Department of Plant Pathology, University of the Free State, Bloemfontein 9300, South Africa
PANNAR (Pty) Ltd., P.O. Box 19, Greytown 3250, South Africa
USDA ARS Foreign Disease-Weed Science Research Unit, 1301 Ditto Ave., Fort Detrick, MD 21702
In February 2001, rust caused by Phakopsora pachyrhizi Syd. was detected for the first time on soybean (Glycine max (L.) Merr.) near Vryheid in northern KwaZulu-Natal, South Africa. As the season progressed, the disease was also observed in other parts of the province, and epidemic levels were reached in the Karkloof, Cedara, Howick, and Greytown production regions. In affected areas, infection foci gradually increased in size and caused premature yellowing and defoliation of soybean crops, usually after the flowering stage. Typical rust symptoms (3) were produced predominantly on the lower surface of soybean leaves. Soybean rust subsequently spread to Amsterdam and Ermelo in the Highveld region of South Africa. Following emergency registration of triazole compounds, fungicides were commonly used to control soybean rust, especially in the more humid eastern production areas. Available yield data suggested a reduction in kernel mass between 4 and 23%, depending on the cultivar and host growth stage at the time of infection. Urediniospores from the original collection (isolate PREM 57280, Plant Protection Research Institute, Pretoria, South Africa) were 23 to 33 × 15 to 22 μm, indicating that spore dimensions fell within the known range for P. pachyrhizi (3). To confirm pathogenicity, 10 to 15 plants of each of the South African soybean cvs. Pan 589, Pan 780, Pan 854, Octa, and Prima were inoculated with isolate PREM 57280. Primary leaves were sprayed with a suspension of spores in light mineral oil (approximately 1 mg of spores per ml) before incubating plants in the dark in a dew chamber for 16 h. Large, sporulating uredinia, producing typical soybean rust urediniospores, developed on all inoculated plants. Classical and real-time fluorescent polymerase chain reaction assays as well as sequence analysis of the internal transcribed spacer regions verified the identity of isolate PREM 57280 as P. pachyrhizi (2). Since the disease is known to occur in Zimbabwe, Mozambique, and several other African countries (1,3,4), inoculum was most likely introduced by air currents from countries to the north of South Africa. It is highly probable that soybean rust will successfully overwinter in South Africa based on experience in other southern African countries.
References: (1) O. A. Akinsanmi and J. L. Ladipo. Plant Dis. 85:97, 2001. (2) R. D. Frederick et al. (Abstr.) Phytopathology 90 (suppl):S25, 2000. (3) G. L. Hartman et al. eds. Compendium of Soybean Diseases, 4th ed. The American Phytopathological Society, St. Paul, MN, 1999. (4) J. B. Sinclair and G. L. Hartman, eds. Soybean Rust Workshop, Publ. 1 College of Agricultural, Consumer, and Environmental Sciences, National Soybean Research Laboratory, Urbana, IL. 1996.