August 19, 2005
From: ProMED-mail <promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of soybean rust caused by Phakopsora pachyrhizi in Uruguay
S. Stewart, Instituto Nacional de Investigacion Agropecuaria (INIA), la Estanzuela, Ruta 50 kM 11, 70000, Colonia, Uruguay; E. A. Guillin, Instituto Nacional de Tecnologia Agropecuaria (INTA), Castelar, Villa Udaondo-Moron, 1712, Buenos Aires, Argentina; and L. Diaz, Direccion General de Servicios Agricolas (DGSA), Ministerio de Ganaderia Agricultura y Pesca, Av. Millan 4703, 12900, Montevideo, Uruguay. Plant Dis. 89:909, 2005; published on-line as DOI: 10.1094/PD-89-0909C. Accepted for publication 12 May 2005.

_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 x 26.3 micrometers (range 18 to 24 and 22 to 30 micrometers, 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 Tecnologia 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 8 plants, while 2 plants were sprayed only with oil as controls. Plants were placed in a dew chamber at 20 C and 100 percent relative humidity for 20 h and then returned to prior conditions.

Symptoms of the disease were reproduced 10 days after inoculation. 2-3 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.

--
ProMED-mail
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[Laura Karlen of Ag Advisory, an Algona, Iowa, crop consulting firm, returned in early April after visiting Argentina and Uruguay. ASR had been detected in a few spots in both countries before planting. ASR subsequently blanketed Uruguay and central Argentina, where much of that nation's soybean production occurs. Excellent preparation by farmers, agronomists, plant pathologists and others has helped them weather ASR.

There are differences between the 2 countries. According to Karlen, "They have a different climate and wind patterns and we can learn from them. The growing climate of these countries more closely resembles that of the
United States than does the steamy tropical environment of Brazil. It's more like Arkansas and Missouri, where frost occurs in the winter," says Karlen.

Similarities also include spore arrival via wind. In the fall of 2004, hurricanes blew ASR spores northward into the United States. It was the opposite situation in Argentina and Uruguay, where northward winds blew spores in from the ASR hotbed of Brazil. Once ASR was initially detected in Uruguay and central Argentina, it quickly surfaced all over. Soybeans double-cropped after wheat in Uruguay were the most prone to infection, as they were in the R4 (full pod) and R5 (beginning seed) stages. These fields were more likely to be infected and treated than early-planted soybeans that were further along in the R6 (full seed) stage.

Farmers treat infected fields with curative fungicides or a mixture of curative and preventive fungicides. These mixtures include fungicides from both the triazole and strobilurin families, which also are approved for use in the United States.

Links:
<http://www.stopsoybeanrust.com/viewStory.asp?StoryID=258>
<http://www.eppo.org/QUARANTINE/Alert_List/fungi/soybean_rust.htm>
- Mod.DH]

[see also in the archive:
Soybean rust, Asian strain - USA (FL): 1st report 2005 20050309.0693 Soybean rust, Asian strain - USA (GA) 20050429.1196 Soybean rust, Asian strain - USA (GA) 20050505.1245 Soybean rust, Asian strain - USA (LA) 20050624.1769 Soybean rust, Asian strain - USA (FL) (02) 20050620.1731 Asian soybean rust, Asian strain - USA (multistate) 20050724.2133 Soybean rust, Asian strain - USA (AL) 20050715.2029 Soybean rust, Asian strain - USA (multistate)(03) 20050714.2005 Soybean rust, Asian strain - USA (multistate)(02): Florida 20050708.1938 Soybean rust, Asian strain - USA (multistate) 20050702.1868 Soybean rust, Asian strain - USA (SC) 20050818.2414 Soybean rust, Asian strain - USA (FL)(03) 20050815.2386 Soybean rust, Asian strain - USA (Multistate)(06) 20050808.2318 Soybean rust, Asian strain - USA (multistate)(05): susp. 20050806.2291 Soybean rust - USA (multistate)(04) 20050806.2289]