A
ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases
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: August 14, 2007
Source: Cattlenetwork [edited]
<http://www.cattlenetwork.com/content.asp?contentid=152135>
Over the past couple of weeks, we have had several reports and
soybean samples of top dieback. The last time we had a
widespread outbreak of this disease in Iowa was from 1997-1999.
This season [2007], the disease has been found in eastern and
central Iowa.
Differences in susceptibility among varieties have been noticed.
In addition, affected plants are often defined by rows --
although patches of top dieback, not unlike those you would
expect to see with soybean cyst nematode damage, have also been
reported.
Top dieback symptoms are usually seen between late July and
mid-August when most soybean plants are in the reproductive
phase.
Initial symptoms are characterized by yellowing and death of
leaves in the upper portion of the plant canopy, followed by
discoloration of internodes. Plants die prematurely when the
disease is severe. An Ohio study showed that fungal species
belonging to the genera _Phomopsis_ and _Diaporthe_, a group of
plant pathogens that cause various diseases in soybean, are
associated with top dieback.
No systematic study has been carried out on this disease, and as
a result, we do not know when this disease strikes or what
factors contribute to its appearance. It has been speculated
that potassium deficiency may be associated with top dieback
symptoms because they somewhat resemble those of potassium
deficiency. Several years ago, it was observed in southern Iowa
that the disease ceased after an application of potassium.
Soybean cyst nematodes are also frequently found in these
fields. In collaboration with Greg Tylka, who specializes in
plant-parasitic nematodes, we will be collecting soil samples
from affected and non-affected areas of a field showing top
dieback symptoms to investigate if there may be a relationship
between the 2 pathogens.
If top dieback is severe, consider changing soybean varieties
for the next growing season. Do not save seeds from fields with
severe top dieback because the pathogens causing it can be
seedborne. If necessary, soil and plant tissue analyses could be
conducted to ascertain the involvement of potassium deficiency
in symptomatic fields.
[Byline: XB Yang and Alison Robertson, Department of Plant
Pathology]
--
Communicated by:
J. Allan Dodds
Former ProMED-mail plant disease moderator <dodds@ucr.edu>
[The _Diaporthe_/_Phomopsis_ fungal complex implicated in top
dieback shows wide genetic diversity and includes pathovars of
great economic importance to soybean cultivation. For example,
northern and southern stem canker, as well as pod and stem
blight, are caused by different pathovars of _Diaporthe
phaseolorum_. Although fungi from this species complex are
commonly found in soybeans, pronounced symptoms are not always
present. These pathogens can overwinter in plant residue and may
be spread with infected seed and plant material.
In many cases, top dieback has also been reported to be
associated with soybean cyst nematode (SCN; _Heterodera
glycines_), which is one of the most destructive pathogens
affecting soybeans. It commonly causes yellowing and stunting of
plants, which are often unevenly distributed throughout the
field.
Map of US states:
<http://www.census.gov/geo/www/us_regdiv.pdf>
Pictures
Top dieback leaf symptoms on soybean:
<http://www.ent.iastate.edu/imagegal/plantpath/soybean/topdieback/topdiebackleaves.html>
and
<http://www.ent.iastate.edu/imagegal/plantpath/soybean/topdieback/topdieback.html>
Soybean field with top dieback:
<http://www.ent.iastate.edu/imagegal/plantpath/soybean/topdieback/fieldtopdieback.html>
Links:
Information on top dieback of soybean:
<http://swroc.coafes.umn.edu/SWMNPEST/06News/issue11/issue11.htm#top>
Top dieback outbreak Iowa 1999:
<http://www.ipm.iastate.edu/ipm/icm/1999/9-13-1999/soytopdi.html>
Genus _Diaporthe_ taxonomy:
<http://www.indexfungorum.org/Names/genusrecord.asp?RecordID=1497>
Genus _Phomopsis_ taxonomy:
<http://www.indexfungorum.org/Names/genusrecord.asp?RecordID=9365>
Information on soybean cyst nematode:
<http://www.soybeans.umn.edu/pdfs/DC3935.pdf>.
- Mod.DHA]
[see also in the
archive:
2006
----
Northern stem canker, soybean - USA (ND): 1st report
20060427.1222
2004
----
Diaporthe spp., stem canker, soybean - USA (WI) 20040519.1340
2002
----
Diaporthe spp. soybean stem canker - Argentina 20021201.5944
2001
----
Diaporthe spp., soybean stem canker - Argentina 20010227.0387] |
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