Asian Soybean Rust -
Frequently Asked Questions II:
Identification, Biology, and Ecology
Author: Erik Stromberg, Extension Plant Pathologist, Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech; Elizabeth Bush, Special Research Associate, Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech; David Holshouser, Extension Soybean Specialist, Tidewater Agricultural Research and Extension Center and Pat Phipps, Extension Plant Pathologist, Tidewater Agricultural Research and Extension Center
Publication Number: 450-302, posted March 2006
What does the Asian soybean rust (ASR)
look like?
The first symptoms usually appear as pinpoint-size
lesions on the upper leaf surface of the lower leaves
of the plant. These early lesions may first appear as
yellow flecks and with age turn tan to brown or redbrown.
The lesions may be angular or circular and may
cluster near leaf veins.
As the disease progresses, lesions may be numerous.
Pustules form, mostly on the lower leaf surface on soybeans.
These pustules erupt to release urediniospores
through circular holes, which when examined with a
hand lens, resemble microscopic volcanoes. The urediniospores
may be powdery and tan or lightly pigmented.
Lesions and pustules may also develop on petioles,
stems, and pods. Chlorosis or mottling on leaves
may also be associated with lesion development.
As the soybean matures and sets pods, ASR spreads
rapidly. Lesions can be found on petioles, pods, and
stems, but are most abundant on leaves.
Especially at the early stages, it is easy to confuse
soybean rust symptoms with the symptoms of three
other soybean leaf diseases: brown spot caused by the
fungus, Septoria glycines, bacterial pustule caused
by Xanthomonas axonopodis pv. glycines, and bacterial
blight (also called angular leaf spot) caused by
Pseudomonas savastanoi pv. glycinea.
See the disease identification section of the Virginia
Soybean Rust website at http://www.ppws.vt.edu/ipm/
soybeanrust/id.htm for more detailed descriptions of
symptoms and other look-a-like soybean diseases.
How does Asian soybean rust affect soybeans?
When ASR colonizes soybean plants, leaves turn yellow
and premature defoliation occurs. Premature defoliation
results in decreased pod production, seed fill,
and/or smaller seed size. ASR can progress very rapidly
and defoliate a soybean plant within four to six
weeks after the initial infection.
The yield loss due to defoliation will depend on the
developmental stage of the soybean. The most critical
stages for maintaining leaf area are the late pod (R4)
to full seed (R6) developmental stages. For instance,
complete defoliation during early seed formation (R5)
could reduce yield more than 75 percent. There will
be progressively less yield loss at earlier (R1-R4) or
later stages (R6). Only after the soybean plant reaches
physiological maturity (R7) will defoliation cause no
yield loss.
Does ASR affect any other crops grown in
the United States?
Some common leguminous crop plants and weeds are
also hosts, including yellow sweet clover (Melilotus officinalis),
vetch (Vicia dasycarpa), medic (Medicago
arborea), lupine (Lupinus hirsutus), green and kidney
beans (Phaseolus vulgaris), and lima and butter beans
(Phaseolus lunatus).
Of most concern may be the snap bean (Phaseolus
vulgaris) industry on Virginia’s Eastern Shore and
in other parts of Virginia. Although the snap bean is
listed as a host species of ASR, the snap bean is not as
susceptible as the soybean. According to observations
by researchers on snap beans and soybeans in South
Africa, it is very difficult to find ASR in fields of snap
beans, even when adjacent soybean fields are severely
diseased. However, some snap bean cultivars are more
susceptible than others.
Corn and other grain crops, alfalfa, cotton, and peanuts
are not hosts to ASR.
Are there soybean cultivars with resistance
to ASR?
Currently, there are no soybean cultivars with acceptable
levels of resistance to ASR. A commercial soybean
cultivar for use in the United States is probably
five to ten years away. The good news is that much
groundwork has been done. Partial resistance and yield
stability are likely to be the most effective resistance
strategies. Planting cultivars with resistance to rust
pathogens has been very successful in managing rust
in wheat and corn and is expected to be similarly successful
with ASR.
Why do I need to know the development
stages of soybeans?
Soybean development is classified as vegetative or
reproductive. Vegetative stages are designated numerically
as V1, V2, V3, through V(n), which refers
to the number of fully developed nodes on the main
stem. Reproductive stages are divided into flowering
(R1, R2), pod development (R3, R4), seed development
(R5, R6), and maturity (R7, R8). Details on soybean
development stages can be found at http://www.ppws.
vt.edu/ipm/soybeanrust/index.htm. The developmental
stage of the soybean crop is used to determine whether
or not the crop is at risk from ASR.
Fungicide application is not recommended during vegetative
growth stages. From the R1 through R6 reproductive
stages, soybeans are at risk (i.e. yield loss will
occur). The most critical period of plant development
in terms of yield is the late pod (R4) through seed development
(R5 and early R6) stages. Any stress during
these stages can greatly reduce yield.
How does the soybean developmental
stage affect ASR management?
Fungicides are not recommended during the vegetative
stages. If the risk of rust is high during flowering (R1)
through seed development (R6), fungicide applications
will be recommended.
Why aren’t soybean seedlings at risk to
ASR?
The seedling-stage soybean is susceptible to ASR;
however, the fungus does not develop and sporulate as
readily as it does once the soybean enters the reproductive
stages.
Is the soybean more susceptible to ASR
in later reproductive stages?
The developmental stage has little to do with the susceptibility
of the soybean to ASR; ASR can infect
seedling-stage soybeans. However, ASR does not develop
and sporulate as rapidly on soybeans in the vegetative
stages as on soybeans in the reproductive stages.
Therefore, fungicide application will not be cost effective
and not be recommended until the soybean begins
to bloom (R1). In 2005, ASR incidence and severity
progressively increased as the crop aged.
Why does ASR usually appear on the
lower leaves first?
The lower leaves experience longer periods of leaf
wetness, higher humidity, less exposure to UV light,
and cooler temperatures during the day and warmer
temperatures during the night. All of these factors favor
rust development. However, climatic conditions in
Virginia make it likely that other areas of the plant will
experience six to eight hours of leaf wetness and the
optimal temperatures for ASR development.
Rust also tends to develop more rapidly on older leaves.
Therefore, leaves originating from the lower part of the
main stem will likely show more disease than leaves
from branches, regardless of their vertical position in
the canopy. Therefore, scouting efforts should be focused
on the lower leaves originating from the main
stem.
Will dry weather affect the disease?
ASR spores are produced rapidly in large quantities
under favorable environmental conditions (i.e. temperatures
between 59 to 82°F and free moisture). Dry
weather can greatly inhibit disease development due
to lack of free moisture. ASR needs a minimum of six
hours of leaf wetness to sporulate.
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
III: Control with Fungicides, Virginia Cooperative
Extension publication 450-303
Asian Soybean Rust - Frequently Asked Questions
IV: Cropping Systems and Cultural Practices,
Virginia Cooperative Extension publication 450-304
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 Scott Hagood, Brad
Jarvis, Cal Schiemann, and Fred Shokes.
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