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