Michigan State University Extension
Michigan Soybean Information - FACT9803
11/16/00
WHITE MOLD IN SOYBEANS
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Soybean Facts
Michigan State University Extension
This research and fact sheet funded by Michigan
Soybean Promotion Committee Making Your Checkoff Pay Off
Winter 1998
By: Patrick Hart
Michigan State University
Department of Botany and Plant Pathology
White mold, also known as Sclerotinia stem rot, is caused
by the plant pathogenic fungus Sclerotinia sclerotiorum.
The host range for this pathogen is very wide and
includes green beans and dry beans, tomatoes, potatoes,
green peppers, canola, and many others. Once white mold
becomes established in a field it is very difficult to
manage. Because of its wide host range, and its ability
to survive in the soil for many years, rotation
contributes little to control. If possible, only one
white mold susceptible crop should be grown in the
rotation. Rotation with a non-susceptible crop such as
wheat or corn will help reduce the build-up of the
pathogen in the soil but will not eliminate it from a
field. The research results presented in this fact sheet
are based on research supported by the Michigan Soybean
Promotion Committee through the soybean checkoff.
SYMPTOMS
The first symptoms usually observed are wilted and yellow
plants scattered throughout the field. In most years
these symptoms become evident about the middle of August.
Eventually, these plants die and by harvest will have a
bleached appearance, which is a characteristic symptom of
white mold infected plants. Before the foliage symptoms
become visible it is often possible to observe white
mycelium growing on the stem at nodes where blossoms were
first colonized(Vis. 1). If adjacent plants come into
contact with the infected area, they may also become
infected, however, plant to plant spread is minimal and
not as important as blossom infections.
Disease Cycle(Vis. 2)
The white mold fungus, Sclerotinia sclerotiorum, survives
in the soil as hard, black structures called sclerotia
(Vis. 3). Soils continuously wet for 10-14 days favor the
germination of the sclerotia to form apothecia, small
mushroom-like structures that eject spores into the air
(Vis. 4). Generally, three inches of rain over a two-week
period beginning the last week of June and continuing
through the middle of July will provide adequate
moisture, and result in complete canopy closure of the
crop which is critical to maintaining the necessary soil
moisture levels. Daily dews or fogs can also provide the
necessary moisture conditions. The soybean plant is
infected primarily through the blossoms, but occasionally
may become infected through injured plant tissue. After
colonization of the blossom, the fungus will invade and
girdle the stem, shutting down the movement of water and
nutrients to the plant. As the plant matures the fungus
grows extensively inside the stem, and eventually forms
many new sclerotia, which will provide inoculum in the
next crop year (Vis. 5).
AGRONOMIC PRACTICES THAT INFLUENCE WHITE MOLD
Row widths
White mold is more severe in narrow row spacings of seven
to fourteen inches when compared to wider rows of twenty
to thirty inches. A row spacing study at Michigan State
University in 1996 and 1997, showed a significant effect
of decreasing row widths on the development of white mold
(Vis. 6). In this study, both tolerant and susceptible
varieties showed an increase in disease as row width
decreased. This research also demonstrated that even at
low levels of disease significant quantities of sclerotia
can be produced(Vis. 6). The weight of sclerotia shown in
table one are from plants in fifty feet of row. The
difference in the amount of sclerotia produced between
1996 and 1997, is probably a reflection of the cool, wet
conditions the latter part of August, 1997. Since the
total number of sclerotia shown in figure one weigh less
than one gram, even the low sclerotial weights result in
significant numbers of sclerotia produced in a field.
Sclerotia will persist in the soil until the next time a
susceptible crop is grown and the environmental
conditions are favorable. In several field experimental
plots in Michigan, a relationship was found between
maturity group, row spacing and yield. Although only
three soybean varieties were used, two group I varieties
and one group II variety, only the group I variety showed
an increased yield response to decreasing row widths
(Vis. 6) and
(Vis. 7).
This supports the work of others, and should be a caution
to growers that planting soybeans in narrow rows may not
always favor an increased yield, but will generally favor
an increased incidence of white mold. Because of the
increased potential for narrow row widths to increase the
incidence of white mold, growers should adopt this
practice cautiously. Since the potential for white mold
is greater in narrow rows, growers with a limited white
mold problem or no problem should consider the risks
involved when using narrow row drilled soybeans. When the
environmental conditions favor low to moderate disease
levels, wider rows may slow the rate at which the
sclerotia build-up in the field, and delay the need to
implement other management practices that may reduce the
incidence of white mold. When the disease potential is
high due to excessive rains in late June and throughout
July, white mold will occur at any row spacing.
Resistance may be overcome when the inoculum levels are
high, and consideration should be given to practices that
reduce the build-up of selerotia in the soil.
Tillage
Tillage practices have the potential to influence the
development of white mold both positively and negatively.
Reduced tillage with excessive surface residues could
maintain favorable soil moisture levels for sufficient
periods of time to promote germination of the sclerotia.
If large numbers of sclerotia were left on the soil
surface because of reduced tillage after a white mold
epidemic, a subsequent susceptible crop could be at
greater risk. However, when a susceptible crop is planted
into surface residues such as corn, the corn residue may
act as a barrier to limit the movement of spores from the
apothecia into the crop canopy. Tillage studies at the
Michigan State University campus, and in Huron County are
in their second year and it is too early in these studies
to determine if tillage will have an influence on white
mold. Preliminary results indicate that white mold is
more severe whenever soybeans are planted after soybeans,
regardless of tillage (Vis. 8). However, at least one
more year of study is necessary before any conclusions
can be reached on the influence tillage may have on white
mold. The number of sclerotia, and their spatial
distribution in a field will influence the pattern of
diseased plants. Closely associated groups of sclerotia
producing apothecia mostly infect plants in their
immediate vicinity (Vis. 9).
Compensation
Several years of field trials have shown that healthy
soybean plants will compensate to some degree for yield
reductions in white mold infected plants (Vis. 10). The
degree of compensation is dependent on the variety, row
spacing, and disease pattern in the field.
CONTROL PRACTICES
Chemical control with fungicides may be economical in
seed production fields, but is probably not economical in
commercial fields. Benlate and Topsin M have provided
some control, but yield increases have been limited to
20-25%, and economic yield increases have not always been
consistent. Information on the role of the herbicide
COBRA and other compounds that induce resistance can be
found in a separate SOYBEAN FACTS sheet. Planting into a
ground cover such as grass or clover, or into surface
residues of corn, may reduce white mold if enough of the
soil surface is covered so that spores are trapped on the
lower surface of the ground cover or residue. A late
cultivation in early July could be an effective
management tool to reduce white mold by disrupting
sclerotia as they germinate, if the soil is not too wet
to accommodate cultivation. There has been speculation
that reduced use of atrazine is correlated with an
increased incidence of white mold in soybeans. Atrazine
does affect the ability of S. sclerotiorum to develop
normal apothecia, and the number of sclerotia that
germinate in the presence of atrazine in soil is reduced
(Vis. 11). However, field experiments must still be done
to confirm this observation. The potential role of
atrazine on inducing abnormal apothecium development may
have some merit since white mold has traditionally been a
problem in areas where atrazine is not normally used,
such as when dry beans are in the rotation.
RESISTANCE
Although there is no complete resistance, soybean
varieties differ in their response to this disease, and
some varieties are less susceptible. A Disease Severity
index rating (DSI) of the reaction of individual
varieties to white mold can be found in the annual
MICHIGAN STATE UNIVERSITY SOYBEAN PERFORMANCE REPORT.
When a particular field has high levels of white mold,
select a soybean variety that reduces the overall
incidence of white mold as compared to the average of all
the varieties in test, and that yields well. There are
several varieties in the group I maturities that have
performed consistently in this range, but only a few in
the group II maturity. There are no soybean varieties
with complete immunity to white mold. There are several
soybean varieties that have higher DSI ratings, but which
still yield well. These varieties may be considered when
selecting varieties to grow under moderate to high white
mold pressure, but definitely not in narrow row widths.
Sclerotia are produced in infected plants, so any
infection will result in more sclerotia added to the
soil. Finally, private and public lines of soybeans not
included in the MICHIGAN STATE UNIVERSITY SOYBEAN
PERFORMANCE REPORT may also reduce the levels of white
mold, and private company representatives should be
consulted. Before selecting any soybean variety to reduce
the impact of white mold, the variety should meet the
following guidelines:
Was the percent of plants infected with white mold
reduced below that of highly susceptible varieties,
and was the yield at an acceptable level?
Did the yield trial include check varieties that were
highly susceptible and tolerant to white mold? How
did other varieties compare to these?
Was the trial replicated? And was the incidence of
white mold uniform over the entire plot?
SUMMARY
White mold is a widely occurring disease affecting many
crops. A combination of cultural practices and the use of
varieties with higher levels of resistance offer the best
hope of avoiding serious yield losses.
Copies of the Soybean Research Fact Sheet are available
from:
Michigan Soybean Promotion Committee
http://www.michigansoybean.org
MSUE County Office
http://www.msue.msu.edu/msue/ctyentpg/
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