Ames, Iowa
October 6, 2008
Source: Iowa State University
- Integrated Crop Management NEWS
by XB Yang*, Department of Plant Pathology
http://www.extension.iastate.edu/CropNews/2008/0929yang.htm
During the past planting season,
there were many reports of low soybean seed germination rates,
which may have been the result of last year’s wide spread of
Phomopsis. Severe Phomopsis fungus infection can reduce seed
quality. This season, several diseases (Cercospora, downy
mildew, and white mold) were prevalent in different parts of
Iowa; the causal fungi of these diseases can infect seeds. As
harvest begins, it is time to learn about these diseases and
check seed quality.
Cercospora leaf spot. This fall Cercospora leaf blight
was wide spread in some parts of Iowa, particularly central and
northeastern Iowa. Cercospora leaf blight is caused by
Cercospora kukuchii. In late-July and August, the disease is
easy to identify by a mottled purple-to-orange discoloration of
the uppermost soybean leaves. The leaves also have a leathery
appearance. In September, when soybean plants are approaching
maturity, infected leaves turn orange or bronze. This season,
delayed planting and warm weather in the fall seemed to favor
the development of this disease. In fields with high disease
incidence, soybeans are not the color we are familiar with. From
a distance, bronzed leaves of these affected plants can be
mistaken for sudden death syndrome or pod and stem blight. Close
examination shows that only top leaves are affected. Affected
plants may show up in only portions of the fields.
If the disease was found in fields before September or the
disease is severe in the fall, seed infection is more likely.
Infected seeds have a purple discoloration called purple seed
stain. The level of seed infection varies with the level of
foliar infection. Discoloration may not be present in soybeans
where infection developed late in the season. Seed infection by
Cercospora fungi may cause poor seed vigor and reduced
germination. Beans with substantial amounts of discoloration
should not be saved for seed because of the seedborne nature of
the disease. Infected seeds carry the disease, which can cause
foliar infections when planted.
Downy mildew is a late season disease in Iowa. However,
this summer the disease appeared as early as late July. The
disease is prevalent in cool wet seasons. Infected soybean
leaves have regular shape, small lesions defined by a few cells.
The lesions are pale or light yellow in color on the upper
surface of the leaves. On the underside of the infected leaves,
the lesions are grey in color with turf like mycelium which can
be seen without a magnifier. The lesions are found in the upper
plant because the fungal spores are airborne. Defoliation can
occur when the disease level is high. In this situation, downy
mildew will infect the seed, creating white mycelium on the seed
coat.
White mold occurred again in part of eastern Iowa this
season, although its occurrence was less extensive compared with
the last few years. This was due in part to the less dense
canopy from delayed planting. This fungus also infects soybean
seeds. Infected seeds are light, small in size, whitish with a
shrunken appearance. The regular gravity-clean process used in
seed processing plants separates infected seeds from healthy
seed. Therefore, it should not be a production concern when you
purchase seeds from a quality company. However, the fungal
sclerotia, which are similar in color and shape of mice
droppings, could be mixed with seed from a white-mold-infected
field. Soybean saved for seed should be checked for sclerotia if
while mold was present in the field.
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Soybean field with leaf blight late in fall |
* XB Yang is a professor of plant pathology with research and
extension responsibilities in crop diseases.
This article was published originally on 9/29/2008 The
information contained within the article may or may not be up to
date depending on when you are accessing the information. |
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