CROP INSIGHTS

Asian Soybean Rust: Fungicides

by Steve Butzen a, Frederic Beudot b and Bond McInnes c
ways – by their general function (preventative or curative),
Summary mobility in the plant (contact, fixed protectant, penetrant
or systemic), and mode of action or chemistry.
• Fungicides, chemical substances that kill or inhibit the
growth of fungi, have proven very effective at controlling Figure 1. Classification of fungicides by general function –
Asian soybean rust (ASR) in major soybean-producing preventative and curative.
areas of the world.
• Fungicides can be classified by their general function
Fungal Spore
(preventative or curative), mobility in the plant, and mode
of action. Germinating Spore
• Fungicide mobility ranges from immobile on the leaf Fungal Hyphae
surface to translocation within the entire plant. The mode
of action refers to how fungicides affect the fungal Preventative - Fungicide prevents the establishment of
organism, and is determined by their chemistry. an infection. Fungicides that are preventative only must be
• To increase fungicide effectiveness, growers should select applied before infection.
the fungicide with the highest level of control and apply
before disease pressure is too severe.
• Four families of fungicides are available for ASR
management – triazoles, strobilurins, chloronitriles, and
carboxamides. Triazoles are both curative and preventive,
with longer residual properties than most fungicide groups.
Curative- Fungicide interrupts the development of an
• Calculation of true cost of fungicides must include the established infection before visible symptoms and
component of residual activity, which allows determina- production of spores.
tion of cost of fungicide protection per day.
These classes are not discreet -- some active ingredients are
Introduction both preventative and curative. In addition, some fungicides
have two active ingredients, one preventative and one cura-
Now that Asian soybean rust (ASR) has entered the southern tive. However, neither class of fungicides is effective after
and southeastern U.S., soybean growers in the U.S. and development of visible disease symptoms. For that reason,
Canada must be prepared to manage the disease. At this time, timely application before establishment of the disease is
the only effective means of management is immediate important for optimum control and preservation of yield.
fungicide application when the disease is detected. A
previous Crop Insights (Asian Soybean Rust1) has addressed Fungicide Mobility
disease biology and identification, including scouting and
sampling procedures. A concurrent article (Asian Soybean Fungicide mobility ranges from immobile on the leaf surface
Rust: Fungicide Application Technology2) provides more to translocation within the entire plant, with categories in-
information on spraying technology. This article will address between these extremes as well. Contact fungicides adhere to
fungicide types, selection and use for Asian soybean rust the leaf surface but do not go deeper. These products are
management. Guidelines for use of different fungicide classes most susceptible to removal by rain and other weather
in sequential applications are included. effects. Fixed protectants move from the leaf surface into the
waxes or cuticle, which increases their longevity on the leaf.
Fungicide Classification Penetrants are able to penetrate beyond the cuticle and into
the treated leaf tissue itself, but technically do not trans-
Fungicides are chemical substances that kill or inhibit the locate. Systemic fungicides are absorbed into the leaf tissue,
growth of fungi. Fungicides can be classified in a number of and then translocate from their point of entry to other tissues.

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Penetrants and truly systemic products both move into the timely manner with good spray coverage, can effectively
leaf tissue, but their mobility differs from there. Penetrants manage ASR and protect yields.
are locally systemic (“translaminar”) within the leaf where
Four families of fungicides are available for ASR
placed, but do not move to, or protect, other leaves. Truly
management – triazoles, strobilurins, chloronitriles, and
systemic products translocate with plant fluids in the xylem
carboxamides (Table 2). While these families are not all
tissue to other plant parts. Since xylem tissue only moves
equal in effectiveness or value, they are all presented here as
fluids upwards, leaves above the point of application may be
options to include in a fungicide program that protects both
protected, but not leaves below.
soybean yield and fungicide efficacy over time.
Table 1. General properties*of contact, penetrant/locally Table 2. Properties of fungicides for ASR management*.
systemic and systemic fungicides.
Triazoles Strobilurins
Contact Penetrant / Locally Systemic
Fungicides Systemic Fungicides Fungicides MOA Prevent production of Stop energy production in
sterols critical for cell the fungus (interfere with
Preventative Preventative Preventative and membrane formation production of ATP)
function function curative function
Site Single site - inhibit one Single site - inhibit one
Multi-site biotic Single-site biotic Single-site biotic specific enzyme specific enzyme
inhibitors inhibitors inhibitors
Mobility Xylem systemic Locally systemic
Less resistance Greater resistance Greater resistance
Function Curative (affect fungal Preventative (inhibit spore
chance chance chance
growth and reduce germination and very early
Higher doses Lower doses Lower doses sporulation) infection)
More applications Moderate applications Less applications Common Myclobutanil, tebucona- Azoxystrobin,
Name zole, propiconazole pyraclostrobin
Low residual Low to medium Medium to high
residual residual Brand Folicur5, Laredo6, Tilt7, Quadris7, Headline9
Name Propimax6, Bumper8
*These are general properties and exceptions may exist.

Chloronitriles Carboxamides
Mode of Action
MOA Stop energy production in Stop energy production in
The mode of action (MOA) of fungicides refers to how they the fungus (interfere with the fungus (interfere with
affect the fungal organism, and is determined by their production of ATP) production of ATP)
chemistry. The oldest chemical fungicides were simple sulfur Site Multi-site inhibitors Single-site (inhibit enzymes
and copper compounds that were general fungal and bacterial containing sulfur)
inhibitors at multiple biotic sites. Organic fungicides that
Mobility Contact Contact
followed (e.g., mancozeb) were much more effective but
were limited by their immobility and non-specific mode of Function Preventative (inhibit Preventative (inhibit spore
action. spore germination) germination)

In the 1960s, the first systemic/curative fungicides (e.g., Common chlorothalonil boscalid
benomyl) were introduced. These fungicides were the first to Name
target a specific, known biological function in fungi such as Brand Bravo7, Echo10, Chloronil One of two chemicals in
mitosis. In the 1980s and 1990s, many new fungicide families Name Pristine9
were introduced with very specific, targeted modes of action.
These families, such as the sterol inhibitors (e.g., triazoles) * Compiled by Alison Robertson, Extension Plant
and quinone outside inhibitors (e.g., strobilurins), include the Pathologist, Iowa State University.
most effective products used today for managing Asian
soybean rust.
Triazoles
Research on Asian soybean rust in Africa and South
Fungicides for Managing ASR America has demonstrated the effectiveness of triazole
fungicides in managing this disease. Triazoles are sterol
Growers should only consider the most effective fungicides inhibitors – they inhibit the biosynthesis of sterol
to combat the potentially devastating Asian soybean rust (especially ergosterol) critical for integrity of fungal cell
disease. Years of experience under intense disease pressure in membranes. This results in destruction of the cell
Brazil have shown that certain fungicides, when applied in a membrane. Triazoles are both curative and preventive,
usually with longer residual properties than most other

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fungicide chemistries. Triazoles arrest established ASR in- movement throughout the growing season, to help you
fections by contacting the fungal hyphae filling cell spaces, determine if spraying is needed.
destroying new fungal cells and preventing branching.
Within this large group of fungicides, however, there is high
variation by active ingredient on activity against ASR and
even translocation within the plant (Figures 2 and 3).

Figure 3. Control of soybean rust on trifoliate leaf by

labeled rate of fungicide placed in circle shown on leaflet.

Figure 2. Translocation of a very mobile triazole fungicide Fungicide Costs

as demonstrated by carbon-14 labeling.
Calculation of actual fungicide cost must take into account
the control period, which varies significantly by fungicide. A
In Figure 2, three droplets of a labeled rate of triazole fungi- fungicide that lasts for only 14 days, for example, will need
cide were applied to a trifoliate as shown at top left. Leaves to be applied more often than one that provides control for 21
were harvested at 1,3,7, 14 and 21 days after application. At days, increasing both the product and application costs by
1 day after application (DAA) fungicide covered the entire 50% for the shorter residual product. Table 3 is a sample
leaf. At 3 and 7 DAA, fungicide concentration is increasing worksheet for determining true cost of control per day based
throughout the leaf. At day 14, fungicide concentration was on the control period for each class of fungicides.
still near peak. At day 21, the leaf is still protected despite
some dissipation of the fungicide.
Use Guidelines for Fungicide Classes
In another experiment, homogeneous soybean field plants with
5% soybean rust incidence on lower canopy leaves were If multiple fungicide applications are likely, growers should
selected. Labeled rates of several different triazole fungicides have a pre-determined plan for sequential applications that
were placed on individual plants in a restricted area of a includes the best-performing products. The spray guidelines
single leaflet as shown (Figure 3). Leaves were marked with a shown in Table 4 are based on soybean rust status before
tag and monitored for disease development. As the figure shows, each application (adapted from Monte R. Miles, USDA-ARS.
there is significant variation in control of soybean rust among the 2005 University of Illinois Pesticide Conference).
four treatments, from very good with triazole 1, to poor with
triazole 4. (Growers would normally try to achieve better leaf
coverage than in this experiment, but some leaves would References
inevitably receive similar restricted coverage.)
1. Schuh, W. and S. Butzen, 2004. Asian Soybean Rust. Crop
These experiments demonstrate the importance of selecting a Insights Vol.4 no. 20. Pioneer Hi-Bred. Johnston, IA.
product with favorable translocation properties and good http://www.pioneer.com/usa/agronomy/soybeans/asian_soy
efficacy against soybean rust. While there were only a few bean_rust.htm
registered triazole products for soybeans in 2004, a relatively
large number of products are awaiting section 18 approval, 2. Butzen, S., A. Marcon, B. McInnes, and W. Schuh. 2005.
and many of these will likely be available for use in 2005. Asian Soybean Rust: Fungicide Application Technology.
Your Pioneer sales professional can help you select a product Crop Insights Vol. 5 no. 1. Pioneer Hi-Bred. Johnston, IA.
that is effective against soybean rust and available in your http://www.pioneer.com/usa/agronomy/soybeans/soybean_r
area. Your Pioneer representative will also have access to the ust/asian_soybean_fung_app.htm
latest maps and information indicating soybean rust

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3. McInnes, B., A. Marcon, and R. Gueddens. 2003. DuPont Table 3. Sample worksheet of total daily cost* of fungicides.
Crop Protection Fungicide Training Guide DuPont Crop
Protection. Wilmington, DE. Fungicide Control Daily
Family Period Cost/Acre
4. McInnes, B., A.Marcon, R. Kczmarczyk, and M. Martin.
Chlorothalonil 7 days
2004. DuPont Crop Protection Fungicide Application
Technology Training Guide on Soybeans. DuPont Crop Product cost $6
Protection. Wilmington, DE. Application cost $4

5
Total cost $10 $1.43
Folicur and Stratego are registered trademarks of Bayer.
Strobilurin 14 days
6
Laredo and PropiMax are registered trademarks of Dow Product cost $12
AgroSciences.
7 Application cost $4
Bravo, Quadris and Tilt are registered trademarks of
Syngenta. Total cost $16 $1.14
8
Bumper is a registered trademark of Makhteshim-Agan. Short-residual Triazole 14 days
9
Headline and Pristine are registered trademarks of BASF. Product cost $10
10
Echo is a registered trademark of Sipcam Agro, Inc. Application cost $4
Total cost $14 $1.00
Long-residual Triazole 21 days
a
Agronomy Information Manager. Pioneer Hi-Bred ,
Product cost $10
Johnston, IA.
b
Treatment cost $4
Fungicide Product Manager. DuPont Crop Protection,
Total cost $14 $0.67
Denver, CO.
c
*These costs and control periods are approximate only. See
Fungicide Product Development Manager. DuPont Crop your local supplier for actual costs, and product label for
Protection. Valdosta, GA. re-spray interval.

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 Table 4. Guidelines for Use of the Different Fungicide Classes in the Management of Soybean Rust*
Program type (based on soybean rust status First Application
before first application)

A. Chlorothalonil
Preventative Fungicide Program: Use in late vegetative or early flowering, before any soybean rust is expected
in the field; will not prevent disease if the spores have already infected the
No soybean rust present but is predicted or plant.
expected B. Strobilurin
Use before any soybean rust is expected in the field
C. Triazole
Can be used before and after initial infection or sign of disease
D. Strobilurin-Triazole mixture
Can be used before and after initial infection or sign of disease

Curative Fungicide Program: A. Triazole

Can be used before and after initial infection or sign of disease
Soybean rust is present and lesions are
B. Strobilurin-Triazole mixture
sporulating in the lower canopy, less than
10% incidence Can be used before and after initial infection or sign of disease

Late Curative Fungicide Program Once the disease is in mid-canopy or greater than 10% incidence, yield losses
will occur. The decision to apply fungicide will need to take into account the
Soybean rust is present and lesions are
expected yield in the field and the economics of the crop.
sporulating in the mid-canopy, incidence is
greater than 10%.

Second Application
Use triazole or strobilurin-triazole mixture if disease is present, less than 10% incidence, and limited to lower canopy.
However, if chlorothalonil was used in the first application, and rust is still not present in the field, use a strobilurin.

Third Application
Use triazole or strobilurin-triazole mixture if disease is present, less than 10% incidence, and limited to lower canopy.
Some products may not allow a third application – check product label. A maximum of two total applications using section 18
products collectively is allowed.

Other Guidelines:
Scouting and monitoring of fields with information on regional spread of the disease are critical for management of soybean rust.
Read and follow all instructions on the label of each of the products.
Chlorothalonil products should only be used protectively.
Strobilurin products should only be used in preventative programs. Strobilurin-triazole mixtures can be used in curative programs.
For Section 18 registrations, triazoles can be used twice per season. If a third application is needed, a different triazole can be
used.
Pre-harvest intervals (PHI) differ by fungicide; most range from 14 to 21 days.
Three applications may not be economically viable.
Incidence, as used in this table, is the number of plants in 100 that have any sign of the disease, e.g., 1% incidence = 1 plant in 100
with a lesion.

*Adapted from Monte R. Miles, USDA-ARS. 2005 University of Illinois Pesticide Conference.

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