A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases <http://www.isid.org>
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
----
Frogeye leaf spot, soybean - USA: surveillance 20070627.2067
2002
----
Frogeye leaf spot, soybean - USA (Wisconsin) 20021028.5655]