PM 914 Revised July 2004

Figure 2. Damping off caused byPhytophthora sojae and Pythium spp.

Phytophthora Root andStem Rot of SoybeanPhytophthora root rot of soybeanwas first identified in Indiana in1948 and has spread throughoutsoybean-growing regions of theUnited States and Canada. Thedisease is a widespread problem inIowa, occurring in most areaswhere soybeans are grown.

CauseThe fungus Phytophthora sojaesurvives from season to season asa resting structure called anoospore or as threadlike structurescalled mycelia. The fungussurvives in the soil or on plantdebris associated with theprevious crop. The oospores cansurvive in soil almost indefinitely.Oospores will germinate andproduce hundreds of small,mobile spores called zoosporesthat travel through the water-filledsoil pores and infect soybeanroots. Spores contained in soilparticles deposited on leafsurfaces by the splashing ofraindrops also can infect plants.

SymptomsThe disease can affect soybeansfrom the seedling stage to nearmaturity. Stand reduction occurswhen the disease infects theplants at the seedling stage andcauses seed rot (fig.1) anddamping off (fig. 2). Infection ofolder plants causes interveinalyellowing of leaves and eventualwilting and death. The

leaves turn gray or brown andremain attached to the plant.Infected roots become discolored,rotten, and eventually die. Plantsalso become stunted. Large,brown lesions, advancing from thesoil line up the stem of the plant,

Figure 1. Soybeanseed rot caused byPhytophthora sojae.

may develop (fig. 3). The lesionswill eventually girdle the stem andkill the plant.

Phytophthora root rot can strikeearly in the season and causedamping-off. Stand reductions will

occur, but may be mistakenlyattributed to environmentalconditions or herbicide damage.Herbicide damage will beuniformly distributed in the fieldor follow planting and spraypatterns, whereas Phytophthora-infected plants often occur incircular patches in low spots orscattered throughout the field(Fig. 4).

Environmental ConditionsPhytophthora root rot is prevalentin heavy clay soils or in soils withpoor drainage during wetweather. In seasons where heavyrainfall occurs early and in poorlydrained soils, the disease canbecome severe. Clay containssmall soil pores that quicklybecome filled with water and donot drain well. Periodic rainfallpatterns, such as a week of wetweather followed by dry weather,are ideal for a severe diseaseoutbreak. Warm soil temperaturesof 70 to 77o F are optimum forinfection.

ResistanceResistance is the most effectivetool for disease control. P sojaehas different races that can becontrolled by soybean withspecific resistance genes. Over 70races of Phytophthora have beenidentified in Ohio soils. Studieshave shown that races of P. sojaevary geographically. It is veryimportant to select cultivars withresistance genes that are effectivefor the races present in yourfields. The predominant races in afield also can shift after aresistance gene is used for anumber of years. Although manyraces exist, only a few races are ofmajor economic importance inIowa.

Race 1 was first found in a diseasesurvey in Iowa in 1966. Research-ers at Iowa State Universitysurveyed Phytophthora root rotraces in Iowa from 1991-1994 and2001-2002 by isolatingPhytophthora from either soilsamples or infected soybeanplants. The results of both surveysare given in Table 1. In the 1991-1994 surveys, races 1 (26percent), 3 (39 percent), and 4 (17percent) predominated thepopulation. Other races detectedincluded 2, 8, 13, 15, and 25. Apopulation shift of P. sojae in Iowawas detected in the 2001-2002survey. Three new races wereidentified – races 20, 28, and 35.In addition, the frequencyincreased when race 25 wasdetected. Races 2, 3, 4, 8, and 13were not detected in infectedsoybean plants,although, withthe exception ofraces 8 and 15,all were detectedin soil samples.

The races foundin Iowa canattack severalcommonly usedresistant genes.The gene Rps1k isvery widely usedbecause it is resistant to mostraces found in Iowa (Table 2).However, the 2001-2002 surveyrevealed that races that arevirulent on Rps1k, namely races20, 25, 28, and 35, are becomingincreasingly important. This maybe due to selection pressure byrace-specific resistant varieties.Phytophthora races that are able toovercome Rps1k resistance werefound in Buchanan, Dickinson,Fremont, Greene, Jasper, Johnson,

Figure 3. A soybean plan infected by

Phytophthora sojae. A dark brown lesion

forms on the stem from the soil line

upward. Courtesy of APS Press.

Figure 4. Circle pattern of diseased soybean plants in a

field infested with Phytophthora sojae.

Lee, Lucas, Madison, Marion,Marshall, Sioux, Ringgold andWayne counties (fig. 5).Many soybean varieties grown inIowa use resistance genes Rps1a,Rps1b, Rps1c, or Rps1k. A fewvarieties include Rps6, which isresistant to races 1, 3, 4, 25, 28,and 35. Growers should usevarieties that include Rps1k if theraces in a particular field areunknown.

Table 2. Resistance gene reaction to races of Phytophthora sojae

Resistance gene

Race Rps1a Rps1b Rps1c Rps1d Rps1k Rps3a Rps6 Rps7

1 R R R R R R R s 2 R s R R R R R s 3 s R R R R R R s 4 s R s R R R R s 8 s R R s R R s s13 R R R R R R s s15 R R R R R s R s20 s s s R s s R s25 s s s R s R R s28 s s R R s R R s35 s s s s s R R s

s=susceptible, R=resistant

Table 1. Phytophthora sojae races found in Iowa from 1991-1994 and 2001-2002

Race 1991-1994 2001-2002

1 26 15.4 2 <2 3.8s

3 39 11.5s

4 17 3.8s

8 <2 013 <2p 5.8s

15 <2p 020 - 3.8p

25 2 11.528 - 7.735 - 9.6Unknown 14 17.3

p Only detected in infected soybean plants; s only detected in soil samples

An alternative method of diseasecontrol by soybean cultivars istolerance. Major resistancegenes,such as Rpslk, can beovercome by mutations in thefungus. Varieties that are said tobe tolerant to Phytophthoracontain factors that limit but donot halt infection and diseaseprogress. Tolerant varieties willallow root rot to progress to thebranch roots but the taproot willbe unaffected. Highly tolerantvarieties will yield comparably toresistant varieties despite infection

by Phytophthora. Tolerance scoresfor many varieties grown in Iowaare listed in the Iowa CropPerformance Test-Soybeans (AG18.) published by Iowa StateUniversity Extension and theIowa Crop ImprovementAssociation.

TillageResearch shows that no-tillpractices increase the risk ofPhytophthora, especially disease,for two reasons. First, the

pathogen survives better in no-tillthan in conventionally tilledground. In conventional tillage, thefungi are buried in deep soil. In no-till systems, plant residues are lefton the soil surface; as a result, thefungus is distributed primarily nearthe soil surface and has a greaterchance of contact with seedlings.Second, soil compaction in no-tillresults in poor drainage, which isfavorable to disease development.

. . . and justice for allThe U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, nationalorigin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to allprograms.) Many materials can be made available in alternative formats for ADA clients. To file a complaint of discrimination, write USDA,Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department ofAgriculture. Stanley R. Johnson, director, Cooperative Extension Service, Iowa State University of Science and Technology, Ames, Iowa.

Figure 5. Iowa counties (in green) where resistance gene reaction to races ofPhytophthora sojae has been discovered.

To reduce disease in no-till, it ishelpful to reduce compaction andimprove drainage.Chemical controlChemical seed treatments arerecommended in fields with a highrisk of Phytophthora. Thefungicides chloroneb, fludioxonil,mefenoxam, and metalaxyl are

Revised by Alison Robertson, assistant professor, and X.B. Yang, professor, Department of Plant Pathology

File: Pest Management 2-5

very effective for the control ofPhytophthora. Chloroneb,fludioxonil, and metalaxyl can beused as a seed treatment (tradenames: Apron XL LS, ApronMAXX RTA, Apron MAXX RTA +Moly, Maxim XL, and WardenRTA) to control damping-off.Metalaxyl can also be used as a

furrow treatment (trade name:Ridomil Gold EC) to reduce rootrot.