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Date: Sat 14 Mar 2009
Source: Clarin [in Spanish, machine trans., edited]
<http://www.clarin.com/suplementos/rural/2009/03/14/r-01876664.htm>

During January and February [2009], many soybean crops -- mainly varieties of the short and intermediate height groups -- were attacked by frogeye leaf spot in various regions of the country.

The province of Cordoba is the epicenter, with many fields severely infected by this disease. The "unexpected outbreak is in the south eastern part of the province, and never before has it had such a serious effect. The high temperatures and rainfall have contributed, together with the susceptibility of the varieties and the [presence of] inoculum, so that the epidemic was unleashed," says an Aapresid [Argentinean No-till Farmers Association] report. Thus, many company experts and [farming] consultants, are "setting up tests to check the effectiveness of control with fungicides, especially in soybeans in 2nd crops," said one of the specialists.

The symptoms of this disease usually occur after flowering, but become more evident from R3 [growth stage]. Frogeye is recognised by small circular patches of 1 to 5 millimeters [on leaves] in the form of an eye. They are characterised by a grey necrotic centre surrounded by a reddish brown halo, but without chlorosis. The absence of a yellow halo around the lesion distinguishes frogeye from other diseases.

--
Communicated by:
ProMED-mail Rapporteur Susan Baekeland

[Frogeye leaf spot is caused by the fungus _Cercospora sojina_, which can infect leaves, stems, and seeds of soybean. The pathogen is found worldwide but is only problematic under warm, humid conditions.
Significant yield losses of 10 to 50 percent are commonly associated with frogeye epidemics. The fungus is spread via infected plant material and can also be seed borne leading to inferior seed quality.
It is most likely to become a problem if infected seed is planted or if infected residue of a previous soybean crop serves as a source of inoculum (thus the expectation above that the disease may be more difficult to control in 2nd crops). Disease management includes use of disease-free seed, planting of resistant varieties, crop rotation, tillage, and fungicide applications.

It is also mentioned elsewhere in the source that a serious frogeye epidemic "caused havoc" in the province of Tucuman several seasons ago, but that the disease had not been reported from the south of Cordoba province before. Apparently all regions are concerned about the health of this 2008-2009 season's soybean crop and expect lower than normal yields.

The system of soybean growth stages divides plant development into vegetative (V) and reproductive (R) stages. The vegetative stages are numbered according to how many fully developed trifoliate leaves are present. The reproductive (R) stages begin at flowering and include pod development, seed development, and plant maturation. The stages can overlap. R3 refers to the stage of early pod formation.

Maps
Argentina:
<http://media.maps.com/magellan/Images/ARGENT-W1.gif>
<http://healthmap.org/r/007a>
Cordoba province:
<http://www.zonu.com/argentina_maps/cordobax.htm>

Pictures
Frogeye symptoms, upper leaf surface:
<http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy003/frogeye2.jpg>
Frogeye symptoms, leaf underside:
<http://extension.missouri.edu/explore/images/g04442frogeyebottom.jpg>

Links
Frogeye information:
<http://extension.agron.iastate.edu/soybean/diseases_frogeye.html>  and
<http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy003/soy003.htm>
_C. sojina_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=119516>
Description of soybean growth stages (with pictures):
<http://extension.agron.iastate.edu/soybean/production_growthstages.html>
Aapresid:
<http://www.aapresid.org.ar/>.  - Mod.DHA]

[see also in the archive:
2007
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Frogeye leaf spot, soybean - USA: surveillance 20070627.2067
2002
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Frogeye leaf spot, soybean - USA (Wisconsin) 20021028.5655]