Phytophthora diseases of soybean, present wherever soybeans are grown, is outlined in this NebFact

Christensen, Jane A.

Giesler, Loren J.

Zwiener, Christopher M.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Historical Materials from University of Nebraska-Lincoln Extension Extension 2002 NF518 Management of Phytophthora Diseases of Soybeans Loren J. Giesler University of Nebraska-Lincoln, lgiesler1@unl.edu Jane A. Christensen Christopher M. Zwiener Follow this and additional works at: https://digitalcommons.unl.edu/extensionhist  Part of the Agriculture Commons, and the Curriculum and Instruction Commons Giesler, Loren J.; Christensen, Jane A.; and Zwiener, Christopher M., "NF518 Management of Phytophthora Diseases of Soybeans" (2002). Historical Materials from University of Nebraska-Lincoln Extension. 94. https://digitalcommons.unl.edu/extensionhist/94 This Article is brought to you for free and open access by the Extension at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Historical Materials from University of Nebraska-Lincoln Extension by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. C-10NF518Management of Phytophthora Diseasesof SoybeansLoren J. Giesler, Extension Plant PathologistJane A. Christensen, Research Technologist, Plant PathologyChristopher M. Zwiener, Student Assistant, Plant PathologyPhytophthora diseases of soybean, caused byPhytophthora sojae, are present wherever soybeans aregrown in Nebraska. The pathogen survives primarily as “resting” spores in the soil or in association withinfested crop debris. Symptoms associated withPhytophthora sojae infections include seed rots, pre-and post-emergence damping off of seedlings and stemrot of plants at various growth stages. Disease develop-ment is most rapid at soil temperatures above 60oFwith high soil moisture. This disease is most commonin low-lying areas of a field, in poorly drained or com-pacted soils, and in soils with high clay content. It alsomay occur on well-drained hillsides during growingseasons with excessive moisture availability. Currently55 races of the pathogen have been identified. In 2000,Phytophthora sojae race surveys for soybean wereinitiated with funding from the Nebraska Soybean Board.As a result of ongoing survey efforts, races 1, 3, 4, 25,and 28 have been recovered from soybean fields inNebraska. Previous surveys were conducted in easternNebraska in 1980 and 1981 and resulted in recovery ofraces 1, 3, 9, 14, 18, and 23.Once P. sojae is established in a field, it cannot beeradicated. The disease must be “managed” with the useof seed treatment fungicides and resistant varieties.Knowledge of the races present in the state and howvarieties with different resistance genes have performedin a grower’s field is critical to variety selection formaximum profitability.Seed TreatmentSeed treatment fungicides can be used for manage-ment of early season seed rot and damping off causedby Phytophthora sojae. The compounds labeled forcontrol are metalaxyl and mefenoxam. Mefenoxam isone of the chemical compounds which has been isolatedfrom metalaxyl. A list of products containing these com-pounds and their recommended rate of application isprovided in Table I. In general, all varieties of soybeangrown in problem fields should be treated since condi-tions favoring Phytophthora sojae also favor Pythiumspp., which commonly cause seedling problems inNebraska.Financial resources for the Nebraska SoybeanDisease Surveys cited in this publication wereprovided by the Nebraska Soybean Board.File under: PLANT DISEASESC-10, Field CropsIssued April 2002Extension is a Division of the Institute of Agriculture and Natural Resources at the University of Nebraska–Lincoln cooperating with the Countiesand the United States Department of Agriculture.University of Nebraska–Lincoln Extension educational programs abide with the nondiscrimination policies of the University of Nebraska–Lincolnand the United States Department of Agriculture.© 2005, The Board of Regents of the University of Nebraska on behalf of the University of Nebraska–Lincoln Extension.  All rights reserved.Table I. Seed treatment fungicides1 labeled for Phythophthora sojae control on soybean.2Application rate forPhytophthoraProduct Name Common Name control ManufacturerAllegiance LS Metalaxyl 1.2-2.4 fl. oz./100 lbs. seed GustafsonAllegiance FL Metalaxyl 0.75-1.5 fl. oz./100 lbs. seed GustafsonAllegiance Dry Metalaxyl 1.5-2.0 oz/100 lbs. seed Trace Chemicals LLCApron FL Metalaxyl 0.75-1.5 fl. oz./100 lbs seed GustafsonApron Flowable Mefenoxam 0.82-1.64 fl. oz./100 lbs. seed Wilber-EllisApron Maxx RTA3 Mefenoxam and Fludioxonil 5.0 fl. oz./100 lbs. seed SyngentaApron Maxx RTA+ Moly3 Mefenoxam and Fludioxonil 5.0 fl. oz./100 lbs. seed SyngentaApron TL Metalaxyl 2.0-4.0 fl. oz./100 lbs. seed Wilber-EllisApron XL LS Metalaxyl 0.16-0.64 fl. oz./100 lbs. seed SyngentaSystem 3 Pentachloronitrobenzene 4-6 oz/bu Helena(PCNB), Metalaxyl, andBacillus subtilits GBO3Warden RTA Mefenoxam and Fludioxonil 5.0 fl. oz./100 lbs. seed Agriliance LLC1Product list is intended for information purposes only. No criticism is intended for products not listed nor endorsement for products listed. Always readand follow label directions when applying any pesticide.2Application rates on the high end of the labeled amount are generally necessary for adequate Phytophthora sojae control.3In fields with a history of high phytophthora pressure, the label recommends using Apron Maxx RTA or Apron Maxx RTA + Moly at the recommendedrate of 5.0 fl. oz./100 lbs. seed in combination with Apron XL LS at a rate of 0.16-0.48 fl. oz./100 lbs. seed.Genetic ResistanceUse of resistant varieties is the most effective means ofmanaging phytophthora diseases in soybean. Genetic resis-tance in the host is expressed in terms of Rps (Resistance toPhytophthora sojae) genes. The genes are denoted as Rps 1a,1b, 1c, 1d, 1k, 3, 6, and 7. The pathogen exists in races thatinteract with these genes. A race is identified by its interactionwith the eight known Rps genes. In a resistant reaction, theplant survives when infection occurs; susceptible varieties arekilled when infection occurs (Table II).Table II. A list of the races of Phytophthora sojae identifiedin Nebraska over the last 20 years and the Rps genereaction to these races.Race Rps Resistance Genes1-a 1-b 1-c 1-d 1-k 3 6 7  1 R R R R R R R s  3 s R R R R R R s  4 s R s R R R R s  9 s R R R R R s s14 R R s R R R R s18 R R s R R R R R23 s s R R R R s s25 s s s R s R R s28 s s R R s R R ss = susceptible or plant killed R = resistant or plant not killedSoybean varieties are marketed on the basis of theirgenetic make-up in relation to the predominate Phytoph-thora sojae races in a given area. The predominant races inNebraska are 1 and 3. The most widely available resistancegenes in the Midwest are 1-c and 1-k, commonly referredto as c or k in seed company literature. Gene 1-c protects againstraces 1, 3, 9, 23, and 28, where gene 1-k protects against races1, 3, 4, 9, 14, 18, and 23. Gene 3 is the onlygene that protects against all known races that occur inNebraska. Currently 40 percent of the maturity group 2 and63 percent of the group 3 soybean varieties marketed inNebraska contain some resistance to Phytophthora sojae.Resistance within these maturity groups is listed below.Maturity Percent Nebraska Marketed VarietiesGroup With Rps Gene Resistance1-a 1-b 1-c 1-d 1-k 3 6 7 1k/1aGroup 2 5 0 20 0 72 1.5 0 0 1.5Group 3 9 0 34 0 54 3 0 0 0Growers should consult local seed company represen-tatives to request varieties with different Rps genes thanmarketed in a specific area.

NF518 Management of Phytophthora Diseases of Soybeans

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NF518 Management of Phytophthora Diseases of Soybeans

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