Soybean Rust: What Happened in 2005?Whats Ahead?Gregory ShanerPurdue University

A brief historyAsian soybean rust appeared first in Western Hemisphere in 2001, in Paraguay and southern BrazilOver the next 2 years it spread to nearly all of Brazils soybean production areasRust reached the continental U.S. in fall of 2004Found in 9 southeastern states, including TN and SE MOOnce there were killing frosts, the fungus was probably eradicated from most of these sitesThe fungus was known to overwinter only in central and southern Florida

Path of Hurricane Ivan4-16 Sep 2005Winds at the periphery of this storm are thought to have carried spores from Colombia, Central America, or the Caribbean to the U.S.

Estimated spore deposition: September 19, 2004

Soybean Rust in the U.S. Nov 10 - Dec 1, 2004

Reasons for concern about soybean rustDestructive potentialYields can be reduced up to 80%Crop over a large area can be affectedPathogen is airborneCapable of rapid, long-distance dispersal

In anticipation of rustKnowing it was a matter of when, not if, rust would reach U.S., pathologists began developing strategies for control Educational programsPublicationsApplications for Section 18 exemptions for fungicidesSystem for early detection and monitoring rust progressUSDA Web siteSentinel plotsDiagnosticsField research

Educational programsCrop Management WorkshopsCounty PARP meetingsISB-Purdue workshopsDTC spray workshopsDTC summer programsRegional programs with Dr. Tadashi YorinoriSpecial programs (ICIA, Fluid Fertilizer School, Kentuckiana, etc.First detector training

Publications

Fungicides labeled for use against soybean rust

Section 18 products for soybean rust control

Early detection and monitoringSentinel plotsPurdue research farms (PACs)Seed company plotsFarmer-county educator plotsCertified Crop AdvisorsSpore trappingMicroscope slidesvisual identificationRain collectorsPCRUSDA web siteP&PDL web site800-number phone line

USDA-APHIS web sitehttp://www.sbrusa.net

This site has frequent updates on rust, nationally, and state by state

Its possible to go back through the season to see how rust developed

Commentary for Indiana on USDA soybean rust web site

Individual states on USDA soybean rust web siteThis shows Alabama, with counties outlinedRed counties are where rust was found as of Nov 23, 2005A state commentary appears below the map

Purdue Plant & Pest Diagnostic LabThere is a direct link from PPDL home page to soybean rust page

Field researchFungicide evaluation (Shaner and Buechley)Evaluation of 39 treatments at 2 locations for rust controlFungicide application technology (Hanna, Conley, Shaner)Effect of:row spacingfungicide timingspray volumeon: spray canopy penetrationrust controlyield

Asian soybean rust in 2005Not nearly as destructive as we fearedSpread slowly in SouthDid not reach Midwest

What did we learn this year?Even over the area where rust was found, it was not epidemicRust development was initially focalLittle data on comparative fungicide performanceIts critical to treat before rust has much chance to develop

Little learned about reaction of adapted varietiesSome are being evaluated in a late-planted nursery in the South

Purdue fungicide trialsProduct comparisonsNo rust, so no data on efficacy against that diseaseFrogeye leaf spot came in lateMany fungicides suppressed disease, but no effect on yieldTreatments were applied earlier than would have been optimalFarm-scale trialsRow width (30, 15, or 7.5-in.) had no effect on spray penetration into the canopyYields for 7.5- and 15-in. spacing were similar and greater than for 30-in. spacingSprayer wheel tracks reduced yieldRepeated passes in the same tracks had no further effect on yield

Spray canopy penetration

Effect of row spacing on yieldAt 7.5-in. and 15-in. row spacing, the wheel track area yielded 5 bu/A less than the non-wheel track areaAt 30-in. row spacing, the wheel track area yielded 1.5 bu/A less than the non-wheel track areaAveraged over row spacing, application only at R1 reduced yield 2.1 bu/A in the wheel track areaAveraged over row spacing, applications at R3 or R5 reduced yield by 5 bu/A *The wheel track area was a 15-ft swath that included both sprayer wheel tracks

Brazil fungicide trialsValues are based on 5 trials conducted in Brazil in 2003/04. Fungicides were applied twice.

Changes for 2006Sentinel plotsMore of them, with an early and normal MG variety at each siteDefer intense scouting until R1 or laterSubmit all spotted leaves to lab for microscopic examinationMaintenance of some plots for detailed monitoring of disease progressWeb site and phone lineFungicide trialsUniform trial to compare productsApplication experiments, including plant health issueAdditional Section 18 exemptions?Fungicide use recommendationsWait until rust is close?

Differences between Brazil and U.S. Phakopsora pachyrhizi can only survive on living host plantFungus has a broad host rangeThere are more green hosts year round in Brazil than in the U.S.Fungus survives winter throughout BrazilIn U.S., fungus will probably only overwinter south of frost zone

Rust spreads rapidly in Brazil

Rust rapidly defoliates plants

Will we have rust in the Midwest in 2006?No one can sayPartial fallacy of it takes awhile for the pathogen to become establishedWas weather in 2005 unfavorable?Cold winter restricted overwintering sitesEarly part of summer was dry in much of SouthIs there something about the soybean rust fungus?U.S strain might be less aggressive than Brazilian strainsSpores may be very sensitive to UV light

How to prepare for 2006It is possible that rust will reach the Midwest in 2006Look at USDA web site, PPDL web site, or call the toll-free phone line to find out:What rust is doing in southern statesWhat sentinel plots in Indiana revealRecommendations for controlThere is a possibility that fungicides will be needed to protect the crop

The colored area extending from Colombia to the southeastern U.S. shows the pattern of winds on one day during Hurricane Ivan. The colors depict spore density in the air, as the cloud dilutes. Predicted densities were 10^3 to 10^5 spores per hectare in the southeastern U.S.States where soybean rust was found in fall of 2004.Nov 11 - LA; Nov 12 - AK ; Nov 17 - FL; Nov 18 - AL, GA, MS; Nov 19 - MO, SC; Dec 1 TNIn Brazil, rust spread rapidly throughout the soybean-producing areas. Each year it is found on virtually all soybean plants not protected by fungicides in the major soybean-producing areas. Based on this behavior in Brazil and the general correspondence between summer weather in that country and the southern and central US, it is reasonable to assume that soybean rust poses a serious threat to soybean here.Altogether there were more than 50 meetings, that reached more than 1500 farmers and others concerned with this disease.Shaner contributed to a regional publication about soybean rust and its management with fungicides. This publication is available from Ohio State University. Staff at Purdue also prepared a publication that concisely covered the diseaseits biology, diagnosis, management, and economic considerations.These products are available for use in Indiana under a Section 18 quarantine exemption, through 10 Nov 2007.Indiana, as part of national system, had several sentinel plots. Plots at the PACs included several forage legumes as well as soybean. The Syngenta microscope-slide spore traps were not deployed in Indiana (these were mostly in the South). A major problem with these was the reliance on visual identification of rust spores under the microscope. There are many species of rust fungi, and it is impossible to distinguish many of them solely on the basis of urediniospore morphology. Evaluation of rain samples uses a PCR reaction with primers specific for the ITS region of ribosomal DNA of P. pachyrhizi. Spores were found in rain samples from several central and northern states, including Indiana. This test was incapable of determining the viability of these spores.Information about sentinel plot scouting and recommendations for rust control were posted regularly on the Indiana site of the USDA soybean rust web site. Information about soybean rust diagnosis and management was posted on the Plant and Pest Diagnostic Laboratory web site, with links to many other rust web sites. A toll-free line was set up during the summer, where callers could hear the status of soybean rust and recommendations for control.This page shows the national map and national commentary about soybean rust for 25 Aug 2005. From here, users can select individual states for specific commentary and rust management recommendations. The various buttons (green or red) direct the user to more information about soybean rust. This web page is maintained largely by Amy Deitrich (BTNY extension secretary) and Gail Ruhl (PPDL co-director).In 2005, Greg Shaner and George Buechley established trials in Tippecanoe and Jennings Counties to evaluate all Section 3 and Section 18 fungicides, plus several experimental products for control of rust. No rust developed in these trials, but they did obtain some data on control of frogeye leaf spot and on phytotoxicity.Also, Shawn Conley, Greg Shaner, and graduate student Shane Hanna conducted field-scale trials at three locations (NEPAC-Whitley Co.; DPAC-Randolph Co.; SEPAC-Jennings Co.). Soybean was planted at three row spacings. Fungicides were applied with a commercial sprayer fitted with Twin Jet tips. The main objective was to look at the influence of row spacing on spray penetration of the canopy at R1, R3, and R5.Row spacing had no effect on spray penetration. This map is taken from the USDA web site, and shows rust progress, from winter through fall. The green counties show where rust was looked for. Red counties depict where rust was found.From initial infections, rust became severe in a very local area (a few feet in diameter) before spreading out. If this focal development is typical, this complicates scouting for rust in large fields.In one experiment in GA, where rust did develop, when treatments were not applied at the earliest stages of rust, it was difficult to halt the epidemic.We evaluated all registered and Section 18 products, as well as additional materials. Treatments were applied at R2 and about 2 wk later, which was the time considered appropriate for rust control. The only disease that developed was frogeye leaf spot. No treatment increased yield significantly over the control.Penetration was not good deep into the canopy, but was fairly good 12 inches into the canopy. If the first spray is applied when rust is just developing, getting down 12 inches into the canopy may target leaves where many initial infections are developing.The overall loss in yield from wheel tracks will depend on the sprayer boom width, and hence the number of passes required to cover a field.If the threat of rust is not great until it is very close, a denser network of sentinel plots seems justified. Experience in the South showed that field diagnosis of early symptoms, when there might be more than one pustule on 100 leaves, is very difficult. It is better to send all leaves with any spots to a lab where they can be examined under a dissecting microscope. To gain essential knowledge about the effect of weather on rust development, disease must be allowed to develop in some plots. There has been concern that such plots would be a source of contamination for nearby fields. Experience indicates that if fields are sprayed with fungicide, the risk from these research plots will be minimal.In 2005 there was great diversity in fungicide trials among states. For 2006 we will try to conduct trials with the same core set of products, applied at standard stages of growth, so that a large body of data can be pooled. Farm-scale experiments with a strobilurin fungicide may also be conducted to further investigate the plant health phenomenon.EPA is considering Section 18 applications from a couple of states for additional fungicides, some of which are not registered yet for any use in the U.S. We will decide soon whether to submit an application from IN for these products.In 2005, had rust started moving in the South (e.g., Arkansas and Mississippi), this may have prompted some midwestern growers to spray their fields. It may be possible next year to wait until rust is much closer before treating. This will depend on how rapidly rust spreads north. Spraying too early means that if rust eventually appears, a second application will be necessary.This is an unsprayed portion of a field in Brazil. Plants were at the R3 stage of growth and were nearly defoliated. Rust may not have to spread far from any overwintering source in Brazil to reach a field.Premature defoliation during early seed development stages can essentially destroy a crop. Growth stage at which disease becomes established has a big influence on damage.Unlike endemic pathogens such as soybean cyst nematode, the sudden death syndrome fungus, or the corn gray leaf spot fungus, P. pachyrhizi does not overwinter where green host tissue is killed by frost. Thus, each year the epidemic must start from overwintering sites in South. Thus, the widescale occurrence of rust in southern states this fall will have little bearing on overwintering because in most of this region killing frosts will destroy host plants. In fall of 2005 rust was found in Louisiana and east Texas, along Gulf Coast. If it survives there, spores are in a better position to be carried up Mississippi Valley into IN and IL.If spring and early summer are wetter, rust could get started in soybean in the South much earlier, increasing the likelihood that spores would reach Midwest during summer.Even though spores were detected in rain samplers as far north as MN, they may not have been viable. Spores carried long distances by tropical storms need to survive in upper air to pose a threat to Midwest soybean. If spores cant survive long distance dispersal, disease will have to move northward stepwise, field to field, which would likely delay the onset of disease here until late in the season.