Asian Soybean RustGregory ShanerDept Botany and Plant PathologyPurdue University
Revised: 1/30/2006
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.
Australasia -1902 through the 1960
Uganda - 1997Zimbabwe - 1998S. Africa - 2001
Paraguay - 2001Brazil -2002Argentina - 2002
Hawaii1994
Continental U.S. - 2004
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
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
Tan and Red Brown reactions to Phakopsora pachyrhizi
(USDA-APHIS Web site)
Premature defoliation Poor pod filling Smaller seed Rusts are obligate parasites, so
they do more than just remove photosynthetic leaf area
Rust can reduce yields 10-80%
Disease triangle
Host Pathogen
Environment
Soybean rust is a polycyclic disease
Pustule from primary infection
Spores from primary pustules cause more infection
About 9 days are required for an infection to mature into a pustule
These infection cycles begin whenever conditions allow spores on leaves to germinate and penetrate
Pustule numbers increase exponentially
Under favorable conditions, pustules develop on all leaves
When infections become old, leaf tissue dies
Spore production in soybean rust pustules
Day 1
Day 27
Day 6
Day 13
Soybean rust can develop very rapidly
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
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
Possible hosts of soybean rust in Indiana
Cultivated Crops:SoybeansKorean and Japanese
cloverWhite cloverKidney beansGarden peaOrnamental plants:Hyacinth beanWild hosts: KudzuCrown-vetchYellow sweet cloverKudzu with
soybean rust
Soybean rust on kudzu
www.planthealth.info
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
7.4% 5.2%6.1%
7.6%
10.5%
36.8%
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
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
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
It’s not known if this resistance is sufficient to provide protection
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
Fungicides approved under Section 18
Myclobutanil (Laredo) Tebuconazole (Folicur, Orius, Uppercut) Propiconazole (Tilt, Bumper, Propimax) Propiconazole + Trifloxystrobin
(Stratego) Tetraconazole (Domark)
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
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
Effects of fungicides on rust development
PenetrationMycelialgrowth
Sporulation
Strobilurins
Triazoles
Highly effective Little or no effect
Pre-sporulation
Chlorothalonil
Sporegermination
Strobilurin plus triazole:Twin 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
TriazoleInhibits sterol biosynthesis. Sterols are importantcomponents of the cell membrane.Good curative activity
...... also of benefit for resistance management
Soybean rust fungicides: Biokinetics (bioassay)
Folicur (tebuconazole)
Untreated
Alto (cyproconazole)
Opus (epoxiconazole)
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
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
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
Application technology
Droplet size <220 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
Spraying conditions
Don’t spray when: Temperature is above 86 °F Relative humidity is below 55% Wind speed is greater than 5 mph
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 won’t be protected
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
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
Rust on trifoliolate leaves
Five days after infection
Soybean rust lesions on leaves, seen with backlighting
Nine days after infection
Twenty-five days after infection
Bacterial pustule may be confused with soybean rust
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.
Brown spot
Common in Indiana
Appears soon after plants emerge
Spots don’t reach upper leaves until late in the summer
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
First detectors
Purdue has a program to train “first detectors”
This is coordinated through the Plant and Pest Diagnostic Laboratory (PPDL)
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
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
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
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