February 9, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of Soybean rust caused by Phakopsora pachyrhizi on dry beans in South Africa
E. D. du Preez, N. C. van Rij, and K. F. Lawrance, KwaZulu-Natal Department of Agriculture and Environmental Affairs, Private Bag X9059, Pietermaritzburg 3200, South Africa; M. R. Miles, USDA ARS, National Soybean Research Center, University of Illinois, Urbana 61801; and R. D. Frederick, USDA ARS Foreign Disease-Weed Science Research Unit, 1301 Ditto Ave., Fort Detrick, MD 21702. Plant Dis. 89:206, 2005; published on-line as DOI: 10.1094/PD-89-0206C. Accepted for publication 9 Nov 2004.

During April 2004, a 150-sq-m dry bean (_Phaseolus vulgaris_) plot growing adjacent to rust-infected soybean (_Glycine max_) at Cedara Agricultural Research Farm (29 deg 32 min S x 30 deg 16 min E) was observed to be infected with 2 distinct rust types. Common bean rust (caused by_Uromyces appendiculatus_) with reddish brown uredinia and black telia was readily identified. A 2nd rust with smaller sporulating uredinia (1.0 to 1.5 sq mm), which were gray in appearance, was also found. Visual rust severity on the dry bean plants, which were in mid pod-fill, was high (approximately 30 to 40 percent disease incidence).

20 plants were examined and observed to be infected with both rusts. With microscopic examination of no fewer than 20 leaves per plant, the urediniospores from the smaller lesions were determined to be morphologically similar to _Phakopsora pachyrhizi_ (3). Real-time fluorescent polymerase chain reaction assays on 6 leaves and sequence analysis of the nuclear ribosomal internal transcribed spacer region 2 (1) verified the identity of the urediniospores as _P. pachyrhizi_.

Although _P. vulgaris_ is a known host of _P. pachyrhizi_, to our knowledge this is the 1st time since the arrival of soybean rust in 2001 that _P. pachyrhizi_ has been observed on an alternate host plant in South Africa (2). Since dry beans are grown all year in frost-free areas, the implications are that dry beans may serve as an important overwintering host and source of inoculum for seasonal soybean rust outbreaks.

References:
(1) R. D. Frederick et al. Phytopathology 92:217, 2002.
(2) Z. A. Pretorius et al. Plant Dis. 85:1288, 2001.
(3) J. B. Sinclair and G. L. Hartman. Soybean Rust. Pages 25-26 in: Compendium of Soybean Diseases, 4th ed. G. L. Hartman et al. eds. The American Phytopathological Society, St. Paul, MN, 1999.

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[Asian soybean rust (ASR) has an extensive natural host range that includes several major food crops, including pigeon pea (_Cajanus cajan_), lima bean (_Phaseolus lunatus_), garden pea (_Pisum sativum_), azuki bean (_Phaseolus angularis_) and cowpea (_Vigna unguiculata_). The fact that ASR has appeared on common bean in South Africa would indicate that dry rust is likely to become a factor in dry bean production. Dry beans are the most important leguminous food crop in Africa. Africa is considered to be a secondary center for bean genetic diversity. In Africa, women on small farms are the primary bean growers. Farmers plant about 3 million hectares of beans annually in eastern, central and southern Africa, usually as a mixture of varieties.

The common bean has been, and continues to be, the most important grain legume for direct human consumption in the world. Its total world production exceeds 12 million tons, of which some 7 million tons are produced in Latin America and Africa. This tonnage represents almost twice that of chickpea, which is the 2nd most important grain legume. -Mod.DH]