August 12, 2005
From: ProMED-mail <promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes [edited]

Morphologic and pathometric characterization of the Asian soybean rust (Phakopsora pachyrhizi) in Santa Fe Province, Argentina
R. N. Pioli, Fitopatologia, Facultad Ciencias Agrarias. Universidad Nacional Rosario. P.O. Box 14, 2123 Zavalla, Santa Fe, Argentina; M. V. Cambursano, Fisiologia Vegetal, Facultad Ciencias Agrarias. Universidad Nacional Rosario. P.O. Box 14, 2123 Zavalla, Santa Fe, Argentina; and E. N. Morandi, Fisiologia Vegetal, CONICET, Facultad Ciencias Agrarias. Universidad Nacional Rosario. P.O. Box 14, 2123 Zavalla, Santa Fe, Argentina. Plant Dis. 89:684, 2005; published on-line as DOI: 10.1094/PD-89-0684B. Accepted for publication 16 Mar 2005.

The Asian soybean rust caused by the fungus _Phakopsora pachyrhizi_ was cited for the 1st time in Argentina during the 2002-2003 growing season (3). During 2003-2004, the disease spread to other northern provinces and
was also observed in north-central Santa Fe, the main producing soybean province of the country. Because the disease appeared at the end of the crop growing season (late March to early April) it had little or no impact on crop yields.

The objectives of this study were to characterize morphologically and pathometrically the disease on soybean and check the presence of _P. pachyrhizi_ on volunteer soybean plants that could eventually carry the disease to the next growing season. The study was conducted in the San Justo Department, Santa Fe Province (between 30 and 31 deg S latitude), where the presence of the soybean rust was molecularly confirmed by Sistema Nacional Vigilancia y Monitoreo (on-line publication at <http://www.sinavimo.gov.ar>).

3 field locations were sampled and identified as M1, M2, and M3. Transversal cuts of soybean leaves through rust lesions and histo-pathological staining were used for micromorphologic characterization of the developmental stages of _P. pachyrhizi_.

The disease incidence was estimated as the proportion of affected soybean plants and leaves. Average severity, expressed as the percentage of leaf area affected, including chlorosis, was measured on the terminal leaflet of
leaves sampled from the lower 1/3 of the canopy. 3 replicates of 10 plants, randomly chosen, were used. The number of uredinia per square centimeter and per lesion (symptomatic foliar area showing chlorosis and necrosis
caused by the fungus) was measured on the undersides of the sampled leaflets at 40x magnification (1).

Typical signs and symptoms of _P. pachyrhizi_ coexisted on soybean leaves with brown spot (_Septoria glycines_), downy mildew (_Peronospora manshurica_), anthracnose (_Colletotrichum truncatum_), and blight and leaf spot (_Cercospora kikuchii_) and also with bacteria (_Pseudomonas_ and _Xanthomonas_ spp.). Uredinia and telia of the _P. pachyrhizi_ cycle were observed. Uredinia were also observed on soybean petioles.

The average size of urediniospores (n = 60) was 23.3 x 16.6 micrometers. Telia were located adjacent to the uredinia. These telia were dark and crusty with 4 stacked layers of teliospores. Rust incidence in plants was 100 for the 3 fields, while the incidence in leaves was 100 percent for M1 and M2 and 60 percent for M3. Average disease severity was 50.3, 25.6, and 14.8 percent for M1, M2, and M3, respectively.

The mean number of uredinia per square centimeter was 327, 179, and 177, for M1, M2, and M3, respectively. The number of uredinia per lesion ranged from 1 to 6. _P. pachyrhizi_ was also found on volunteer soybean plants that emerged shortly after harvest. On 40 leaflets, the foliar incidence was 25 percent, showing 1-2 lesions with 1-2 uredinios per leaflet (2).

The volunteer soybean plants could constitute a potential early source of inoculum.

References:

(1) M. Marcchetti et al. Phytopathology. 66:461, 1976.
(2) R. Pioli et al. La roya asiatica en Santa. Fe, Arg. XII Cong. AAPRESID,
II Sem. Internac. Soja, Arg. 283-290, 2004.
(3) R. L. Rossi. Plant Dis. 87:102, 2003.

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[The Asian strain of soybean rust (ASR) appears to be firmly entrenched in Santa Fe Province. ASR-Infected volunteer soybean plants are a matter of concern as the authors point out. All commercial soybean varieties
available today are susceptible to ASR. The only management option remaining for producers is application of approved fungicides. -Mod.DH]

[see also in the archive:
Soybean rust, Asian strain - Argentina 20050119.0185
Soybean rust, dry beans - South Africa 20050210.0452
Soybean rust, Asian strain - Americas: alert 20050528.1476 Soybean rust, Asian strain - Argentina (Santa Fe) 20050617.1715 Sudden death syndrome, soybean - USA (KY) 20050811.2350 2004
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Soybean rust - Zimbabwe 20040212.0463
Soybean rust, Asian strain - USA (LA) 20041116.3083
Soybean rust, Asian strain - Brazil 20041222.3374
2003
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Phakopsora sp., soybean, yield loss - Uganda 20030119.0176 Soybean rust - Brazil (Sao Paulo State) 20030124.0214 Soybean rust - Zimbabwe (Arcturus) 20030222.0456 Soybean rust - Brazil (Mato Grosso & Bahia) 20030415.0917 Phakopsora pachyrhizi, soybean - Brazil 20031119.2873 2002
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Phakopsora sp., soybean rust - S. Africa, Argentina 20021231.6167]