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GUIDELINES AND RESOURCES FOR MANAGING SOYBEAN DISEASES

Diseases can and do cause economic losses inmidsouthern soybean systems. Until the early2000's, many diseases could only be managedwith resistant varieties or with cultural practicesthat were marginally effective.

Fortunately, there are now preventive and/orcurative management practices available for mostmajor diseases of soybeans. A list of soybeandiseases and how they can be managed, prevented,or controlled is shown in Table 1 below.

Several important diseases (sudden deathsyndrome [SDS], stem canker, Phytophthora rootrot [PRR], charcoal rot, seed and seedlingdiseases) of soybeans have no curative control;i.e., these diseases may be prevented but not curedonce present. SDS and stem canker can bemanaged or avoided by using less-susceptible orresistant varieties, or rotation to a non-host crop ina field that has a history of a problematicinfestation by one of the diseases. PRR can bemanaged by using resistant varieties. However,PRR appears to be a relatively rare disease andtypically only occurs on clay soils that holdexcessive water when saturated or near-saturated.

Seed and seedling diseases (caused by numerousfungi that likely comprise a “complex” of fungithat includes but is not limited to Cercospora,Fusarium, Phomopsis, Pythium, Phytophthora,and Rhizoctonia solani) can be effectivelyprevented by using the proper fungicide seedtreatment. However, this is not to suggest thatthey will be eliminated with the use of a properlylabeled seed treatment. The environment at timeof planting or shortly thereafter dictates whetheror not a seedling disease will occur.

There are no known resistant varieties (only

moderately resistant germplasm and some tolerantvarieties) or fungicides for charcoal rotmanagement. Additionally, it is likely that themajority of germinating seed are infected with thecausal organism Macrophomina phaseolinashortly after the cotyledon emerges from theplanted seed. Charcoal rot will manifest itself ininfected plants if and when a condition such asdrought or poor irrigation management causesstress to plants. Thus, it is the disease that ispresently considered the most problematic.

Foliar fungicides can be applied to prevent severalprominent soybean diseases. Preventivefungicides (i.e. strobilurins [QoIs] such asazoxystrobin [Quadris] or pyraclostrobin[Headline]) are most effective when applied priorto or at the earliest appearance of a disease.

The general suggestion is that the first applicationshould be made at R3 or beginning of podset. Fungicide application during early reproductivedevelopment to prevent foliar diseases insoybeans has been proven over the past decade tobe an economical management practice in themidsouthern US.

Soybean rust can be managed with preventive andcurative (i.e. triazole [demethylation inhibitors(DMI)] such as flutriafol [Topguard] ortetraconazole [Domark]) applications of foliarfungicides timed according to occurrence of rustin sentinel plots. Based on the last five years’experience, soybean rust may be avoided in theMidsouth by planting early-maturing varietiesearly so that R6 or full seed stage is reachedbefore August 1. Additionally, the R3/R4fungicide application utilized in Midsouthproduction systems has likely provided someprevention of soybean rust in areas where the

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disease has occurred.

Scouting should be used to detect the firstoccurrence of disease(s) or to accuratelydetermine the reproductve stage recommended forthe most effective preventive fungicideapplication prior to disease presence. A May2016 Plant Management Network (PMN) webinarentitled “Integrated Approaches to FungicideApplications in Soybean” by Dr. Hillary L. Mehlof Virginia Tech Univ. provides coverage of thewhen’s and how’s of scouting for foliar diseases.

Cost and effectiveness of fungicide productsshould be evaluated when choosing options fordisease management. Resistant varieties shouldbe chosen based on level of pest tolerance andyield and grown in those areas with a knownhistory of a particular disease (e.g. frogeye leafspot [FLS]).

Information in Table 1 below provides a summaryof the important points for managing prominentdiseases. In the table:

• Click the varietal resistance heading to findthe most recent ratings of resistance publishedin midsouthern states’ variety trialpublications. Varietal resistance is the mostwidely used and effective management tool forsoybean diseases.

• Click the foliar fungicide heading to getrecommendations from the University ofArkansas. Also, a 2015 Universty of Missouripublication (p. 134-149) gives foliar fungicidesuggestions with a very detailed description ofapplication guidelines.

A recent survey indicated that losses to thediseases in Table 1 are significant in most years. However, some portion of the losses to these

diseases can be prevented every year if availablecontrols are used. Keep in mind that themanifestation of plant diseases will be mostdependent on the environment that is encounteredeach growing season as well as the over-winteringpotential for organisms such as the soybean rustfungus that has to blow into the Midsouth soybeanproduction area from more southern locationseach year.

To better assist in selecting foliar fungicides forcontrol of the above diseases, the North CentralRegional Committee on Soybean Diseases and theRegional Committee for Soybean Rust Pathologydeveloped information about foliar fungicideefficacy for control of major foliar soybeandiseases in the United States. Results from thatcompilation are in Table 2 below.

Specific considerations for soybean fungicidemanagement are:

• An R3/R4 strobilurin or strobilurin + triazolefungicide application is made at that timeregardless of the presence of disease. Itproduces best results when applied in apotentially high-yielding soybean crop (e.g.early planted, irrigated soybean followingsoybean).

• Applying a product that contains a stand-alonetriazole should be delayed until foliar diseaseis present. They should be relied on formanaging against yield loss as a result of FLSor soybean rust infestations.

• Fungicides in the strobilurin class are bestsuited for when diseases are not present; i.e.,used on a preventive basis. The residual effectin this case should be about 21 days.

• Even though triazole fungicides have the

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ability of being curative and can be applied tomanage a present disease, they perform bestwhen applied prior to the onset of visibledisease symptoms. Their residual effectgenerally lasts about 14 days.

• The systemic activity of both strobilurin andtriazole fungicides is limited to movementaround the area of the leaf where a spraydroplet is deposited. Fungicides in bothclasses should not be considered to movethroughout the plant from the point of entry.

• Growing varieties that are susceptible to FLS

may increase the likelihood of developing

fungicide-resistant FLS biotypes.

• If an FLS-tolerant/resistant variety is grown,

relying on a stand-alone strobilurin fungicide

is an acceptable practice to manage other

diseases or as an automatic fungicide

application.

• If an FLS-susceptible variety is grown and

FLS has been detected, applying a labeled

triazole fungicide could reduce yield loss.

• With the onset of strobilurin-resistant

FLS,triazoles should be considered to manage

the disease.

Dr. Heather Kelly with UT Extension postedScouting for Soybean Diseases and Deciding onFungicides on the utcrops.com website. In thisarticle, she provides a link to her Soybean Diseaseand Nematode Identification Guide that isavailable from UT Extension. This guidedescribes soybean disease symptoms (withaccompanying pictures) and management optionsfor those diseases. She also presents results shownin Table 3 below that are a summary from 2013-

2015 FLS (caused by Cercospora sojina) fieldtrials using FLS-susceptible varieties. It wasdetermined from these data that there is a strongcorrelation between percent FLS control and seedyield (R = 0.8923). Dr. Kelly has also designed a2

mobile-friendly version of the above guide as wellas an online interactive soybean diseasemanagement guide that includes videos to helpwith identifying symptoms of the various diseasesthat affect soybeans.

The Pioneer.com website contains a guide thatprovides detailed information about the majorsoybean diseases, including symptoms (narrativeand pictorial) and facts about each disease, eachdisease’s cycle, and practices that can be used tomanage each one.

PMN’s Soybean Fungicide Resistance Hub is acentral destination for up-to-date information onsoybean fungicide use and management practicesthat should be considered to ensure the prolongedeffectiveness of present and forthcomingfungicide products. The hub includes a “FeaturedWebcasts” section with open-access videos onfungicide resistance (FR) management, a“Fungicide Resistance Tracking” section withmaps of yearly distribution of FR plant diseases,and a “Fungicide Resistance Resources” sectionwhich contains information on FR management insoybean. The following posts are especiallynoteworthy.

Fungicide Classification is a poster that shows theFRAC code and mode of action of soybeanfungicides and fungicide premixes presented bythe Fungicide Resistance Action Committee(FRAC).

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Fungicide Resistance in the Cercospora LeafBlight and Purple Seed Stain Pathogen of Soybeanis a webcast presented by Dr. Trey Price with theLSU AgCenter. In his presentation, Dr. Pricediscusses the symptoms of the disease caused byCercospora kikuchii pathogen, and how thesymptoms are manifested in the foliar and matureseed stages. Severe infections in soybean canresult in defoliation (Cercospora leaf blight) andpoor seed quality (purple seed stain) that willresult in dockage at the elevator. He also showshow fungicide efficacy against this pathogen hasdeclined over the years to the point that thevarious classes of fungicides that are availablenow provide only limited efficacy against thepathogen, and no yield protection. Finally, hestates that control measures consist of choosingtolerant varieties based on ratings taken in fieldtrials (there are no resistant varieties) and earlyplanting.

Principles of Fungicide Resistance is authored byDr. Carl Bradley (Univ. Of Kentucky), Dr.Clayton Hollier (LSU AgCenter), and Dr. HeatherKelly (UT Extension). The authors definefungicide resistance, and describe how disease resistance to fungicides develops followed by thesubsequent loss of fungicide efficacy. They alsodiscuss the factors associated with fungalpathogens and fungicides that affect resistancedevelopment. They show the primary chemicalclasses of fungicides commonly applied tosoybean, and how the FRAC code can be used to distinguish these different fungicide classes aswell as determine the risk level of fungicideresistance developing to each of the fungicidegroups (Table 4 below). And finally, they presentmanagement practices that will prevent or delaydevelopment of fungicide resistance in order toretain fungicide efficacy over a long period oftime.

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Table 1. Major midsouthern soybean diseases and potential methods of prevention.

DiseaseVarietal

resistance Foliar

Fungicide* Additional information

Anthracnose No Yes Use seed treatment to reduce damping off

Soybean rust Yes Yes Resistant germplasm has been identified; however, there arepresently only two commercially-available soybean varieties

Cercospora leaf blight,purple seed stain

No Yes Use seed treatment to reduce early season damping off frominfested seed, and foliar fungicide for some late-seasonprevention; foliar fungicides provide only limited efficacyand no yield protection

Charcoal rot No No Prevent/reduce plant stress; some tolerant varieties may becommercially available

Frogeye leaf spot (FLS) Yes Yes Resistance to strobilurin fungicides has been identified; rotatefungicide chemistries and apply mixed mode of actionproducts to susceptible varieties; plant resistant varieties

Phytophthora root rot Yes No Use seed treatment (early season)

Phomopsis seed decay No Yes Use seed treatment to prevent early season seedling diseasesas a result of Phomopsis-infested seed

Pod and stem blight Yes Yes Fungicides, although labeled, may not be as effective

Pythium seed decay,damping off

No No Use seed treatment

Aerial blight No Yes Use less-susceptible varieties if available

Stem canker Yes No Varietal resistance is very effective

Sudden death syndrome Yes No Use less-susceptible varieties; monitor for the presence ofsoybean cyst nematode (SCN); use seed treatment

Target Spot Yes No Foliar fungicide use not economical

*Click for Univ. of Ark. list of foliar fungicides for control of indicated diseases in this table, and a table thatgives efficacy ratings for the various foliar fungicides against the shown diseases.

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Table 2. Fungicide efficacy for soybean disease management. NR = not recommended; NL = not labeled;P = poor; F = fair; G = good; VG = very good; E = excellent.

ProductRate/acre

(fl oz)Aerial web

blightAnthrac-

noseCercosporaleaf blight

Frogeyeleaf spot

Pod/stemblight

Soybeanrust PHI*

Strobilurins-QoI (Group 11)

Aftershock 480 SC 2.0-5.7 VG G ** P ** ** R5

Aproach 2.08 SC 6.0-12.0 VG G F P ** G 14 days

Evito 480 SC 2.0-5.7 VG G F P ** ** R5

Headline 2.09 EC/SC 6.0-12.0 VG VG F P ** G-VG 21 days

Quadris 2.08 SC 6.0-15.5 VG VG F P ** G-VG 21 days

Triazoles-DMI (Group 3)

Alto 100SL 2.75-5.5 ** ** F F ** VG 30 days

Domark 230 ME 4.0-5.0 NL VG F VG ** VG-E R5

Proline 480 SC 2.5-3.0 NL NL NL VG NL VG 21 days

Tilt 3.6 EC 2.0-4.0 P VG F VG ** VG R5

Topguard 1.04 SC 7.0-14.0 ** VG F VG ** E 21 days

Thiophanates-MBCs (Group 1)

Topsin-M 10.0-20.0 --- --- F VG --- G 21 days

Premixes (Mixed Mode of Action)

Avaris 1.66 SC 14.0-20.5 ** ** ** G ** VG 21 days

Evito T 3.99 F 4.0-6.0 ** F ** F ** ** 30 days

Priaxor 4.17 SC 4.0-8.0 E VG F VG ** E 21 days

Priaxor D (A + B)*** 4(A)+4(B) ** ** F VG ** ** 21 days

Quadris Top 2.72 SC 8.0-14.0 ** ** F VG ** VG 14 days

Quilt 1.66 SC 14.0-20.5 ** ** F G ** VG 21 days

Quilt Xcel 2.2 SE 10.5-21.0 E VG F VG ** VG R6

Stratego 250 EC 10.0 G-VG VG F VG ** VG 21 days

Stratego YLD 4.18 SC 4.0-4.65 VG VG F VG ** VG 21 days

*PHI = pre-harvest interval in days, or no later than shown R stage.**Insufficient data for efficacy statement.***Priaxor D is a combination product offered by BASF that includes: Component A = Priaxor and ComponentB = Domark. One case of Priaxor D contains a 2.5-gal. jug each of Priaxor and Domark.Labels for above fungicides can be found on the Crop Data Management Systems Labels/MSDS page or atAgrian’s Labels/MSDS page.

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Table 3. Results from 2013-2015 FLS field trials in Tenn. using FLS-susceptible varieties.

Fungicide Rate (fl oz/acre) % FLS Control* Yield (bu/acre)

Stratego YLD 4.65 55a 52.3a

Quadris Top SB 8 52a 51.5a

Aproach Prima 6.8 62a 51.3ab

Overrule or Topsin XTR 20 59a 51.1ab

Priaxor + Domark 4 + 4 62a 50.9ab

Topsin 20 45a 49.2ab

Topguard 7 47a 48.9ab

Priaxor 4 32b 48.3abc

Headline 6 22b 47.8abc

Bravo 6 23b 46.2bc

Non-treated --- 0 44.5c

P-value <0.0001 0.0337

Treatment values with the same letter are not significantly different.

All products were tested in 4-row-wide plots that were 30 ft long in randomized plots with 4 replicates in allyears at a total of 10 locations with the exception of Priaxor + Domark (tested 2 years at 6 locations), Priaxor(tested 2 years at 8 locations), and Stratego YLD (tested 2 years at 3 locations).

*Weighted by FLS severity in non-treated.

Table 4. FRAC determination of risk level of fungicide resistance development to commonly usedfungicide groups applied as foliar fungicides to soybean.

FRAC code Fungicide Group Risk of Resistance Development

1 Methyl benzimidazolecarbamates (MBC)

High

3 Dimethylation inhibitors(DMI, includes triazoles)

Medium

7 Succinate dehydrogenaseinhibitors (SDHI)

Medium to High

11 Quinone outside inhibitors(QoI, includes strobilurins)

High

M5 Chloronitriles Low

Composed by Larry G. Heatherly (MSPB - larryheatherly@bellsouth.net) and Tom Allen (MSU-DREC –TAllen@drec.msstate.edu), Revised/Updated Jan. 2017.

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