Asian Soybean Rust Gregory Shaner Dept Botany and Plant Pathology Purdue University

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Asian Soybean RustGregory ShanerDept Botany and
Plant PathologyPurdue University
Revised 1/30/2006
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Two species of rust fungi infect soybean

• Phakopsora pachyrhizi
• Asian Soybean Rust
• Extremely damaging
• First reported in continental U.S. in Nov 2004
• Phakopsora meibomiae
• Known for many years from subtropical Americas
• Not as aggressive
• Not known in U.S.

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Continental U.S. - 2004
Hawaii 1994
Australasia - 1902 through the 1960
Uganda - 1997 Zimbabwe - 1998 S. Africa - 2001
Paraguay - 2001 Brazil -2002 Argentina - 2002
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How did Phakopsora pachyrhizi get to the U.S.?

• Most likely hurricane winds in autumn of 2004
• South America
• Central America
• Land bridge
• Gradual movement from South America to Central
  America, Mexico, southern U.S.
• Short-distance movement on susceptible plants

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Soybean rust is a foliar disease

• Fungus infects mainly leaf blades
• Infections may also occur on leaf petioles and
  pods
• Most pustules are on the underside of the leaf

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Tan and Red Brown reactions to Phakopsora
pachyrhizi
(USDA-APHIS Web site)
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Rust can reduce yields 10-80

• Premature defoliation
• Poor pod filling
• Smaller seed
• Rusts are obligate parasites, so they do more
  than just remove photosynthetic leaf area

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Disease triangle
Pathogen
Host
Environment
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Soybean rust is a polycyclic disease
Pustule from primary infection
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Spores from primary pustules cause more infection
About 9 days are required for an infection to
mature into a pustule
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These infection cycles begin whenever conditions
allow spores on leaves to germinate and penetrate
Pustule numbers increase exponentially
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Under favorable conditions, pustules develop on
all leaves
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When infections become old, leaf tissue dies
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Spore production in soybean rust pustules
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Day 1
Day 6
Day 13
Day 27
Soybean rust can develop very rapidly
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Winter survival of soybean rust

• This is the bottleneck
• Rust fungi do not persist in crop residue
• Soybean rust is not seed borne
• It survives on green host plants
• Unlikely to survive the winter in most U.S.
  soybean areas
• Probably will survive the winter on the Gulf
  Coast or Caribbean

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The soybean rust fungus has a broad host range

• Phakopsora pachyrhizi is known to infect 35
  species of legumes in addition to soybean
• Among these are several weeds and crops grown in
  the U.S.
• Most of these other host species occur in the
  South

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Possible hosts of soybean rust in Indiana

• Cultivated Crops
• Soybeans
• Korean and Japanese clover
• White clover
• Kidney beans
• Garden pea
• Ornamental plants
• Hyacinth bean
• Wild hosts
• Kudzu
• Crown-vetch
• Yellow sweet clover

Kudzu with soybean rust
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Soybean rust on kudzu
www.planthealth.info
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Epidemiology scenario

• Overwintering in South
• Annual migration of spores into Midwest

• Damage will depend on
• How early spores arrive from South
• How favorable local weather is for spread

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36.8
10.5
6.1
5.2
7.4
7.6
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Weather that Favors Rust

• Summer climate in Midwest will probably be
  generally favorable for rust
• Temperatures from 59 to 82 F
• Dew for several hours is sufficient for infection

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Management of soybean rust

• Resistant cultivars
• This would be the desirable approach, but there
  are none
• Cultural practices
• Tillage, rotation, date of planting will not
  likely have any effect
• Fungicides
• There are several effective fungicides

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Resistant cultivars

• USDA has been screening germplasm in a
  containment facility and in other countries
• So far, not much resistant germplasm has been
  found in Glycine max
• Some germplasm lines have partial resistance
• Its not known if this resistance is sufficient
  to provide protection

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Fungicides

• Fungicides will be the only control option for
  several years to come
• Effective fungicides currently fully labeled
• Chlorothalonil (Bravo, Echo 720)
• Azoxystrobin (Quadris)
• Pyraclostrobin (Headline)
• Topsin M, also labeled for use on soybean, does
  not control rust

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Fungicides approved under Section 18

• Myclobutanil (Laredo)
• Tebuconazole (Folicur, Orius, Uppercut)
• Propiconazole (Tilt, Bumper, Propimax)
• Propiconazole Trifloxystrobin (Stratego)
• Tetraconazole (Domark)

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Systemic vs. non systemic fungicides

• Most fungicides that are, or will be, labeled for
  soybean rust are systemic
• They are absorbed by plant and move in
  transpiration stream (toward leaf tips)
• Products differ in degree of systemicity
• These products may show some curative activity
• Non systemic fungicides provide a protective
  barrier on the leaf surface

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Types of fungicide action

• Protectant
• Applied before disease onset
• Kills spores as they begin to infect
• Curative
• Can kill established infections before symptoms
  appear
• Eradicant
• Inhibits development of an established,
  symptomatic infection
• May have antisporulant activity

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Effects of fungicides on rust development

Spore germination
Mycelial growth
Pre- sporulation
Penetration
Sporulation
Chlorothalonil
Triazoles
Strobilurins
Highly effective
Little or no effect
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Strobilurin plus triazoleTwin biochemical modes
of action
Strobilurin Inhibits electron transfer in
cytochrome bc1 complex of mitochondria.
Therefore, disrupts energy production by the
fungus. Powerful preventative action
Fungal cell
Triazole Inhibits sterol biosynthesis. Sterols
are important components of the cell
membrane. Good curative activity
...... also of benefit for resistance management
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Soybean rust fungicides Biokinetics (bioassay)
Folicur (tebuconazole)
Alto (cyproconazole)
10 x 5 microlitre droplets (1.9 g ai/ha 0.027
oz av ai/A assumed spray volume of 200 l/ha
21.4 US Gal/A) applied to all leaflets in zone
between the two white lines 1 day before
inoculating lower leaf surface
Opus (epoxiconazole)
Untreated
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An effective fungicide product is only part of
the story

• Application must be timely
• Good coverage is important
• This includes getting fungicide down in the
  canopy
• By the time pustules break out on infected
  leaves, one or more leaves will have formed above

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Application timing

• Fungicides work better as protectants than as
  therapeutics
• First spray should be applied before incidence
  exceeds 5
• This means only a few pustules at mid-canopy on
  no more than 1/20 of the plants
• If application is delayed until most plants have
  rust, control will be unsatisfactory
• Incubation period is what can trip you up

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Application technology

• Droplet size lt220 ?m (fine to medium)
• 60 drops per cm2 of leaf (387/in2)
• Nozzles 1 ft above canopy
• 15 to 20 gpa spray volume
• 5 gpa for aerial application

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Spraying conditions

• Dont spray when
• Temperature is above 86 F
• Relative humidity is below 55
• Wind speed is greater than 5 mph

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Will more than one application be needed?

• Rust may start before plants have produced all
  their leaves
• A plant at R1 has developed only half its nodes
• New growth wont be protected

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Should growers scout their fields?

• Initial infections may be focal
• These may be overlooked
• Sentinel plots may be used
• Several plots in each state
• Sown 2 wk before normal sowing time
• Monitored twice weekly for rust
• Once rust is found, a general alert will be issued

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Diagnosis and detection

• Early infections can be difficult to see
• Sporulation is on underside of leaf
• Several look-alike diseases
• Bacterial pustule
• Bacterial blight
• Brown spot

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Rust on trifoliolate leaves
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Five days after infection
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Soybean rust lesions on leaves, seen with
backlighting
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Nine days after infection
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Twenty-five days after infection
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Bacterial pustule may be confused with soybean
rust
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Bacterial blight is another rust look-alike
Lesions produced by Pseudomonas savastanoi pv
glycinea. These may superficially resemble rust,
but inspection with a hand lens will reveal the
difference.
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Brown spot
Common in Indiana Appears soon after plants
emerge Spots dont reach upper leaves until late
in the summer
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Who is looking for rust?

• Scouting activity in Indiana will begin in the
  spring
• Plant pathologists at Purdue and county extension
  educators will be looking for soybean rust
• First detection of rust will also depend on
  others
• Soybean growers
• Crop advisors
• Seed company staff
• Crop certification inspectors
• Survey entomologists

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First detectors

• Purdue has a program to train first detectors
• This is coordinated through the Plant and Pest
  Diagnostic Laboratory (PPDL)

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What to do with a suspect sample

• Take sample to County Extension Office
• Submit sample to the Purdue Plant and Pest
  Diagnostic Laboratory (PPDL)
• County extension offices have submission forms
• Flatten symptomatic leaves between pieces of dry
  paper and double-bag in plastic
• Exact location of field is critical

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Web site

• There are several Web sites about soybean rust
• The Purdue Plant and Pest Diagnostic Laboratory
  Web site has current information about soybean
  rust and links to other authoritative sites
• http//www.ppdl.purdue.edu

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Conclusions

• Rust will be a serious soybean disease in
  Indiana, but not necessarily every year
• Rust can be controlled
• Fungicides will be the first line of defense
• Ultimately, resistant cultivars will probably be
  developed
• Purdue CES is working with other governmental and
  commodity organizations to provide accurate and
  timely information about this disease