda, Nigeria, Zambia, Mozambique and Zimbabwe. The fungus produces spores that can be transported over long distances by air currents and the appearance in South Africa is most likely to be the effect of recent cyclone activities bringing in inoculum from Zimbabwe and Mozambique. No evidence of seed transmission has ever been found.

Soybean rust is known to regularly cause yield losses of 50-60% while complete losses are possible where early infection and environmental conditions favour epidemics. Conditions for infection are temperatures between 15-25ƒC and prolonged periods of leaf wetness produced by dew, mist and light rain. Infection under these conditions takes place within 6 hours and the first sporulating rust pustules appear within 10 days. Symptoms are first observed as water-soaked spots on lower leaves which progresses to reddish brown lesions, then rust pustules. These rust pustules are more pronounced and numerous on the lower than upper leaf surface. Infected leaves turn bronze/yellow and these patches can clearly be seen in infected fields. Walking into these severely infected patches also releases a visible cloud of rust spores. Premature defoliation occurs as result of infection and this affects the number of pods and the seed weight.

Early recognition is crucial to guarantee successful control and fungicides should be applied as soon as the disease is observed. The South African Department of Agriculture is currently busy with emergency registration of triazole fungicides. Further information can be obtained from PANNAR Research in Greytown, the Department of Agriculture in KZN at Cedara or the ARC in Potchefstroom.