Interaction of the mycoparasite Pythium oligandrum with other Pythium species

Interactions of Pythium oligandrum and four plant‐pathogenic Pythium spp. (P. ultimum, P. vexans, P. graminicola and P. aphanidermatum,) were studied in vitro by (i) video microscopy of hyphal interactions on water agar films, (ii) counting of host and mycoparasite propagules in different regions of opposing colonies on sunflower‐seed extract agar films and (Hi) ability of P. oligandrum to overgrow plates of potato‐dextrose agar previously colonized by Pythium spp. Pythium oligandrum typically coiled round the hyphae of Pythium hosts and penetrated the host hyphae after approximately 50 min from the hyphal coils, causing disruption of host hyphal tips up to 1.2 mm ahead of contact points. The relative growth rates of mycoparasite and host hyphae, timing of penetration and distance (sub‐apical) at which penetration led to host tip disruption were used to assess the potential of mycoparasitism by P. oligandrum to prevent the growth of Pythium hosts. P. aphanidermatum was unique among the ‘host’ Pythium spp. in being largely unaffected by P. oligandrum and in antagonizing the mycoparasite by coiling and penetrating the mycoparasite hyphae. Other host Pythium spp. apparently differed in susceptibility, the most susceptible being P. vexans and P. ultimum, whereas P. graminicola was more resistant. The results are discussed in relation to the role of P. oligandrum as a biocontrol agent, especially for limiting the ability of other Pythium spp. to increase their propagule populations in crop residues.     

Effectiveness of Four New Pyrazole-pyrimidines on Phytopathogens: Ultrastructural Evidences on Pythium ultimum

Four newly synthesized molecules derived from pyrazole-pyrimidine were assayed on Botrytis cinerea Micheli, Fusarium moniliforme Sheld and Pythium ultimum Trow. All proved effective in inhibiting the growth of the phytopathogens at all of the test concentrations (10, 20, 50, 100 μg/ml). The most effective compound was 1-(3)nitrophenyl - 6 - trifluoromethylpyrazolo[3,4 - d ]pyrimidine 4(5H)-thione (CF₃3). Ultrastructural studies on P. ultimum treated with CF₃3 revealed alterations in the normal hyphal shape and, at high concentration, plasmolysis and damage to the wall texture was observed. At 20 μg/ml different vesicles were seen in the cytoplasm: some appeared quite dense, and specific cytochemical reactions indicated that they were most likely peroxysomes; other vesicles seem to be vacuoles of varying content. In some cases there was disintegration of the nuclear envelope. The effects on membrane lipids and interference in protein synthesis are hypothesized as possible mechanism of action of the molecule.     

Proteins Related to the Biocontrol of Pythium Damping-off in Maize with Trichoderma harzianum Rifai

Induced resistance has been evidenced as one of mechanisms of Trichoderma to control plant diseases, however, no study showed the change of host proteomics in Trichoderma-induced resistance of maize against damping-off caused by Pythium ultimum Trow. The mechanism of Trichoderma harzianum Rifai for controlling maize seedling disease caused by Pythium ultimum Trow was investigated firstly by proteome technique and the result suggested that T. harzianum strain T22 was not only able to promote seedling growth but also protein accumulation. One-dimensional electrophoresis assay showed that more bands appeared on the gel with T22 or T22 combined with P. ultimum （T22 ＋ P. ultimum） treatment than with other treatments. Enzyme assay showed that two chitinases of the root sample were more activated in the treatments with T22 than in the other treatments without T22. Proteins in the seedling roots from the various treatments were separated through protein extraction and 2-D electrophoresis technique. In the seedlings produced from the T22-treated seeds, there were 104 up-regulated proteins and 164 down-regulated proteins relative to the control, and 97 and 150, respectively, aftel treatment with T22 ＋ P. ultimum; however, with P. ultimum alone the values were much lower than with the other two treatments. The correlation coefficient values were 0.72, 0.51 and 0.49 for the comparison of protein spot distribution on gel among control with T22, P. ultimum and T22 ＋ P. ultimum, respectively. So it seemed that P. ultimum infection was more effective than T22 in interfering with the host proteome profile. Furthermore, analysis with MALDITOF-MAS showed that some important proteins associated with defensive reactions were identified in T22 or T22 ＋ P. ultimum treatments, including endochitinase, pathogenesis-related protein PRMS （pathogenesis-related maize seed）, GTP-binding protein, isoflavone reductase and other proteins related to respiration. All those proteins are probably part of the network of resistance or development-related proteins. Interestingly, P. ultimum treatment resulted in elimination of pathogenesis-related protein PRMS on gel, and therefore damping-off could be in part attributed to inhibition of the expression of this protein by P. ultimum infection. Some unknown proteins are also related to the defensive reaction of the host.     

Methods used to study Pythium oligandrum, an aggressive parasite of other fungi

Isolates of Pythium oligandrum Drechsler and P. acanthicum Drechsler behaved similarly in a range of experiments and in a manner consistent with mycoparasitism. They grew on cellulose in association with some fungi - notably Botryotrichum pilulferum Sacc. & March. and Phialophora radicicola Cain var. radicicola (IMI 187786) - and markedly reduced cellulolysis by these species; however, they had little effect on cellulolysis by Gaeumannomyces graminis (Sacc.) Arx & Olivier var. graminis and made little growth on cellulose in the presence or absence of this fungus. Pythium oligandrum and P. acanthicum also grew rapidly across plates of potato-dextrose agar precolonised by Phialophora radicicola var. radicicola, whereas isolates of P. echinulatum Matthews, P. mamillatum Meurs, P. megalacanthum de Bary, P. spinosum Sawada and P. ultimum Trow did not grow under these conditions. Precolonised agar plates were therefore used as a selective medium to isolate Pythium oligandrum and similar fungi from soils. For this, small pieces of soil organic matter were transferred to precolonised agar plates and incubated for 4–6 (-12) days, when the presence of P. oligandrum or similar fungi was assessed by their production of spiny oogonia across the host colony. The technique proved successful in isolating these fungi from a wide range of cultivated soils, even if the soils had been stored air-dry. However, boiled hemp seeds buried in soil and subsequently transferred to precolonised agar plates usually gave rise to phytopathogenic rather than mycoparasitic pythia. Pieces of wheat straw precolonised by P. oligandrum decomposed at the same rate as virgin straws or straws precolonised by P. ultimum or Mucor hiemalis Wehmer, when all were buried in nitrogen-supplemented soil. However, different cellulolytic fungi appeared to colonise straws in these respective treatments, Fusarium spp. being less common and Stachybotrys atra Corda more common in the presence of P. oligandrum than in its absence. When subsequently opposed to P. oligandrum on agar plates, Fusarium spp. appeared to be parasitised by this fungus and made very little growth across the agar, whereas S. atra grew through the colony of P. oligandrum and was clearly unaffected by the presence of this fungus. Our results demonstrate the widespread occurrence of P. oligandrum in cultivated soils in Britain and also suggest that this species might influence the activities of other soil fungi.     

Effect of Storage on the Survival and Biocontrol Activity of Pythium oligandrum in Pelleted Sugar Beet Seed

Seeds of sugar beet were pelleted with oospores of Pythium oligandrum and stored for 6 years at 8 20IC. Mycelium of P. oligandrum grew from pelleted seed when plated on cornmeal agar (CMA) within 48 h from 100% of seeds stored for 0, 2 and 4 years, and from 93% of seeds stored for 6 years. The germination of oospores removed from pelleted seed immediately after pelleting was 30% on CMA after 18 h of incubation, but storage gradually reduced germination to only 16% after 48 h of incubation for oospores removed from seed stored for 6 years. The biocontrol activity of P. oligandrum -pelleted seed was also tested after 6 years of storage in mixes of soil naturally infested with Pythium spp. and Aphanomyces cochlioides , and sand. P. oligandrum -pelleted seed had no effect in reducing damping-off due to the combined effect of Pythium spp. and A. cochlioides in 5 and 1% (v/v) soil-sand mixtures. However, in the 1% (v/v) soil-sand mixture, P. oligandrum significantly reduced Pythium spp.-induced damping-off from 33 to 26%.     

Microbial suppression of in vitro growth of Pythium ultimum and disease incidence in relation to soil C and N availability

Experiments were designed to examine effects of the soil microbial community, C and N availability on in vitro growth of Pythium ultimum and its infection of cotton seedlings by manipulating the stage of cellulose decomposition, size and activity of microbial populations, and N availability. In comparison to the untreated control (CONT), cellulose addition alone (CELL) reduced soil nitrate by 35–80 fold, but had no significant effect on soil ammonium. Soil microbial biomass C (SMBC) increased over 2 fold in 14 days following cellulose addition, but significantly decreased in the following 10 days due to N limitation. Addition of both cellulose and N (NCELL) resulted in sustained SMBC for 24 days and significantly reduced in vitro P. ultimum growth and disease incidence. In vitro growth of P. ultimum and disease severity were consistently reduced in the order: CONT > CELL > NCELL. In vitro growth of P. ultimum was lower in soils previously incubated for 24 days than in those incubated for 14 days, and was most closely correlated to cumulative soil CO₂ evolution (CO2T). Correlations between P. ultimum growth rates and NO₃-N or total available N were substantial (p < 0.05), but much less significant than those between the growth rates and SMBC, microbial activity measured as CO₂ evolution rates or CO2T (p<0.0001). Addition of available N (NH₄NO₃) and C (glucose) just before the assays did not increase the in vitro growth of P. ultimum or disease severity on cotton seedlings, suggesting that time-dependent microbial processes or microbial metabolites significantly contributed to suppression of P. ultimum growth.     

Seedling mortality of metal hyperaccumulator plants resulting from damping off by Pythium spp.

Seedling mortality of Alyssum serpyllifolium ssp. lusitanicum and A. murale , both nickel hyperaccumulators, was reduced by increasing concentrations of metal within plant tissues when inoculated with the fungi Pythium mamillatum or P. ultimum , both of which cause damping-off disease of seedlings. Pythium mamillatum , isolated from nickel-rich serpentine soil, was more tolerant of nickel than P. ultimum , isolated from low-metal control soil, and was more pathogenic than P. ultimum towards seedlings containing high concentrations of metal. These results support the hypothesis that metal hyperaccumulation by plants is closely linked to increased protection against disease.     

Differential inactivation of seed exudate stimulation of Pythium ultimum sporangium germination by Enterobacter cloacae influences biological control efficacy on different plant species

This study was initiated to understand whether differential biological control efficacy of Enterobacter cloacae on various plant species is due to differences in the ability of E. cloacae to inactivate the stimulatory activity of seed exudates to Pythium ultimum sporangium germination. In biological control assays, E. cloacae was effective in controlling Pythium damping-off when placed on the seeds of carrot, cotton, cucumber, lettuce, radish, tomato, and wheat but failed to protect corn and pea from damping-off. Seeds from plants such as corn and pea had high rates of exudation, whereas cotton and cucumber seeds had much lower rates of exudation. Patterns of seed exudation and the release of P. ultimum sporangium germination stimulants varied among the plants tested. Seed exudates of plants such as carrot, corn, lettuce, pea, radish, and wheat were generally more stimulatory to P. ultimum than were the exudates of cotton, cucumber, sunflower, and tomato. However, this was not directly related to the ability of E. cloacae to inactivate the stimulatory activity of the exudate and reduce P. ultimum sporangium germination. In the spermosphere, E. cloacae readily reduced the stimulatory activity of seed exudates from all plant species except corn and pea. Our data have shown that the inability of E. cloacae to protect corn and pea seeds from Pythium damping-off is directly related to its ability to inactivate the stimulatory activity of seed exudates. On all other plants tested, E. cloacae was effective in suppressing damping-off and inactivating the stimulatory activity of seed exudates.     

Effects of the humic substances of de-inking paper sludge on the antagonism between two compost bacteria and Pythium ultimum

We investigated the in vitro influence of humic substances (HS) extracted from de-inking paper sludge compost on the inhibition of Pythium ultimum by two compost bacteria, Rhizobium radiobacter (Agrobacterium radiobacter) and Pseudomonas aeruginosa. When low concentrations (5 or 50 mg l(-1)) of HS were added to the culture medium, fungal inhibition by R. radiobacter significantly increased (P<0.01) by 2-3%. In contrast, these low levels of HS had no effect on P. ultimum inhibition by P. aeruginosa. The Fe, chelated by HS, was in part responsible for the decrease of P. ultimum inhibition by the bacteria when increasing amounts of HS were added in the culture medium. The addition of 500 mg l(-1) of humic acids isolated from de-inking paper sludge compost or from fossil origin completely eliminated the inhibition of P. ultimum by R. radiobacter. This Fe effect also stimulated growth of R. radiobacter and reduced its siderophore production in a minimal medium supplemented with HS as sole source of Fe. The results showed that HS influence microbial antagonism when added to a culture medium. However, this effect varies with different factors such as the type of bacteria, concentration of HS, molecular weight and Fe content.     

腐霉属的一个新种和二个新记录

本文报告作者1982—1983年在北京和广东一些地区土壤中用已知方法(余永年,1975)分离到的几种腐霉,其中1个新种(Pythium borealis sp.nov.)、2个国内未报道的种(P.oligandrum和P.myriotylum)和1个已知种的变异株(P.ultimum variant)。文中对以上各菌进行了形态学描述。     

Biocontrol of Rhizoctonia solani and Pythium ultimum on Capsicum by Trichoderma koningii in potting medium.

Two isolates of Trichoderma koningii were evaluated for efficacy in control of damping-off diseases in seedlings of Capsicum annuum grown in pasteurized potting medium in a glasshouse. A selected isolate of binucleate Rhizoctonia and two fungicides were also included as standards for control of Rhizoctonia solani and Pythium ultimum var. sporangiiferum. Both isolates of T. koningii reduced seedling death caused by R. solani in one of two experiments, and by P. u. sporangii-ferum in two of three experiments. Neither isolate of T. koningii suppressed damping-off caused by either pathogen as consistently as the binucleate Rhizoctonia or fungicides. The implications of these results for commercial disease management are discussed.     

Comparative Physiology and Behaviour of the Mycoparasites Pythium acanthophoron, P. oligandrum and P. mycoparasiticum

Pythium acanthophoron (two isolates) was as aggressive as P. oligandrum in hyphal interactions with Fusarium oxysporum on water agar films. It caused rapid post-contact lysis or cytoplasmic coagulation of the host, and often branched and penetrated the host at contact points. P. acanthophoron grew across a range of fungi on pre-colonized agar plates; the range was narrower than for P. oligandrum but broader than for P. mycoparasiticum. In chemically defined liquid media, P. acanthophoron was unique among the six known mycoparasitic Pythium spp. in requiring organic nitrogen but not thiamine. It also grew on mannitol as the sole carbon source in the presence of calcium or sterols or both. However, individual isolates of the three mycoparasitic species showed different responses to calcium and the sterols ergosterol, cholesterol and beta-sitosterol when grown on mannitol. All three mycoparasites required components of molasses, carrot extract or sunflower seed extract to produce oogonia in culture; they formed few oogonia on potato extract, with or without dextrose, even when supplied with sterols. The three mycoparasites were less tolerant of high NaCl levels in culture than were three phytopathogens (P. aphanidermatum, P. ultimum, P. graminicola), in contrast to a previous report for P. oligandrum and P. ultimum. These in vitro studies suggest that P. acanthophoron has potential for use as a biocontrol agent instead of, or in addition to, P. oligandrum.     

Rhizosphere persistence of three Pythium oligandrum strains in tomato soilless culture assessed by DNA macroarray and real-time PCR

In tomato soilless culture, plant-disease optimal control and growth promotion are achieved when the rhizosphere is heavily colonized by the biocontrol agent Pythium oligandrum. Discrepancies in performance are generally attributed to the poor persistence of P. oligandrum on roots. In this study, three selected strains of P. oligandrum were introduced into the rhizosphere of greenhouse-grown tomato plants, and their persistence was assessed by DNA macroarray hybridization and real-time PCR. The experimental data from DNA detection and plate counting were compared. PCR-based methods detected P. oligandrum throughout the 6-month growing season, whereas plate counting indicated its presence only over the first 3 months. Moreover, the DNA array method provided information about the various Pythium species present in the rhizosphere: P. dissotocum was frequently detected on roots of plants, without distinction between plants inoculated or not inoculated with the antagonist. The detection of other Pythium species was noticed sporadically (P. ultimum, P. sylvaticum and P. intermedium), independent of the treatment. Even though the yield enhancement is not significant throughout the entire growing season, data obtained from epidemiological studies demonstrate an enhancement of P. oligandrum persistence on the rhizosphere of plants and less use of mycoparasitism.     

Biocontrol of Damping-off Diseases Caused by Rhizoctonia solani and Pythium ultimum with Alginate Prills of Gliocladium virens, Trichoderma hamatum and Various Food Bases

Alginate prills were formulated with the biomass of isolates of Gliocladium virens and Trichoderma spp. and various food bases (wheat bran, corn cobs, peanut hulls, soy fiber, castor pomace, cocoa hulls and chitin). Alginate prills with G. virens (Gl-21) biomass and all food bases except cocoa hull meal significantly reduced the damping-off of zinnia in a soil-less mix caused by Rhizoctonia solani and Pythium ultimum. The prills with bran, soy fiber, castor pomace or chitin resulted in stands similar to those in the non-infested control. In soil, prills with all the food bases and Thrichoderma hamatum (TRI-4) biomass controlled the damping-off of cotton caused by R. solani and gave stands comparable to, or better than, those in the non-infested control soil. Prills with all the food bases resulted in a proliferation of Gl-21 in a soil-less mix and of Gl-21 and TRI-4 in soil. Prills with food bases and TRI-4 biomass reduced the survival of R. solani in infested beet seed to less than 30%, with bran and chitin being the most effective food bases; prills with Gl-21 biomass and all food bases also reduced the survival of R. solani in beet seed, but not as much as did prills with TRI-4 biomass. In prills containing wheat bran, soy fiber or chitin, the biocontrol isolate Th-58 (T. harzianum) was almost as effective as TRI-4, but isolate Gl-3 (G. virens) was less effective. There was no significant interaction between the biocontrol fungus and the food base. The results suggest that the intrinsic properties of a selected fungus isolate are more important than some formulation variables in biocontrol.     

Potassium phosphite affects growth,antioxidant enzymes activity and alleviates disease damage in cucumber plants inoculated with Pythium ultimum

In the present study, the impact of potassium phosphite on response of cucumber plants inoculated with Pythium ultimum var. ultimum was assessed. Variations in the accumulation of both antioxidant enzymes and growth parameters were investigated. The results revealed that fresh and dry weights of shoot and root exhibited up to 2 fold increase in potassium phosphite-treated plants. The concentrations of peroxidase, superoxide dismutase and catalase were significantly higher in the plants treated with 4 g L^?1 of potassium phosphite compared to other treatments, at 48 h after inoculation. It was demonstrated that both 2 and 4 g L^?1 treatments could alleviate the disease damage to a high extent, while control plants were severely damaged by the pathogen. The results of this study suggest that the increased induction of antioxidant enzymes (2.2, 2.8 and 4 fold increase for superoxide dismutase, catalase and peroxidase, respectively) might have alleviated damping-off symptoms leading to increased plant growth and yield.     

Prof. Dr. Lothar Behr

A damping-off disease of wheat was shown in a wheat field in Kidwan village, El-Minia city, Egypt, during December, 2000. Pythium diclinum was the causal agent of such disease and this is the first reported work of its isolation as a disease to wheat. Wheat seedlings collected from that field showed browning lesions at the basal part and wilting followed by damping-off. Examination of root pieces and other infected parts yielded only Pythium diclinum. The pathogen was characterized by its typically filamentous zoosporangia, diclinous antheridia and aplerotic thick-walled oospores. Pathogenicity of this fungus was determined on wheat under greenhouse conditions and P. diclinum was proved to be pathogenic on wheat. Two isolates of each of Gliocladium roseum and Trichoderma harzianum were tested for their bio-control activity against damping-off disease of wheat caused by P. diclinum. Incorporation of G. roseum or T. harzianum isolates into carboxymethylcellulose seed coating successfully eliminated pre-emergence damping-off of the wheat caused disease, whereas post-emergence damping-off was prevented by adding inocula of each of the two fungi separately to the infested soil with P. diclinum.     

Integration of Pythium nunn and Trichoderma harzianum isolate T-95 for the biological control of Pythium damping-off of cucumber

Two biological control agents, Pythium nunn and Trichoderma harzianum isolate T-95, were combined to reduce Pythium damping-off of cucumber in greenhouse experiments lasting 3–4 weeks. T. harzianum T-95, a rhizosphere competent mutant, was applied to seeds and P. nunn was applied to pasteurized and raw soils naturally and artificially infested with Pythium ultimum. Some treatments were also amended with bean leaves to enhance the activity of P. nunn. The biological control of Pythium damping-off was evaluated in a Colorado soil (Nunn sandy loam) and an Oregon soil mix, which were replanted twice after 2 and 3 months. Interactions between P. nunn and T-95 were detected in the Colorado but not the Oregon soil. No consistent evidence of antagonism between P. nunn and T. harzianum was seen, and significant interactions were detected in the Colorado, but not the Oregon soil. In the first planting of some treatments, the combination of P. nunn and T. harzianum gave greater control of damping-off than either applied alone. P. nunn was most effective in soils that were pasteurized or amended with bean leaves. T. harzianum controlled Pythium damping-off in the Colorado, but not the Oregon soil. In both soils, disease declined over time in treatments amended with bean leaves but without P. nunn or T. harzianum added. This suppression was greater in the Colorado soil, which contained an indigenous population of P. nunn. This work demonstrates that two compatible biological control agents can be combined to give additional control of a soil-borne plant pathogen.     

Solid formulations of binucleate Rhizoctonia isolates suppress Rhizoctonia solani and Pythium ultimum in potting medium

Two isolates of binucleate Rhizoctonia spp., previously selected for efficacy in suppression of Rhizoctonia solani and Pythium spp., as well as plant growth promotion, were incorporated into various solid substrate formulations. These formulated products were assayed at three doses in three glass-house experiments for biocontrol of damping-off diseases in Capsicum annuum. R. solani anastomosis group 4 or Pythium ultimum var. sporangiiferum were incorporated into pasteurized potting medium with each formulated binucleate Rhizoctonia product. All formulations were effective against both pathogens in at least two experiments, but some formulations of one isolate of binucleate Rhizoctonia did not give consistent control of R. solani in one experiment. The most consistent formulation, which provided control of both pathogens at all doses of binucleate Rhizoctonia, was the simple substrate of rice hulls. The implications for commercialization of a biocontrol product are discussed.     

Mutation of rpiA in Enterobacter cloacae decreases seed and root colonization and biocontrol of damping-off caused by Pythium ultimum on cucumber

Strains of Enterobacter cloacae show promise as biocontrol agents for Pythium ultimum-induced damping-off on cucumber and other crops. E. cloacae A145 is a mini-Tn5 Km transposon mutant of strain 501R3 that was significantly reduced in suppression of damping-off on cucumber caused by P. ultimum. Strain A145 was deficient in colonization of cucumber, sunflower, and wheat seeds and significantly reduced in colonization of corn and cowpea seeds relative to strain 501R3. Populations of strain A145 were also significantly lower than those of strain 501R3 at all sampling times in cucumber, wheat, and sunflower rhizosphere. Populations of strain A145 were not detectable in any rhizosphere after 42 days, while populations of strain 501R3 remained at substantial levels throughout all experiments. Molecular characterization of strain A145 indicated mini-Tn5 Km was inserted in a region of the E. cloacae genome with a high degree of DNA and amino acid sequence similarity to rpiA, which encodes ribose-5-phosphate isomerase. In Escherichia coli, RpiA catalyzes the interconversion of ribose-5-phosphate and ribulose-5-phosphate and is a key enzyme in the pentose phosphate pathway. Ribose-5-phosphate isomerase activity in cell lysates from strain A145 was approximately 3.5% of that from strain 501R3. In addition, strain A145 was a ribose auxotroph, as expected for an rpiA mutant. Introduction of a 1.0-kb DNA fragment containing only the rpiA homologue into strain A145 restored ribose phosphate isomerase activity, prototrophy, seedling colonization, and disease suppression to levels similar to those associated with strain 501R3. Experiments reported here indicate a key role for rpiA and possibly the pentose phosphate pathway in suppression of damping-off and colonization of subterranean portions of plants by E. cloacae.     

Compost-induced suppression of Pythium damping-off is mediated by fatty-acid-metabolizing seed-colonizing microbial communities

Leaf composts were studied for their suppressive effects on Pythium ultimum sporangium germination, cottonseed colonization, and the severity of Pythium damping-off of cotton. A focus of the work was to assess the role of fatty-acid-metabolizing microbial communities in disease suppression. Suppressiveness was expressed within the first few hours of seed germination as revealed by reduced P. ultimum sporangium germination, reduced seed colonization, and reduced damping-off in transplant experiments. These reductions were not observed when cottonseeds were sown in a conducive leaf compost. Microbial consortia recovered from the surface of cottonseeds during the first few hours of germination in suppressive compost (suppressive consortia) induced significant levels of damping-off suppression, whereas no suppression was induced by microbial consortia recovered from cottonseeds germinated in conducive compost (conducive consortia). Suppressive consortia rapidly metabolized linoleic acid, whereas conducive consortia did not. Furthermore, populations of fatty-acid-metabolizing bacteria and actinobacteria were higher in suppressive consortia than in conducive consortia. Individual bacterial isolates varied in their ability to metabolize linoleic acid and protect seedlings from damping-off. Results indicate that communities of compost-inhabiting microorganisms colonizing cottonseeds within the first few hours after sowing in a Pythium-suppressive compost play a major role in the suppression of P. ultimum sporangium germination, seed colonization, and damping-off. Results further indicate that fatty acid metabolism by these seed-colonizing bacterial consortia can explain the Pythium suppression observed.