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First Report of Soybean Rust Caused by Phakopsora pachyrhizi in Uruguay

Phakopsora pachyrhizi is a fairly new pathogen in South America and has become a serious threat for soybean production in the region (3). During May 2004, soybean (Glycine max) leaves with rust symptoms were observed on an experimental trial at La Estanzuela, National Institute for Agricultural Research in Colonia, southwestern Uruguay, on late-maturing genotypes (R7 stage). Small, necrotic, reddish brown lesions, suggestive of soybean rust, were detected on the upper surface of leaves. Uredinia and urediniospores were found on the underside of the leaves. The National Service of Plant Health (DGSA) was informed immediately. There the genus Phakopsora was confirmed on the basis of urediniospore morphology. These spores were minutely equinulated and measured 21 × 26.3 μm (range 18 to 24 and 22 to 30 μm, respectively), which was within the range described by Ono et al. (2). Leaf samples showing rust symptoms were submitted to the Instituto Ewald A. Favret (Instituto Nacional de Tecnología Agropecuaria, Argentina) for polymerase chain reaction assay using primer pairs Ppa1/Ppa2 (P. pachyrhizi) and Pme1/Pme2 (P. meibomiae) (1). Results showed P. pachyrhizi as the causal agent of soybean rust, while P. meibomiae tests yielded negative results. Pathogenicity tests were carried out on 10 V4 soybean plants, cv. Don Mario 5800, grown in the greenhouse at 20 to 22°C and a 14-h photoperiod. Urediniospores were collected with a cyclone spore collector into glass tubes, which were then filled with nonphytotoxic light industrial oil. Spore suspension was atomized onto eight plants, while two plants were sprayed only with oil as controls. Plants were placed in a dew chamber at 20°C and 100% relative humidity for 20 h and then returned to prior conditions. Symptoms of the disease were reproduced 10 days after inoculation. Two or three sporulating uredinia were observed only on the inoculated plants. Soybean rust caused by P. pachyrhizi was officially recognized as present in Uruguay in August 2004.

References: (1) R. D. Frederick et al. Phytopathology 92:217, 2002. (2) Y. Ono et al. Mycol. Res. 96:825, 1992. (3) J. T. Yorinori. Page 447 in: Proc. World Soybean Res. Conf. 7th, 2004.