Plant Pathology Fact Sheet PP-53

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

DISEASES OF AGRONOMIC ANDVEGETABLE CROPS CAUSED BY PYTHIUMTom Kucharek, Professor and Extension Plant Pathologist; Dave Mitchell, Pro-fessor, Plant Pathology Department, University of Florida, Gainesville FL 32611.February, 2000

THE ORGANISMS

Pythium is a generic term used herein to representnumerous species of fungal-like organisms within theGenus Pythium. Pythium spp. occur throughout theworld and are common in association with roots of plantsand other plant parts in contact with or in the proximityof soil. When environmental conditions are favorablefor these organisms and adverse for the plant, infectionof plants may occur. Infections typically occur in roots,lower stems, fruit near or in contact with the soil, andsoft plant tissues which exist naturally in seedlings of plantsprior to or shortly after emergence from the soil. Pythiumspp. that are not parasitic in plants have caused diseasesin fish, crustacea, and even other Pythium spp.

Over time, taxonomists (scientists who study sys-tematics, the system of classification) have changed theclassification position of Pythium spp. from being withinthe kingdom Planta to being within the kingdomChromista. The newest classification schemes placePythium spp. as being more closely related to diatomsand brown or golden algae than fungi. In the newestclassification systems, based upon single evolutionarylines and unique morphological and biochemical char-acteristics, most of the other fungi, such as Fusariumspp. or Rhizoctonia spp. that were formerly in the plantkingdom, are now placed in the kingdom Eumycota(named Mycota or Fungi by some authorities). A fewof the fungi, such as Plasmodiophora spp., are in thekingdom Protozoa. All of these organisms, however,may still by referred to by the common or colloquialterm fungi because they are all spore-bearing eukary-

otes, without chlorophyll, that typically have a walled,filamentous or plasmodial thallus and the ability to ab-sorb their food. Thus, although different fungi are clas-sified in three biological kingdoms, they all retain the com-mon name (fungus; fungi, pl.) based upon common lifeforms and functions. The reader should realize that theseacademic developments have not changed the life cyclesof Pythium spp. nor have they effectually influencedour approaches used for control and management of thediseases caused by Pythium spp. This information pro-vides the reader with some of the dynamic turns andtwists that occur in science.

The most common species of Pythium that causeimportant plant diseases in Florida are Pythiummyriotylum and P. aphanidermatum. Other speciesof Pythium that are sometimes associated with dysfunc-tional plants in Florida are P. splendens, P. irregulare,and P. vexans. It is generally considered that P.myriotylum and P. aphanidermatum abound in Floridabecause they are adapted to high soil temperature.Growth of these later two species can occur from 40 or50° F to 105o F, but the optimum temperatures for theirgrowth and infection of plants range between 86 and98o F. In other locations in the world, other species ofPythium cause plant diseases. For example, in the north-ern United States, P. ultimum prevails in some situa-tions. Pythium ultimum prevails in cooler to cold soils;it can grow across as wide a range of temperatures asthe other two, but its optimum for growth is between 77and 86o F. Interestingly, the optimum for infection of planttissue by P. ultimum is slightly lower (74o F or slightlybelow). Typically, Pythium spp. are composed of mi-croscopic, slender, round tubes (strands) called hyphae

(Fig.1). A mass of these hyphae is called a mycelium(mycelia, pl.) and is white. Sometimes large mycelial colo-nies can be seen growing on infected tissue (Figs. 18,19, & 20). A colony of mycelium can grow more thanan inch per day when moist and warm conditions pre-vail.

Multiplication of Pythium spp. is done by growth ofthe hyphae and production of microscopic spores. Fourtypes of spores are produced, sporangia, zoospores,oospores and chlamydospores. Chlamydospores areformed as thickened segments of hyphae, but they arenot considered important for Pythium spp. that causeplant diseases in Florida. Sporangia can germinate toform new hyphae, but typically the role of sporangiawithin the disease cycle is to produce vesicles in whichzoospores are formed.

Zoospores are fragile, non-sexually produced sporeswithout cell walls and they have two flagella (tails) thatpropel the organism in water. Zoospores are producedin microscopic vesicles (bulb-like, unwalled structures)that are attached to sporangia (also a bulb-like or lobe-like structures, with cell walls ) which are in turn formedon stalks or formed as enlarged hyphae (Fig 1). Wateris necessary for production of sporangia, vesicles, andzoospores, and thus, Pythium spp. have been referredto commonly as belonging to the Awater molds.@ Con-sequently, standing water in fields, ditches, ponds, lakes,and hydroponic systems are places that Pythium spp.can exist, multiply to great numbers and spread. To il-lustrate the small size of zoospores, it would take about2500 zoospores in a line to equal one inch. Zoosporescan swim for 20 to 30 hours and move three or moreinches through soil. Without free water they die rapidlyor sometimes they can encyst by forming a thick walland may survive up to 7 days in soil. Zoospores swimagainst gravity and after reaching the surface of floodedsoil, may be carried to new roots if the water is moving.The zoospores are then attracted by chemicals to roottips where infection typically occurs. Natural or artifi-cial movement of water infested with Pythium spp. pro-vides a method by which rapid spread of these plantpathogens can occur over greater distances.

An oospore (Fig. 2) forms after mating by male andfemale portions of the thallus (the entire mass of the or-

ganism). Most species of Pythium are hermaphroditic(homothallic) and thus are self fertile. Because oosporesare the result of sexual mating, the organism has a mecha-nism for recombining genes to enhance variation withinthe species. Also, oospores provide an effective methodfor survival of Pythium spp. because oospores typi-cally have thickened walls. Oospores survive in old cropdebris such as undecomposed roots and stems that wereinfected earlier and they survive in soil or old containersor on implements that contain infected plant tissue orinfested soil. Oospores are extremely durable and havesurvived for more than 10 years. Pythium spp. havesurvived passage through the intestinal tracts or earth-worms, snails, and birds.

The primary habitat for Pythium spp. is in the soil orprevious crop debris, such as roots and stems. Pythiumspp. can be found typically in the upper 12 inches ofsoil, but they have been found as deep as 12 feet.Pythium spp. can also exist in old plant debris in cullpiles or areas where infected plants are discarded. Thesefungi commonly exist in ponds, lakes, or other sourcesof standing water. Soil and these bodies of water arethe most common sources of inocula (spores or hyphalfragments) for pathogenic species. Pythium spp. arenot seedborne, but they can reside in clonally propa-gated stock such as seed pieces of potatoes, sweet po-tatoes, sugarcane or transplants of strawberry. Trans-plants of any crop can carry Pythium spp. in colonizedtissue or by infestation of associated soil and transplanttrays.

Most infections from Pythium spp. occur by thepenetration of the root (particularly root tips) or othersusceptible tissue from germ tubes that arise from thezoospores. Because zoospores can be produced inextremely high numbers, visible disease appears to oc-cur Aalmost overnight.@ In fact, one generation (i.e.zoospore to zoospore) can occur in 18 hours or less.Hyphal infections can occur also, but they are less effi-cient and they would require significantly more time toattain an equivalent amount of disease to that resultingfrom zoospores. When these organisms colonize andthen penetrate plant tissue, hyphae absorb nourishmentfrom their living or non-living substrate. Colonization ofplant tissue by P. aphanidermatum is assisted by theability of this organism to produce pectolytic or cellulytic

enzymes that breakdown tissue. Pythium myriotylumalso may produce toxins that assist in breakdown of thehost tisuue. Often, infections from Pythium spp. have agreasy or water-soaked appearance in the plant tissuebecause of leakage of the moist cellular contents fromplant cells that occurs from the enzymatic activity men-tioned earlier. Because the enzymatic activity fromPythium spp. Abreaks down@ the soft (parenchymous)tissue, it is common to see the outer portion of dysfunc-tional roots rotted and the harder vascular tissue in thecenter of the root remains intact (Fig. 3 ). This can bedetected in the field by trying to slip the outer portion ofthe root off of the central vascular tissue (stele). Con-trast this effect with a healthy appearing root seen mi-croscopically in Fig. 4.

Infection is promoted as the population of a Pythiumsp. increases. Factors that influence the population ofPythium spp. in the soil include soil water content, soilatmosphere (O2, CO 2, and volatiles), soil temperature,soil pH, nutrient status of soil, usage of certain pesti-cides in the soil, and other microorganisms. For ex-ample, microflora in the soil, such as the fungi Tricho-derma spp. and actinomycotic organisms such as Acti-nomyces spp., can interfere with the life cycles ofPythium spp. Roots or plant parts that are infectedwith Pythium spp.may be infected with other parasiticorganisms. For example, infection with other organisms,such as Rhizoctonia solani or Fusarium oxyspsorumand nematodes, such as root knot nematode or stingnematode, can occur in the same tissue as that infectedwith Pythium spp. Conversely, some bacteria or fungi,such as the fungus Trichoderma, may colonize rootsalong with Pythium spp. and reduce damage.

The host range for Pythium spp. is extremely large.Broadleaf and grass plants are susceptible to multiplespecies of Pythium. For example, P. myriotylum andP. aphanidermatum are both capable of infecting broa-dleaf plants such as tomatoes, cucurbits, peanuts, orornamentals, as well as grasses such as wheat, corn, orturfgrasses. The crop species that have been commonlyinfected with Pythium spp. in Florida include foliageornamentals, turfgrasses, Bermudagrass used in pastures,peppers, tomatoes, beans, soybeans, southern peas, andsmall grains (e.g. wheat, oats, rye). Solanaceous weeds,Crotalaria spp., many weedy grasses, and other weeds

may be hosts of Pythium spp. and serve as importantsources of inocula.

With the anticipated loss of methyl bromide as a soilfumigant in 2005, it is likely that crops that are now com-monly grown on methyl bromide/chloropicrin-fumigatedfield soil, such as tomatoes, peppers, strawberries, andmany others, will incur greater incidences of visible dis-ease from Pythium spp. Infection of roots by Pythiumspp. has occurred to some degree in crops that havebeen grown in methyl bromide/chloropicrin-fumigatedsoil or treated with some other chemical, but the level ofinfection has generally been at a lower level and thus,symptoms have been suppressed. Chloropicrin, whichis commonly formulated with methyl bromide, tends tobe more fungicidal than methyl bromide but the combi-nation of methyl bromide plus chloropicrin has had asynergistic effect on soilborne organisms such as Pythiumspp., Rhizoctonia spp., Fusarium spp. and many oth-ers.

DIAGNOSIS AND SYMPTOMS

For accurate diagnoses of plants suspected of hav-ing Pythium-induced dysfunctions, it would be best tosubmit quality samples to a reputable diagnostic labora-tory. With continued experience, you should be able tolearn which situations will require submission of samples.You should realize that discolored roots or tissue arenot always associated with parasitic organisms. Excessfertilizer, flooded soils, insect feeding, and nematodefeeding are just a few of the other causes of dysfunc-tional roots. Generally, all dysfunctions where root rotis a portion of the problem should have a laboratoryassessment to determine if Pythium spp. are involvedby themselves, or in conjunction with infections fromother organisms such as Rhizoctonia spp., Fusariumspp., Macrophomina spp. or a myriad of other possi-bilities. Diagnosticians use microscopic techniques andaseptic culturing to assess for the presence of Pythiumspp. Some serological kits are available for use in thefield but the user should realize that because of the di-versity of Pythium spp. in the soil and in plant tissue,and because of the likelihood of mixed infections withother organisms, serological techniques for Pythiumalone are not likely to provide a complete diagnosis un-less a complete array of test kits are used for the one

situation at hand.

Pythium spp. are often associated with seedlingblights for many crops. Both grass crops and broadleafcrops incur this type of disease. Seedlings can be in-fected prior to (pre emergent damping off) or after (postemergent damping off) emergence from the soil. Forthe spinach and soybeans pictured herein (Figs. 5 & 6),the seedling blights were both pre emergent and postemergent for both crops and the damage was the resultof infection by both Pythium spp. and Rhizoctoniasolani. It is risky to decide what pathogens are involvedwith seedling blights, root rots, and lower stem rots with-out assessments in the laboratory, but one of the char-acteristics of tissue infected with Pythium spp. is thepresence of water-soaked or greasy appearing tissue.In Figure 6, the water soaked tissue of soybean seed-lings is caused by the infection with a Pythium sp. andthe distinct, orange to red to dark, sunken lesions werecaused by Rhizoctonia solani.

Cultures of a Pythium sp. were isolated from thewater-soaked, lower stem tissue (Fig. 7) of these hy-droponically grown tomatoes. The symptoms in theleaves include dried and necrotic (brown) areas that beginnear the tips or margins of the leaflets. When roots orlower stems are infected with Pythium spp. or otherpathogens, symptoms in the upper parts of plants areexpressed indirectly because the effect from root andstem rots is to partially or totally inhibit the movement ofwater and nutrients from the soil to the upper portions ofthe plants. In this situation with tomatoes, the leavesturned brown, but as shall be seen later, chlorosis (yel-lowing) of leaves is another common indirect symptom.

In small plants planted thickly, such as in transplanttrays in the greenhouse, Pythium spp. can infect andcolonize the plants with the result that the entire plant isdestroyed (Fig. 8). Note the water-soaked tissue in thissituation with lettuce (Fig. 8). It is common to see white,mycelial growth associated with dead or dysfunctionalplants that are infected with these pathogens. Mycelialgrowth was present in this situation, but a hand lens wasnecessary to see the hyphal strands.

Another major symptom in plants that possess rootand lower stem rot caused by Pythium spp. is yellow-

ing of leaves. Typically, the yellowing begins on thelower leaves first, but cases where yellowing begins onupper portions of the plant are not rare. In some situa-tions, the entire plant will become yellow. This leaf yel-lowing occurs in both broadleaf plants and grasses (Figs.9, 10, 12, 13, and 14). Lower leaf yellowing can alsobe caused by a deficiency of nitrogen.

The level of infection can be so severe that plants die(Figs. 9 and 13). In some situations, infections fromPythium spp. along with another pathogen may pro-duce similar symptoms, as is the case with wheat in-fected with Pythium and soilborne wheat mosaic virusat the same time (Fig. 14). Field examination of rootsinfected with Pythium spp. may reveal an abbreviatedroot system, as was the case with the millet picturedherein (Figs. 10 and 11). Reductions in grain and fruityield can occur and the quantity and quality of animalfeed can be reduced. If Pythium root rot occurs early inthe life of the crop, it may cause an infected root toproduce multiple roots that are smaller in diameter, asoccurs in carrot (Fig. 15). Such carrot roots are obvi-ously unmarketable.

Infection with Pythium spp. can cause wilting of nu-merous crop species. Peppers commonly wilt when in-fected with Pythium spp. (Fig. 16). In peanut, Pythiumroot rot can cause wilt of the entire plant or a dark brownto black discoloration of the pods (Fig. 17). Sometimesinfections by Pythium spp. are accompanied by infec-tion with root knot nematode (Fig 17).

If wet and warm conditions prevail because of fre-quent rains or irrigation, moist and overcast days, or thesite is typically wet, the white mycelia of Pythium spp.may become evident in the field. A common name forthis disease is Acottony leak.@ The cottony portion ofthe name is from the white mycelial growth and the leakportion of the name is because the infected tissue is of-ten greasy and water soaked as a result of tissue break-down from the enzymes produced by Pythium spp. Anenclosed crop canopy is another situation where mois-ture is retained, but if ambient conditions are wet, anenclosed canopy is not necessary for cottony leak tooccur. Some of the crops where cottony leak is seeninclude bean (Fig 18), squash (Fig. 19), cucumber (Fig.20), and salad crops such as lettuce.

CONTROL AND MANAGEMENT

Suppression of plant diseases caused by Pythiumspp. requires a different sequence of tactics for differ-ent situations. For example, suppression of seedlingblights, caused by Pythium spp., requires different tac-tics than suppression of cottony leak, caused by Pythiumspp., in cucumber fruit. It is best to use all the tacticsavailable for any situation because no one tactic is a pana-cea. A planned, systematic approach is best for con-trolling any plant disease. Resistant cultivars to Pythiumdo not exist and thus, the control program for Pythiumspp. must be done without this important control mea-sure. Hardened and woody tissue of all plants tend tobe more resistant that soft and young tissue.

For all field situations, crop rotation with unrelatedcrops should be used. Many plant pests, such as nema-todes, fungi that cause wilts, and some leaf spot patho-gens, are significantly suppressed by crop rotation. Be-cause Pythium spp. infect and colonize many broadleafand grass crops, crop rotation may not provide the levelof control attained with those pests that are more cropspecific, such as a fungus that causes Fusarium wilt.However crop rotation may prevent a given Astrain@ of aPythium sp. from increasing to unmanageable levels ona crop that is grown on the same soil over and over.Keep in mind that crop rotation refers to years and notseasons. For example, alternate cropping wheat andsoybeans on the same land within the same year is con-sidered double cropping and not crop rotation. The plant-ing of wheat or soybean, in this case, one year afteranother is generally not considered an adequate periodof time to suppress soil populations of Pythium spp. orother soilborne pests, such as other fungi or nematodes.

Crops should be planted in well drained soils be-cause Pythium spp. thrive in moist soils. They multiplyand move in free water with the production and move-ment of the swimming zoospores. One is most likely toencounter Pythium-induced diseases at sites that havestanding water or saturated soils for long periods of time.However, zoospores are produced rapidly and may bedetected within half an hour after a site is flooded andremains inundated. The sandy soils in Florida are gen-erally considered quite well drained because of their

porosity, but certain conditions can counteract this natu-ral porosity. If enough rain or irrigation occurs, watercan stand in portions of fields with sandy soils eitherbecause those portions of the fields are low and wateraccumulates or hard pans occur below the soil surface.Hard pans in soils are formed either from compactionby repeated use of tractor and implement usage or be-cause of an underlying layer of a finer sand or clay thatcongeals. Hard pans may exist on new land. One of thecharacteristics of the Aflatwoods@ soils in peninsularFlorida is the natural existence of a hard pan. Subsoilingequipment is available to break hard pans; this practicewill enhance drainage and deeper penetration of roots.Where seepage irrigation is used on a natural hardpan,destruction of this hardpan would be counterproduc-tive. Unnatural hardpans above this natural hardpan canform and may need to be disrupted.

Seed or transplants should not be placed too deep.The longer it takes for seedlings to emerge, the morelikely seedling blights will occur. Prior to planting everyeffort should be made to suppress nematodes and in-sects in the soil. Damage from these pests to the rootsor lower stems provides entry points for Pythium spp.and other pathogens.

High density plant populations should be avoided.While high density plantings have some advantages forsome crops in some situations, they do create more fa-vorable environments for growth of Pythium spp. andcertain other pathogens because aeration and dryingwithin the canopy is reduced.

Mechanical cultivation of an established planting forweed control can influence diseases caused by Pythiumspp. and other pathogens. If the cultivating implementdamages the roots of the crop, it would not be unex-pected to have increased root or lower stem rots later.Also, when cultivating, avoid moving soil onto the es-tablished plant. The soil deposited onto the stem cancontain disease-causing organisms which can infect thetender stem. An older and hardened stem is more resis-tant to such infections. Depositing hot soil on stems hascaused heat scald in the stems, which was followed byinfections by several parasitic organisms.

The time of year when a crop is planted may influ-

ence the qualitative and quantitative aspects of Pythium-induced diseases. For example, the planting of smallgrains, such as wheat, oats, or rye, early in the fall (Sep-tember to mid October) in northern Florida, when thesoil is still quite warm, is desirable for using the crop forlivestock forage. However, such early plantings are com-monly damaged by Pythium aphanidermatum. whichcauses seedling blights and root rots. Planting those smallgrains in November to early December progressivelyreduces the risk from Pythium spp. Vegetables, suchas collards, turnips, and mustard, that are planted in earlyfall can incur the same problem as they grow best atcooler temperatures. For vegetables that grow best inwarmer soils, such as okra, planting in warm soils pro-motes earlier emergence and reduces the possibility ofseedling blight caused by Pythium spp.

To reduce problems with Pythium spp. in cropsplanted early in the fall, some control techniques are avail-able in addition to the previously mentioned tactics. Forthe small grains planted in early fall, certain seed treat-ment chemicals are available. Some of the broad spec-trum, non-systemic chemicals, such as captan and thiram,provide low levels of suppression. However, certainsystemic chemicals (e.g mefenoxam) provide longer pro-tection as the seedling enlarges. For the early fall-plantedvegetables or vegetables planted at any time, one canbypass the highly susceptible seedling crop growth stageby using healthy transplants. Transplants can be infectedwith Pythium spp. or other pathogens during their pro-duction; thus, it is imperative to use pest-free transplants.Ask your supplier if the transplants were certified pestfree by an authorized government agency. Although suchan inspection and certification is not a guarantee for per-fect plants, it is better than no inspection at all. If certi-fied plants are not available from your supplier, startdealing with a supplier who provides quality plants.

Production of healthy transplants is the result of thedeliberate use of sanitary practices. If the plants areproduced outdoors in ground level beds, such as thoseused for tobacco in northern Florida for over 100 years,several tactics are available. Do not use the same siteeach year for producing transplants. Isolate the trans-plant production area from crop production fields.Choose a site that has adequate drainage for water. Whenpreparing the site with tillage equipment, plow under old

weed and crop debris at least 30 days prior to seedingthe bed, particularly if a soil fumigant is not to be used.This interval of time allows beneficial organisms to buildup in the soil and reduce parasitic fungi, such as Pythiumspp. and Rhizoctonia spp. that can abound on recentlyplowed down plant debris. Wash off soil and crop de-bris from tractor tires and implements when moving froma field known to be infested with Pythium spp. or otherpests. The finished transplant bed should be raised abovethe perimeter area by at least six inches to allow for runoff of excess water and reduce the chance of the pre-pared bed from being further contaminated with disease-causing organisms. If a soil fumigant is used to reducesoilborne pests, such as Pythium spp. and Rhizoctoniaspp., the use of a raised bed is even more important.Fumigated, sterilized or pasturized soil have minimal orno beneficial, soilborne organisms to compete againstpathogenic organisms. Without competition, Pythiumspp. and Rhizoctonia spp. tend to grow and multiply atfaster rates when compared to their growth in the pres-ence of competitors. Do not use pond or ditch waterfor irrigation; they are commonly infested with patho-genic organisms.

For transplants that are grown in a greenhouse-typesituation in containers or trays, several sanitary tacticsare available to minimize infestations and subsequent in-fection from Pythium spp. and other parasitic organ-isms. A soil mix that has been sterilized or sanitized,such as MetroMix, should be used. Use a loose mixthat allows excess water to drain. Many commercialbrands of such soil mixes are available or you can blendyour own soil mix and then sanitize it with either dryheat, steam or an available soil fumigant, such as methylbromide. However, note that at the time of this writing,methyl bromide is scheduled to be totally unavailable forany use except for quarantine purposes by the year 2005.

The site of the transplant production area should beaway from production fields and the perimeter aroundthe site should be a well maintained grass. The trans-plant production trays or containers should be on raisedbenches to minimize the chance of contamination withinfested soil. They should never be placed on the flooranywhere within the premises. Hands should be thor-oughly washed with soap and water before handling theplanting equipment, containers or plants. Specific per-

sonnel should be assigned to the transplant productionarea, and they should not be allowed to return to thetransplant production area if they leave the site and makeany contact with soil until they have washed their handsand forearms. All implements for planting that are likelyto come into contact with the containers or plants shouldbe sanitized. If a tray or plant falls to the ground levelsoil, do not place it back onto the raised bench until ithas been sanitized. The discharge end of hoses or wa-tering devices, and other implements should not be al-lowed to be on the ground where they can become con-taminated.

Walkways and access ways within the productionarea should be concrete or a material that can be easilywashed. The soil under the raised benches or framesshould be void of weeds and volunteer plants. Somegreenhouse production sites have eliminated exposedsoil by pouring the entire site with concrete for easiercleaning and maintenance. All soil mixes and containersshould not be stored in contact with the flooring of thestructure even if it is concrete. They should also be storedin such a way that when the concrete floor is washed,water splash from the floor will not contact the soil orcontainers. Do not use pond or ditch water for washingor irrigation. Cull piles of plants and used containersshould not be near areas used for plant production orplanting purposes and they should be buried or destroyedas soon as possible.

Containers used for transplant production can bereused repeatedly provided they are sanitized betweenuses. All cleaning operations should be away from trans-plant and production areas. Prior to sanitizing the con-tainers or trays, wash off the clinging soil and plant parts.The washed containers can then be sanitized by soakingthem for at least 30 minutes in a commercial bleach(5.25%) solution that is diluted to 10% (one part 5.25%bleach to nine parts of tap or well water). Other sanitiz-ing agents and formulations are available, but the abovesuggestion is made because it is known to be effective.One logistical problem that exists when trying to sub-merge polystyrene trays in a sanitizing solution is theirbuoyancy. It will take significant overhead weight ormechanical force to keep them submerged. Solid, plas-tic trays are becoming increasingly popular because theyare easily washed and they submerge easily. However,

they will not float if one is using a float-type transplantproduction system.

At first sighting of some transplants having disease ofany kind, the container or tray with those plants shouldbe removed from the production area until the cause ofthe problem is determined. If they are not removed, thesefirst groups of plants with disease may serve as a sourceof inoculum for the remainder of your operation. It isbetter to play it safe early rather than having a majorproblem later.

For crops such as tomatoes or cucumbers that areproduced in hydroponic systems in the greenhouse, thesame level of sanitation used for production of trans-plants must be utilized. Contamination of the rootingzone of plants must be eliminated whether the system isan ebb and flow water system or bags of moistened soilare used for growing the plants. The same sanitary tech-niques used for transplant production can be utilized forcrop production in the greenhouse.

Several types of chemicals can be use for suppres-sion of Pythium spp. As mentioned above, sanitizingcompounds such as bleach are available and effective.Many chemical seed treatments have been used for de-cades. At the time of this writing, formulations of seedtreatment fungicides that contain mefenoxam (e.g Apron)are best because they strongly suppress Pythium spp.and they are systemic. One of the negative characteris-tics of mefenoxam and certain other non-broad spec-trum fungicides is that resistance within the population ofthe intended pest develops and renders the product in-effective against those non-sensitive strains. For this rea-son it is advised that a seed treatment be composed of aselective fungicide, like mefenoxam, and a broad spec-trum fungicide, such as captan and thiram.

Pre-plant, soil fumigation with chemicals such asmethyl bromide (available as limited supplies until 2005),chloropicrin and metam-sodium are available. They areeffective if applied correctly. Soil fumigants are mosteffective if applied into well tilled soil that is slightly moist.If the soil is too wet, the vapors of the fumigant do notmove adequately through the air pores in the soil. If thesoil is too dry, the fumigant moves too quickly and theexposure time of the plant pest at any one site in the soil

is not long enough. The linear air pockets from fumigantchisels, disks, or shanks that remain to the soil surfaceafter the fumigation process must be solidly sealed. Fail-ure to do so will allow for an immediate and prematureescape of the fumigant from the soil which in turn, mini-mizes the needed effects from the fumigant. Each fumi-gant has a standard waiting time from time of applicationto time of planting or transplanting. With chloropicirinand metam-sodium, the waiting period can be as long asthree weeks in heavier soils with clay. With sandier soils,slightly shorter waiting periods can be used, but if thesoil is moist, it would be best to wait the full three weeks,even in sandy soils.

Some fungicides for suppression of Pythium-induceddiseases are available as sprays. These products canbe somewhat effective against diseases caused byPythium spp. that occur above the soil surface, such ascottony leak Most fungicides that are applied as sprayswill not adequately suppress disease below the soil sur-face. If the fungicide is water soluble and directed to thesoil surface, the compound will leach into the rootingzone and suppress the fungal activity below the soil sur-face. Such is the case with mefenoxam-containing fun-gicides (e.g. Ridomil Gold) for suppression of seedlingblights and root rots in young plants caused by Pythiumspp. However, if the spray is applied after infectionoccurs, you will be less likely to attain a positive re-sponse. WITH ALL CHEMICALS, READ ANDUNDERSTAND THE INTENTIONS OF THELABEL. IT HAS INFORMATION ON USE PAT-TERNS AND RESTRICTIONS.

Heat can be used for suppression of diseases causedPythium spp. For suppression of Pythium spp., thesoil must be heated to at least 120o F for at least 30consecutive minutes to reduce the population. This canbe done with steam for use in greenhouse operations orsolarization for use in the field or greenhouse. For green-house soil, steam can be released into an enclosed bodyof soil until the temperature of the soil at all sites reaches120o F for at least 30 min. In reality, to control otherfungal pathogens, insects and weed seeds, a tempera-ture of 180o F throughout the soil mass for 30 consecu-tive minutes is recommended. Because the steam maynot be able to permeate deeply into soil from its ejectionsite, smaller batchs of soil should be used to insure com-

plete pasteurization. Start the 30 minute count when allthe soil mass has achieved 180o F as indicated by areliable thermometer. Dry heat can be used but it istypically not as effective as moist heat (steam).

In the field, solarization of soil before planting canincrease the temperature of the soil and reduce popula-tions of soil pathogens, such as Pythium spp. Solariza-tion is done by securing clear plastic over the soil sur-face for up to six weeks. The soil temperature will in-crease provided that direct sunlight is available. Theupper few inches of soil will incur the greatest increasein temperature, so it is likely that solarization will be mostbeneficial while the roots of the plants remain in the up-per few inches of soil. This may provide some suppres-sion of disease through the seedling or young plant stages.If cloudy conditions prevail during the solarization pe-riod, minimal benefit, if any, will result. Solarization hasbeen most effective in drier climates, such as Israel andOklahoma, where the number of cloudless days is greaterthan that in Florida.

Fire can be used to reduce levels of pathogens in thesoil. The burning of brush piles on soil increases its tem-perature and will reduce pests in the soil where brushpiles are burned. This is most likely to be useful in gar-den or small sites. Another advantage of such burns isthe recycling of nutrients back to the soil.

Trellising crops to grow vertically rather than on baresoil will significantly reduce infection from Pythium spp.,particularly in fruit. This technique has been extremelyeffective in reducing cottony leak in fruit of tomatoes,cucumbers, strawberries, and in beans that produce longvines. Similarly, the use of plastic mulch over the soil togrow a crop will significantly reduce fruit rots by simplyhaving a barrier between the soil and the fruit.

Manipulating organic matter, either by the direct ap-plication of amendments or by planting crops for livingor debris mulches, may by beneficial in developing soilswith high levels of microorganisms that are antagonisticto or competitive with Pythium spp. Organic amend-ments, such as sewage sludge or cotton gin trash andmature composts from yard wastes and animal wastes,may be used to reduce damage from Pythium spp. inthe greenhouse or field, but generally, very high, and of-

ten uneconomical, amounts of the amendments areneeded. Solarization of soil that has been amended withaged chicken manure plus aged yard waste has suc-cessfully suppressed Pythium spp. Certain types of peator composted hardwood bark have effectively sup-pressed Pythium spp. in potting media in greenhouseproduction systems.

Planting living mulches may support competing or-ganisms in the field, but unless tested, may result in in-creased populations of the pathogen. Some soils arenaturally suppressive to diseases caused by Pythiumspp. or may become suppressive with increased organicmatter, manipulation of soil pH, or some crop rotations.Combinations of minimum tillage and multicropping re-sulted in lower incidences of damage by Pythium spp.on some crops, such as soybean, rotated with corn withminimum tillage when compared to conventional tillage.Living mulches or plant debris used on the soil surfacefor mulch may restrict the movement of inoculum insplashing water and, thus, reduce spread of the disease.At the time of this writing, many claims have been madeby others in relation to the use of suppressive mulches,soil mixes, and field soils. The positive side of this is thatresearch is being conducted, but the negative side is thatthis desirable tactic is still a long way from being em-ployed reliably.

Several biological control agents, including actino-mycetes and other bacteria, fungi, and even selected

species of Pythium, are available commercially individu-ally or as combinations for suppression of soilborne plantpathogens. Although a great deal of effort has been ex-pended for the development of these products, their suc-cess rate has been variable, at best. The most likelysites for their success are in highly controlled locations,such as hydroponic systems or containerized nurserysystems, that allow the addition of the biological controlagents to a pasteurized or sterilized growth medium priorto the buildup or recontamination by the pathogen, suchas Pythium spp., or prior to a similar buildup of poten-tial microbial antagonists to the biological agents.

Constant plant growth and proper nutrition of the plantare capable of reducing infection from Pythium spp.This is exemplified by increased plant growth when ni-trogen fertilizer (ammonium nitrate) was applied to oatsand millet (Fig. 10 ) that were yellow and slightly stuntedbecause of infections from Pythium spp. in the roots.The nitrogen is not fungicidal; it allows the plant to growquicker if even a slight nitrogen deficiency exists in thesoil. If the plants are severely stunted and near death,this tactic will be useless. The suppressive effects ofvarious nutritive amendments, such as calcium and po-tassium, have been studied for control of Pythium spp.No definitive statements can be made with reliability onthis subject at this time except that some individuals claimthat the addition of calcium or potassium ions have hadsuppressive effects against Pythium spp.

Fig. 1. Hyphae , zoospores in a vesicle, andsporangia of Pythium aphanidermatum.

Fig. 2. Oospores of Pythium sp. In cells ofroot tissue.

Fig. 3. Microscopic view showing sloughingoff of root cortex from infection by Pythium.

Fig. 4. Microscopic view of healthy root withintact cortex.

Fig. .5. Seedling blight of spinach caused byPythium sp. and Rhizoctonia sp.

Fig. 6. Seedling blight of soybeans caused byRhizoctonia sp. and Pythium sp.

Fig. 7. Stem rot of tomato caused by Pythium sp. Fig. 8. Pythium blight in lettuce transplants.

Fig. 9. Yellowing of pepper seedlings fromPythium root rot.

Fig. 10. Yellowing of pearl millet fromPythium root rot.

Fig. 11. Root rot of pearl millet caused byPythium sp. (on left) and healthier root systemon right.

Fig. 12 Yellowing of wheat leaves caused byPythium root rot.

Fig. 13. Death of wheat plants from Pythiumroot rot.

Fig. 14. Yellowing and stunting of wheatcaused by infection by Pythium sp. Andsoilborne wheat mosaic virus.

Fig. 15. Pythium root rot (Brown rot) ofcarrot.

Fig. 16. Wilting of peppers caused by infectionwith Pythium sp. in roots.

Fig. 17. Pod rot of peanut (black) caused byPythium sp. and galls caused by root knot nema-tode.

Fig. 18. Aerial blight (cottony leak) of youngbean plants caused by Pythium sp.

Fig. 19. Aerial blight (Cottony Leak) of fruit ofyellow summer squash caused by Pythium sp.

Fig. 20. Fruit rot (Cottony Leak) of cucumberfruit caused by Pythium sp.