Asian Soybean Rust -
Frequently Asked Questions IV:
Cropping Systems and Cultural Practices
Author: David Holshouser, Extension Soybean Specialist, Tidewater Agricultural Research and Extension Center; Pat Phipps, Extension Plant Pathologist Tidewater Agricultural Research and Extension Center; Erik Stromberg, Extension Plant Pathologist, Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech; and Elizabeth Bush, Special Research Associate, Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech
Publication Number: 450-304, posted March 2006
Are there advantages to either narrowrow
or wide-row planting?
Research does not show differences in Asian Soybean
Rust (ASR) incidence and severity among different
row spacings. In a wide-row situation, it is thought that
there is more turbulence within the rows during a rainstorm,
which would result in greater dispersal of spores
throughout the canopy and field. On the other hand, the
microenvironment is more favorable for rust development
in narrow rows because of longer periods of high
relative humidity compared to wide-row spacing. The
plants require complete coverage with fungicides for
control. Some believe it is more difficult to achieve
coverage in a narrow-row situation. However, if the
correct sprayer, nozzle technology, and application
methods are used, adequate coverage can be achieved
with narrow-row spacing. It is best to choose the row
spacing that gives the best yield and then apply fungicides
according to Virginia Cooperative Extension
recommendations. Scientists at Virginia Tech are conducting
research to address this issue.
Does the planting date affect soybean
rust incidence and severity?
The planting date does not directly affect rust incidence
or severity. However, by planting early, the crop
matures earlier. This potentially reduces the time that
the crop is exposed to ASR, since the plant’s lifetime is
shorter. In Virginia, soybeans can be planted in April,
but there is no advantage compared to planting in May.
Additionally, planting seed into cold soils will delay germination and increase the risk of seedling disease,
which could reduce stand and yield. One should choose
the planting date that will give the best yield and then
implement ASR control measures if needed.
Are double-crop soybeans more at risk of
ASR than are full-season soybeans?
Double-crop soybeans are not inherently more susceptible
to the disease; however, double-crop soybeans face
more disease pressure due to later maturity dates, compared
to full-season soybeans. Since double-cropped
soybeans are planted at the end of June or the first of
July, they are likely to be at an earlier development
stage when colonized by ASR and would, therefore, be
exposed to ASR for a longer period of time.
Will double-crop soybeans require more
fungicide applications than full-season
soybeans?
This will depend on when ASR arrives, weather conditions,
and disease incidence and severity. However,
assuming the disease arrives in August, or later, and
disease pressures are equal, it is more likely that double-
crop soybeans will require more fungicide applications,
because a later maturing crop will have to be
protected longer.
How does maturity group affect ASR infection
and severity?
One maturity group is no more or no less susceptible to
the disease than another. Since ASR cannot overwinter
in Virginia and must move here from the south, latermaturity
groups are more likely to experience exposure
to ASR and/or higher inoculum levels than early-maturity
groups. Early-maturing cultivars will enter into the
reproductive stages sooner; therefore these cultivars
are likely to be in later stages of development when
ASR arrives. Risk of yield loss progressively decreases
as soybeans move past the late-pod development (R4)
stage. Therefore, less yield loss would occur for a maturity
group III in the late-seed (R6) stage, than would
occur for a maturity group IV in the early-seed (R5)
stage. Likewise, a maturity group IV in the early-seed
(R5) stage would incur less yield loss than a maturity
group III in the late-pod (R4) stage.
Later-maturity groups are also more likely to require
multiple fungicide applications. Early-maturity groups
will reach “safe” development stages (R6.5 to R7) earlier
in the year than later-maturing cultivars. Unless
ASR moves into Virginia early (i.e., before any maturity
group flowers), the later-maturity groups will need
to be protected longer. Therefore, fungicide applications
are more likely for later-maturity groups.
Should I begin growing earlier-maturing
cultivars to reduce the risk of ASR?
The best recommendation is to grow cultivars that are
best adapted to your growing area and give the highest
yield. Early-maturing cultivars will not always perform
as well as later-maturity groups, especially with
drought or late planting, which is the norm in doublecrop
systems. More dollars can be lost from poor cultivar
selection than saved by one less fungicide application.
Producers should use the maturity group that has traditionally
yielded highest for their region of the state and
their farm. Continue to select cultivars that yield well
and possess needed traits. At the most, select a cultivar
in the earlier end of the maturity range that is adapted
to the region.
Should I change my cropping system?
Continue to use the cropping system that is most profitable
to your individual operation. Double-crop soybeans
are ten to 15 days behind full-season soybeans
in development and, therefore, more at risk of infection
and yield losses from ASR. If more fungicide applications
are required for double-crop soybeans, then the
double-crop system could be less profitable.
Should I change any agronomic practices?
No, continue to use recommended practices to produce
the highest yielding, most profitable crop. A healthy
crop will tolerate ASR better than a stressed crop.
Will ASR affect full-season and doublecrop
systems equally?
ASR could affect both systems equally if ASR were to
arrive early in the season when full-season soybeans
are in the R1 to R2 stage (e.g. flowering). However,
since double-crop soybeans lag ten to 15 days in development,
they are at a higher risk of exposure to higher
inoculum levels of ASR. However, keep in mind that
dry and hot weather could greatly reduce development/
movement of ASR. Furthermore, double-crop soybeans
produce a smaller, less dense canopy that will not support
disease development as well as the larger, more
dense canopy of full-season soybeans. The amount of
canopy and weather may have more impact on disease
development than planting date or maturity group.
What are sentinel plots?
Sentinel plots are small plots (one-half to five acres)
that are scouted often and thoroughly for soybean rust.
Sentinel plots usually include an early-maturing cultivar
and/or are planted two to four weeks prior to the
normal soybean planting date. In Virginia, this means
planting a maturity group II or III cultivar and/or planting
in April or early May. For example, a maturity
group III cultivar planted in April will begin flowering
in mid-June, four to six weeks earlier than a maturity
group IV or V planted in May. Since ASR develops
more rapidly in reproductive-stage soybeans, it is more
likely to be detected in reproductive stages than in the
vegetative stage of the regular crop. Thus, early planting
or early-maturing cultivars serve as an early-detection
system. Ideally, a sentinel plot will contain two
or more planting dates and/or maturity groups. Laterplanted
or later-maturity groups assist in detecting a
late-season invasion.
The Virginia Soybean Rust and Aphid Monitoring
Program monitors numerous sentinel plots scattered
throughout the commonwealth for early ASR detection.
These plots will be scouted weekly from June
through October. In addition, the United States has established
a coordinated system of sentinel plots which
will be used to track soybean rust as it moves through
all United States soybean-growing regions.
Should I plant my own sentinel plot?
You may plant your own sentinel plots as an early ASR
detection system. However, if these plots are planted
early or contain an early-maturing cultivar, they should
be scouted regularly - at least weekly during the reproductive
stages. If ASR is suspected in these sentinel
plots, contact your local Extension agent, certified crop
advisor, or the nearest Virginia Primary Diagnostic
Center for confirmation. If ASR is positively identified,
the plots should be sprayed with a curative or
premix fungicide. We DO NOT recommend planting
sentinel plots containing early planting dates or
early-maturing cultivars unless they will be scouted
on a regular basis.
How much damage will I do to my soybeans
by running over them when I spray
for ASR?
The amount of damage to the soybean crop from running
over rows will depend on the total number of rows
that are damaged. The wider the spray boom and/or
the narrower the sprayer wheels, the less damage there
is to the crop. Some data are available to answer this
question. In Ohio, yield losses from soybeans plantedw
in 7.5-inch row spacing ranged from 1 percent with
a 120-foot spray boom to 2.5 percent with a 50-foot
spray boom. Indiana data showed a yield loss of 1 percent
with a 90-foot boom and 2.5 percent with a 45-foot
boom when soybeans were planted in either 7.5- or 15-
inch row spacings. Depending on yield potential, losses
could range from $2 to $8 per acre with soybeans valued
at $5.50 per bushel. This monetary loss can greatly
increase with a crop that is marketed at higher prices.
Virginia researchers are currently investigating the effect
of running over soybean rows in full-season and
double-crop systems.
Should I install tramlines or skip rows in
my narrow-row soybean?
Yes, you should install tramlines or skip rows in narrow-
row plantings to avoid running over reproductive-
stage soybeans. Running over one or two rows of
soybeans during the pod and seed development stages
could reduce yield by one-half to two bushels per acre,
depending on width of sprayer; thereby reducing the
cost effectiveness of fungicides.
Other ASR Resources
Asian Soybean Rust website at Virginia Tech -
http://www.ppws.vt.edu/ipm/soybeanrust/index.htm
USDA Soybean Rust Tracking site -
http://www.sbrusa.net/
North American Plant Disease Forecast Center-
http://www.ces.ncsu.edu/depts/pp/soybeanrust/
The Southern Plant Diagnostic Network -
http://spdn.ifas.ufl.edu/soybean_rust.htm
Related Publications
Asian Soybean Rust - Frequently Asked Questions
I: Background and General Information, Virginia
Cooperative Extension publication 450-301
Asian Soybean Rust - Frequently Asked Questions
II: Identification, Biology, and Ecology, Virginia
Cooperative Extension publication 450-302
Asian Soybean Rust - Frequently y Asked Questions
III: Control with Fungicides, Virginia Cooperative
Extension publication 450-303
Asian Soybean Rust - Frequently Asked Questions
V: Monitoring, Tracking, and Scouting, Virginia
Cooperative Extension publication 450-305
Asian Soybean Rust - Frequently Asked Questions
VI: Sprayer and Nozzle Technology, Virginia
Cooperative Extension publication 450-306
Acknowledgments
The authors would like to express their appreciation for
the review and comments made by Keith Balderson,
Joel Faircloth, Ames Herbert, and Matt Lewis.
View this document in PDF format
Visit Virginia Cooperative Extension.