IMPORTANCEPythiumrootrot(FIGURE1)isthemost common disease found in tobacco foat beds in Kentucky and it can cause severe losses or delays in transplanting.Damage caused bythisdiseasecanbeminimizedthrough acombinationofsoundmanagement practices and timely application of fungicide.SYMPTOMSThe frst symptoms of Pythium root rot tend to occur on the roots that extend from trays into the foat water (water roots).Water roots affected by Pythium root rot show a light-to-dark brown necrosis and slough away easily whentouched.Whenatransplanttray containing Pythium-infected plants is turned onitsside,necroticrootsoftenadhere tothetraysbottom(FIGURE2).Infected rootswilleventuallyfallawayandsome re-growthmaybeobserved;however,new growthwilllikelybecomeinfected.Asthe diseaseprogresses,yellowingandstunting oftransplantsiscommonandtendstobe confned to well-defned areas of a foat bay (FIGURE 1).During the outbreak, seedlings wilt and decay can progress into plug roots (FIGURE3)andsometimeslowerstemsof plants. Damping-off, or seedling death, can occur in severe cases (FIGURE 4).CAUSE AND DISEASE DEVELOPMENTThereareseveralspeciesofPythium,a fungus-like organism, that cause root rots on tobacco seedlings.Pythium species (spp.) require water, abundant in the foat system, forreproductionandmovement.Initial infectionslikelyresultfromgerminationof Plant Pathology Fact SheetPythium Root Rot in Tobacco Float SystemsKenneth W. SeeboldExtension Plant PathologistFIGURE1.PLANTSAFFECTEDCHRONICALLYBYPYTHIUM ROOTROTARESTUNTEDANDYELLOWED;MANYTIMES THEY ARE ISOLATED IN WELL-DEFINED AREAS IN FLOAT BEDS. University of Kentucky College of Agriculture Plant Pathology ExtensionPPFS-AG-T-01resting structures (oospores) of Pythium spp., alongwiththeproductionofzoosporangia. Swimming spores (zoospores) are liberated fromzoosporangiaandfndtheirwayto tobaccoroots.Zoosporesencystafter encounteringsusceptibletissueandenter therootsystemtoestablishaninfection. Manycyclesofzoosporeproductionand infectionarepossibleafterinitialinfections occur.Watertemperaturesgreaterthan 72 Ffavorrapiddevelopmentandspread of Pythium root rot in foat systems.ThemostcommonwaysforPythiumspp. tobeintroducedintofoatsystemsare contaminatedwater,infestedsoil,and recycled(andcontaminated)Styrofoam trays.Pythiumspp.arefoundwidelyin oursoilsandsurfacewater,soanything that moves these sources can be a means ofcontaminatingfoatbeds.Pythiumspp. can persist in the tissue of roots that have penetrated Styrofoam foat trays, providing a source of inoculum when the trays are used the following season.DISEASE MANAGEMENTQualitytobaccotransplantsareoneofthe most important parts of a successful growing season.Throughcarefulmanagementit ispossibletoachieveexcellentcontrolof Pythium root rot, as well as good transplant quality and a healthy bottom line!Sanitationisanimportantpartinthe management of Pythium root rot in the foat system.Use new or properly sanitized trays topreventcarryoverofPythiumfromone seasontothenext.Trayscanbesteam- sterilizedbyheatingthemtobetween 165 F and 175 F for at least 30 minutes, orbyimmersingtraysinasolutionof1 parthouseholdbleachto9partswaterfor several minutes.Alternatively, trays can be sprayedwiththebleachsolution,covered overnight, and then rinsed to remove bleach residues.Whether steam or bleach is used tosanitizetrays,trytoreplacetraysevery 3 to 4 years.Trays that have reached this FIGURE 4.LEFT UNCONTROLLED, PYTHIUM ROOT ROT CAN CAUSESEVERESTUNTINGORDAMPING-OFFOFTOBACCO SEEDLINGS.FIGURE2.WHENINFECTEDBYPYTHIUMSPP.,TOBACCO ROOTS THAT EXTEND INTO THE FLOAT WATER ARE GENERALLY NECROTIC AND LIMP, OFTEN ADHERING TO THE BOTTOMS OF FLOAT TRAYS.FIGURE 3.IN ADVANCED STAGES OF PYTHIUM ROOT ROT, NECROSIS EXTENDS WELL INTO PLUG ROOTS.FIGURE 2FIGURE 3agetendtoabsorbexcesswater,resulting inpoorplantgrowthandgreaterpotential for diseases in general.Never use pond or surfacewatertofllfoatbeds,sincewater fromthesesourcesislikelycontaminated withPythiumandotherplantpathogens suchasPhytophthoraorFusarium.Make sure that shoes and tools are cleaned before bringing them into a transplant facility.A fungicide, Terramaster 4EC, is labeled for use in foat systems and is very effective against Pythium root rot when used correctly. Detailed information on this fungicide can be foundontheproductlabel.Youcanalso refer to the following publications: Kentucky-TennesseeTobaccoProductionGuide(ID-160) and Fungicide Guide for Burley and Dark Tobacco(PPFS-AG-T-08).Forpreventive use, apply 0.7-1 fuid ounces of product per 100gallonsoffoatwaterbeginning2to3 weeksafterseeding,orwhenrootsfrst enterthewater.Sequentialapplicationsof 0.7 to 1 fuid ounce per 100 gallons of water can be made at least 3 weeks apart up to 5 days before transplanting; do not exceed 3.8 fuidouncesper100gallonsoffoatwater per season.When using Terramaster, make sure that the product is mixed thoroughly in foat bays to minimize the risk of plant injury (FIGURE 5).RescueapplicationsofTerramaster(1.4 fuidouncesper100gallonsoffoatwater) insystemswithactivePythiumrootrotwill haltfurtherdevelopmentofdiseaseand symptomaticseedlingswilllikelyrecover. However,thehigherratesofTerramaster used in rescue treatments increase the risk of plant injury AND recuperating plants may stillharborPythium.Itmayalsoincrease plantsusceptibilitytoblackshankand Fusarium wilt.For these reasons, preventive useofTerramasterisrecommendedover curative applications of the product.Before using Terramaster, or any pesticide, refer to the label for specifc instructions and safety information.PleasenotethatTerramaster istheonlyproductlabeledforcontrolling Pythiumrootrotontobaccoseedlings growninfoatbeds.Otherproducts,such as Banrot, Rovral, Iprodione 50W, or Ridomil Gold SL, are not approved for this use.ADDITIONAL RESOURCESThefollowingUniversityofKentucky publicationsareavailableatCounty Extension offces, as well as on the Internet. Fungicide Guide for Burley and Dark Tobacco, PPFS-AG-T-08http://www.ca.uky.edu/agcollege/plantpathology/ext_fles/PPFShtml/ppfsagt8.pdf Kentucky-Tennessee Tobacco Production Guide, ID-160 (1.6 MB fle)http://www.ca.uky.edu/agc/pubs/id/id160/id160.pdfFIGURE5.TYPICALSYMPTOMSOFINJURYCAUSEDBY ETRIDIAZOLE(TERRAMASTER).THISTYPEOFINJURYIS COMMON WHEN RECOMMENDED DOSES ARE EXCEEDED, OR IF THE FUNGICIDE IS MIXED OR APPLIED IMPROPERLY. Educational programs of the Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability, or national origin. Issued June 2011Photos by Kenneth Seebold