June 17, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]
<http://link.ixs1.net/s/link/click?rc=al&rti=9209114&si=j89142132>

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 soybean-producing 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 31S 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 histopathological 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 one-third 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 percent 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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[This piece gives a short, but comprehensive, outline of the Asian strain at that time (2003-2004). However, I suggest that use of the word "urediniospores" be used instead of "uredinios" to describe the asexual spores.

Links:
<http://www.stopsoybeanrust.com/viewStory.asp?StoryID=374>
<http://www.eppo.org/QUARANTINE/Alert_List/fungi/soybean_rust.htm>
<http://ohioline.osu.edu/ac-fact/0048.html>
<http://www.nsrl.uiuc.edu/news/nsrl_pubs/sbr1995/summarybiogeoepi.pdf>
<http://www.nsrl.uiuc.edu/news/nsrl_pubs/sbr1995/summarybiogeoepi.pdfurediniospores>
- Mod.DH]