Sclerotinia Stem Rot (White Mold)
Sclerotinia
stem rot is a damaging fungal disease caused by Sclerotinia sclerotiorum. The disease is also called white mold,
because of the visible white mold on the stem and pods of infected plants
(Figure 1).
White mold development
is dependent on weather conditions during flowering
At soybean flowering
stages, the fungus forms tiny mushroom-like structures on the soil surface
called apothecia (Figure 2). Infection takes place when spores from
the apothecia are projected into the air from apothecia and land on
flowers where the fungus enters the plant. The growth and pathogenic
activity of the white mold fungus is favored by a dense crop canopy
and cool wet weather during flowering.
Although symptoms
of white mold may be present 3 or 4 weeks after flowering, signs of
infection are usually not noticed until August. At that time, examine
the stems of symptomatic plants for signs of the disease. Stem symptoms
begin as gray to white lesions at nodes that rapidly progress above
and below nodes. Sometimes they are covered with fluffy, white mycelium.
Foliar symptoms
of white mold are not very diagnostic. Leaves wilt, eventually die and
turn completely brown, but often remain attached to the stem past maturity.
Foliar symptoms can easily be mistaken for brown stem rot, Phytophthora
root rot, sudden death syndrome or stem canker.
Management of
White Mold
White mold is a
disease of high yield potential soybeans. This is because management
practices that promote high yield, such as planting in narrow row widths,
high seeding densities, early planting, and high soil fertility also
promote a dense crop canopy and a favorable environment for Sclerotinia
sclerotiorum.
Although there
is currently no soybean variety completely resistant
to white mold, the use of tolerant varieties is effective in managing
white mold. Avoid planting highly susceptible varieties in fields with
a history of white mold.
Avoid excessive
irrigation during flowering.
Short crop rotations will eventually lead to a build up
of sclerotia (overwintering structure) in the field. Sufficiently long crop rotations with nonhosts such as corn and wheat is effective in minimizing the
pathogen buildup and risk of white mold outbreak. Most sclerotia die
over a three- to four-year period between soybean crops.
Tillage can affect white mold in several ways. Although sclerotia can
survive in deep soil up to 7 years, only sclerotia within 2 inches from
soil surface germinate and produce spores to infect soybean. Therefore,
burying infested residues by deep plowing can prevent the germination
of the sclerotia. However, subsequent cultivation and tillage would
bring sclerotia up to the surface.
Under no-till, or
shallow till, a large portion of the sclerotia germinate under corn
or other nonhost crops, which reduces the amount of pathogen in the
soil. Recent research from Iowa State University shows that in long
run, no-till results in a smaller white mold risk compared with conventional
tillage.
The white mold fungus
infects common weeds like lambs-quarters and pigweed. Use good
weed control practices, especially in rotation years
Do not save
beans from fields infested with white mold if you plan to use
your own seed. Combines cannot separate white mold sclerotia from beans
(Figure 3). Use of contaminated seeds will spread the pathogen from
one field to another. When you combine soybeans, you may want to first
harvest fields free of white mold. If not, you must clean soybean debris
from the combine after you harvest a field infested with white mold.
A
summary of management guidelines based on field history can be found
at the North Central Soybean Research Program web site...see link below.
Risk of Yield
Loss
Field data from
several states indicate that the risk of yield reduction is minimal
when incidence is less than 20%. Yield
loss has been estimated a 4 bu/ha for each 10% increment of diseased
plants. When the kill is light (less than 10 percent of all plants),
expect a minimum loss.
More
web resources on white mold
University of Wisconsin Soybean Plant Heath
- All you need to know about white mold
- Printable PDF file on white mold
The National Sclerotinia Initiative
North Central Soybean Research Program
- Management guidelines based on the field history of white mold
-
Risk Assessment for white mold
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