Use of Oospore Formulations of Pythium oligandrum for Biological Control of Pythium Damping-off in Cress

Oospore preparations of Pythium oligandrum produced by liquid and solid-substrate fermentations were evaluated for biocontrol activity against Pythium damping-off in cress in artificially infested sand and naturally infested soil. Oospore biomass preparations from liquid fermentation of six isolates of P. oligandrum were equally effective in reducing damping-off in sand when tested as seed-coatings, whereas this type of preparation of a single isolate formulated as a kaolin dust, on Perlite and as alginate pellets incorporated into sand gave little or no control. None of the formulations containing oospores produced by solid-substrate fermentation incorporated into sand had any effect. In soil, a formulation containing oospores produced in a barley-Perlite solid-substrate fermentation and all oospore-biomass formulations which were prepared increased seedling survival, but none of these were as effective as a propamocarb HCl drench.     

Antagonism of Rhizoctonia spp. to Pythium oligandrum and Damping-Off Fungi

In paired cultures on corn-meal agar, Rhizoctonia solani, R. cerealis and R. fragariae caused vacuolation, disappearance of cytoplasm, and apparent lysis of hyphae of Pythium oligandrum, P. ultimum, and Aphanomyces cochlioides. Hyphae of Phoma betae were not injured by the Rhizoctonia spp. When sugar-beet seeds dressed with mycelium of R. cerealis, R. fragariae, or an isolate of R. solani nonpathogenic to sugar-beet were planted in soil naturally infested with P. ul-timum, the level of biological control of damping-off was similar to that obtained with captan dressing. In soil artificially infested with P. ultimum, biological dressings were slightly less efficacious than the chemical dressing.     

Isolation of Pythium Acanthicum,P. oligandrum,and P. periplocum from Soil and Evaluation of Their Mycoparasitic Activity and Biocontrol Efficacy Against Selected Phytopathogenic Pythium Species

Mycoparasitic Pythium species with spiny oogonia were surveyed in 50 Palestinian agricultural fields subject to different cropping practices using the Sclerotia Bait Technique (SBT) and the Surface-Soil-Dilution-Plate method (SSDP) with the selective VP3 medium. The mycoparasitic Pythium species were obtained from 21 (42%) soils using the SSDP method and from 37 (74%) soils using SBT. Pythium acanthicum and P. oligandrum were isolated by both methods, whereas P. periplocum was isolated only by the SBT. Using a newly modified dual plate culture method (MDPCM), the three mycoparasites showed varying antagonistic performance against several Pythium host species under a range of in vitro conditions. However, P. periplocum and P. oligandrum were found to be active biocontrol agents against P. ultimum, the damping-off organism of cucumber. This pathogen was antagonized, on thin films of water agar, by the three mycoparasites, and was moderately susceptible to P. periplocum while slightly susceptible to P. acanthicum and P. oligandrum. In direct application method in which antagonistic mycoparasites were incorporated into peat/sand mixture artificially infested with P. ultimum under growthroom conditions, Pythium oligandrum and P. periplocum (at 500 CFUg⁻¹) significantly improved seedling emergence and protected seedlings from damping-off. In the seed coating method, biocontrol by two types of seed dressing (homogenate- or oospore coated seeds), was comparable to that achieved by direct application. This revised version was published online in June 2006 with corrections to the Cover Date.     

Antagonistic actions of Pythium oligandrum and Trichoderma harzianum against phytopathogenic fungi (Fusarium oxysporum and Pythium ultimum var. ultimum)

Abstract

The possible biological control of damping-off fungi, Fusarium oxysporum and Pythium ultimum by Pythium oligandrum or Trichoderma harzianum was in vitro investigated. Results of comparing the antagonistic activity of P. oligandrum and T. harzianum in dual plates against the tested phytopathogens indicated different degrees of antagonism. After 12 days of incubation colony of the phytopathogenic fungus was completely overgrown by the antagonist, except for the interaction between T. harzianum and F. oxysporum which showed no overgrowth or any hyphal penetration by the antagonist. However, growth and proliferation of F. oxysporum colony was repressed. T. harzianum and P. oligandrum produced chitinase and β-1,3-glucanase when they were grown on liquid culture medium supplemented with chitin or fungal dried mycelium as a sole carbon source, and enzyme production was higher by T. harzianum comparing with P. oligandrum under the same condition. Fungal dried mycelium of F. oxysporum was the most selective carbon source for enzyme production, on the other hand, chitinase production was significant locked when P. ultimum dried mycelium was used as a carbon source. Production of volatile compounds by P. oligandrum or T. harzianum against F. oxysporum and P. ultimum was examined using the inverted plates method. F. oxysporum was inhibited by the antagonist volatile compounds and it is inhibited 100% by increasing the amount of inoculum size. Production of potential biocontrol agents provided with economically features and working under field conditions are recommended.     

Biocontrol of black scurf on potato by seed tuber treatment with Pythium oligandrum

The biological control activity of Pythium oligandrum against black scurf of potato caused by Rhizoctonia solani AG-3 was evaluated in field experiments after treatment of potato seed tubers with P. oligandrum. Seed tubers infected with black scurf sclerotia were dipped for a few seconds in a suspension of 10³, 10⁴ or 10⁵ mL^?1 P. oligandrum oospores and were then air-dried. Each level of P. oligandrum-treatment significantly reduced the disease rates of stolon at a level similar to that achieved by chemical control. When P. oligandrum populations adherent to the surface of seed tubers were determined, oospore counts on tubers treated with 10⁴ or 10⁵ oospores mL^?1 were about 540/cm² or about 22,000/cm² just after dipping and decreased to about 170/cm² or 2900/cm² after a 3-week incubation, respectively. Confocal laser scanning microscopic observation with an immuno-enzymatic staining procedure showed that P. oligandrum hyphae had colonized the sclerotia and established close contact by coiling around the R. solani hyphae present on the surface of seed tubers, in a manner similar to that observed in the dual-culture test. Quantification of R. solani DNA by PCR indicated that the R. solani population was reduced on the seed tubers treated with P. oligandrum compared to untreated tubers. Furthermore, the ability of P. oligandrum to induce resistance against black scurf was determined using a potato tuber disk assay. Treatment of tuber disks with the cell wall protein fraction of P. oligandrum enhanced the expression of defense-related genes such as 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase, lipoxygenase and basic PR-6 genes, and reduced disease severity upon challenge with R. solani compared with untreated controls. These results suggest that biocontrol mechanisms employed by P. oligandrum against black scurf involve both mycoparasitism and induced resistance.     

Epicoccum Nigrum as biocontrol agent of Pythium damping-off and root-rot of cotton seedlings

Efficacy of Epicoccum nigrum and its exudate was tested in control the pre- and post-emergence damping-off and root-rot of Egyptian cotton (cv. Giza 83) in vitro and in vivo. Different isolates of Epicoccum nigrum reduced the radial growth of both Pythium debaryanum and P. ultimum significantly, by production considerable inhibition zones. In liquid cultures E. nigrum exudate showed a high fungicidal effect resulting in a significant reduction of the mycelial dry weight of the two investigated Pythium spp. Also E. nigrum exudate inhibited cellulase and pectinase activity by P. debaryanum and P. ultimum. Soaking of cotton seeds in E. nigrum exudate for different intervals resulted in significant reduction of root-rot severity of seedlings as well as the contamination of seeds and seedlings by fungi during and after germination. These treatments also stimulated germination of cotton seeds and enhanced the seedlings vigour significantly. In pot experiments, the use of E. nigrum as a soil mixture or seed dressing significantly alleviated the hazard effect of P. debaryanum. Pythium ultimum seemed to be weak or non pathogen to the used cotton cultivar (Giza 83). Application of E. nigrum or its exudate not only involved in protection of cotton seedlings against Pythium damping-off and root-rot but also enhanced their vigour and growth characteristics. The main conclusion of this study is that E. nigrum could be used successfully as environmentally safe and economic biological control agent to protect cotton (cv. Giza 83) from damping-off and root-rot diseases caused by P. debaryanum.     

Chemical,physical and biological control of carrot seedling diseases

In naturally infested soil containingPythium ultimum, P. acanthicum andPhytophthora megasperma, onlyP. ultimum was associated with root rot and damped-off seedlings. Damping-off was promoted by low soil temperatures and by flooding. Seedling stands were markedly reduced when seed was pre-incubated in soil at 12°C but not at 25°C or 35°C. Dusting carrot seed with metalaxyl significantly increased seedling stands in the field at rates from 1.5–6 g kg⁻¹ seed and in both flooded and unflooded, naturally infested soil at 3.15 g kg⁻¹. In greenhouse experiments using artifically infested soil,P. ultimum andP. paroecandrum caused damping-off of carrot seedlings andRhizoctonia solani reduced root and shoot weights.R. solani caused damping-off in nutrient-enriched soil.P. acanthicum andP. megasperma were not pathogenic to seedlings, although both fungi colonized roots. Soil populations of allPythium spp., particularlyP. ultimum, increased during growth of seedlings and population growth ofP. megasperma was promoted by periodic flooding. Infestation of soil withP. acanthicum did not reduce damping-off of carrot seedlings byP. ultimum orP. paroecandrum, but significantly increased root and shoot weights and decreased root colonization byR. solani P. acanthicum has potential as a biocontrol agent againstR. solani.     

Morphological and molecular identification of some closely related Pythium species in Egypt

Twelve isolates of Pythium species (P. aphanidermatum, P. deliense, P. ultimum var. ultimum and P. ultimum var. sporangiiferum) from different hosts were compared from morphological, pathological and molecular viewpoints. Minimum, optimum and maximum temperatures of P. aphanidermatum and P. deliense were similar while those of P. ultimum var. ultimum and P. ultimum var. sporangiiferum were also similar. All tested isolates were highly virulent against cucumber seedlings with 100% damping-off. RAPD data using three different primers revealed that strains of P. ultimum var. ultimum and P. ultimum var. sporangiiferum are distinct from each other. This data can be used to separate those species from P. aphanidermatum and P. deliense. In contrast, RAPD data cannot be used to separate P. aphanidermatum and P. deliense. Sequence analysis of the ribosomal DNA internal transcribed spacers (ITS) was used to establish phylogenetic relationships among the tested isolates.     

Compost amendments enhance peat suppressiveness to Pythium ultimum,Rhizoctonia solani and Sclerotinia minor

Peat is the most common organic material used for the preparation of potting mix because of its homogeneous and favorable agronomic characteristics. However, this organic material is poorly suppressive against soilborne pathogens and fungicides are routinely used to manage damping-off diseases. In the present study, we investigated the suppressive capability of five compost – peat mixtures towards the plant pathogens Pythium ultimum, Rhizoctonia solani and Sclerotinia minor – Lepidium sativum pathosystems. For all organic media, 18 parameters were measured including enzymatic activities (glucanase, N-acetyl-glucosaminidase, chitobiosidase and hydrolysis of fluorescein diacetate), microbiological (BIOLOG® EcoPlates?, culturable bacteria and fungi), and chemical features (pH, EC, total, extractable and humic carbon, total and organic N, NH₄–N, total protein and water content). In addition, ¹³C-CPMAS-NMR spectroscopy was used to characterize the organic materials. Peat amended with composts reduced disease damping-off caused by P. ultimum, R. solani and S. minor in 60% of the mixtures and compost derived from animal manure showed the largest and most consistent disease suppression. Sterilization decreased or eliminated suppressiveness of 42.8% of the mixtures. The most useful parameters to predict disease suppression were different for each pathogen: extractable carbon, O-aryl C and C/N ratio for P. ultimum, alkyl/O-alkyl ratio, N-acetyl-glucosaminidase and chitobiosidase enzymatic activities for R. solani and EC for S. minor. Our results demonstrate that the addition of composts to peat could be useful for the control of soilborne pathogens.     

Formulation of the Biocontrol FungusCladorrhinum foecundissimumto Reduce Damping-Off Diseases Caused byRhizoctonia solaniandPythium ultimum

Five isolates ofCladorrhinum foecundissimum, added to soilless mix as 10-day-old fresh bran preparations (1.0% w/w), significantly reduced (P≤ 0.05) damping-off of eggplant and pepper caused byRhizoctonia solanistrain R-23. After 4 weeks of growth, plant stands in the biocontrol-amended, pathogen-infested treatments (>80%) were comparable to those in the noninfested controls. Since plant stands were similar at 2 and 4 weeks, most of the disease was preemergence damping-off. The bran preparations also reduced saprophytic growth of the pathogen, and there was an inverse correlation (r²= −0.94) between saprophytic growth and eggplant stand. Added to soilless mix at a rate of 2.0% (w/w), alginate prill containing 20% fermentor-produced biomass of six biocontrol isolates ofC. foecundissimumreduced (P≤ 0.05) damping-off of eggplant caused byR. solani, but only the prill with biomass of isolates Cf-1 or Cf-2 yielded plant stands (>80%) comparable to that in the noninfested control. As with the bran preparations, there was also an inverse correlation (r²= −0.80) between saprophytic growth of R-23 and eggplant stand with the alginate prills. Alginate prill with biomass of Cf-1 or Cf-2 also reduced (P≤ 0.05) damping-off of eggplant and pepper caused by other isolates (195, NG-2, DPR-1) ofR. solani, but only the stands (>80%) of pepper were similar to that in the noninfested control. Alginate prill formulations ofC. foecundissimum(Cf-1, Cf-2, and Cf-3) also reduced (P≤ 0.05) populations of the pathogen and damping-off of eggplant and pepper caused byPythium ultimum(PuZ3). However, although the plant stands in the treatments were not as high as those in the noninfested controls, they were higher than those in the pathogen-infested controls. The treatments also reduced populations ofP. ultimumin the soilless mix so that there were inverse correlations between the pathogen population and eggplant stand (r²= −0.81) and pepper stand (r²= −0.78). Extruded flour/clay granules containing 5.0% biomass of Cf-1 and Cf-2, added toR. solani-infested soilless mix (2.0%), reduced (P≤ 0.05) damping-off of eggplant and pepper. However, only the Cf-2 treatments resulted in stands (>80%) equal to those in the noninfested controls for the crops after 4 weeks of growth. The influence of bran and alginate prill of Cf-1 or Cf-2 on the spatial spread ofR. solaniand its ability to incite damping-off of eggplant showed that prill with Cf-1 or Cf-2 and bran with Cf-2 were equally effective in reducing the spread of the pathogen from the point source of the inoculum to the center of the flats.     

Distribution and Expression of Elicitin‐like Protein Genes of the Biocontrol Agent Pythium oligandrum

Pythium oligandrum has the ability to induce plant defence reactions, and four elicitin‐like proteins (POD‐1, POD‐2, POS‐1 and oligandrin) that are produced by this oomycete have been identified as elicitor proteins. The first three are cell wall protein elicitors (CWPs), and the latter is an extracellular protein. Pythium oligandrum isolates have been previously divided into two groups based on the CWPs: the D‐type isolate containing POD‐1 and POD‐2, and the S‐type isolate containing POS‐1. We identified the genes encoding these elicitin‐like proteins and analyzed the distribution of these genes among 10 P. oligandrum isolates. A genomic fosmid library of the D‐type isolate MMR2 was constructed and genomic regions containing the elicitin‐like protein genes were identified. Southern blot analyses with probes derived from pod‐1 and an oligandrin gene indicated that the 10 P. oligandrum isolates could be divided into the same groups as those based on the CWPs. The D‐type isolates carried pod‐1, pod‐2 and two oligandrin genes, termed oli‐d1 and oli‐d2, while the S‐type isolates carried pos‐1 and one oligandrin gene termed oli‐s1. Phylogenetic analysis of POD‐1, POD‐2, POS‐1, Oli‐D1, Oli‐D2 and Oli‐S1 with the previously defined elicitins and elicitin‐like proteins of Phytophthora and Pythium species showed the specific clade. These genes occurred as single copies and were present in the P. oligandrum genomes but not in the other nine Pythium species (Pythium iwayamai, Pythium volutum, Pythium vanterpoolii, Pythium spinosum, Pythium torulosum, Pythium irregulare, Pythium ultimum, Pythium aphanidermutum and Pythium butleri). Furthermore, RT‐PCR analysis demonstrated that all of these genes were expressed during the colonization of tomato roots by P. oligandrum, supporting the idea that they encode potential elicitor proteins. To investigate the genetic relationships between the D‐type and the S‐type isolates, physical maps of the flanking regions around pod‐1, pod‐2, pos‐1 and the oligandrin genes were constructed. The maps suggest that the D‐type isolates may be derived from the S‐type isolates due to gene duplication and deletion events.     

Suppression of sugar beet damping-off caused by Rhizoctonia solani using bacterial and fungal antagonists

Rhizoctonia damping-off caused by Rhizoctonia solani Kühn, is one of the most damaging sugar beet diseases. It causes serious economic damage wherever sugar beets are grown. Biological control is an efficient and environmentally friendly way to prevent damping-off disease. Suppression of damping-off disease caused by R. solani was carried out by four isolates of Bacillus subtilis (Ehrenberg) Cohn as well as three isolates of each of Trichoderma harzianum Rifai and Trichoderma hamatum (Bonord.) Bainier. The effect of Bacillus and Trichoderma isolates against R. solani was investigated in vitro and tested on sugar beet plants under greenhouse conditions. Isolates of Bacillus and Trichoderma were able to inhibit the growth of R. solani in dual culture. Furthermore, Trichoderma isolates gave high antagonistic effect than isolates of B. subtilis. Under greenhouse conditions, coating seeds by T. harzianum and B. subtilis separately, reduced seedling damping-off significantly. However, applications of T. harzianum increased the percentage of surviving plants more than B. subtilis in comparison to control. The obtained results indicate that T. harzianum and B. subtilis are very effective biocontrol agents that offer potential benefit in sugar beet damping-off and should be harnessed for further biocontrol applications.     

Pythium and Phytopythium species associated with weeds collected in vegetable production fields in Brazil

This study aimed to identify Pythium and Phytopythium species from weeds collected in vegetable fields and test their pathogenicity. Weeds with symptoms of damping-off, root rot or wilt were sampled in the Brazilian states of Ceará, Goiás and Pernambuco, as well as in the Distrito Federal, for isolation and identification of the causal agents. Once isolated, colonies with typical Pythium and Phytopythium characteristics grew in selective V8 medium. Procedures for species identification included morphology and amplification of the ITS and Cox II regions, which were compared with other accessions available at GenBank. The phylogenetic relationships among the isolates and pathogenicity to their original hosts were evaluated. Six Pythium species were identified: P. aphanidermatum, P. oopapillum, P. orthogonon, P. ultimum var. ultimum, P. myriotylum and P. sylvaticum, and two species of Phytopythium, Phy. chamaehyphon and Phy. oedochilum. In the pathogenicity tests, the 10 weed hosts showed symptoms of damping-off or root rot after inoculation, with exception of Portulaca oleraceae in which none of the isolates was pathogenic. Therefore, common weeds in vegetable fields areas can host different Pythium and Phytopythium species and play an important role in the epidemiology of vegetable diseases, in particular on pathogen survival and population increase.     

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.     

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.     

Use of Trichoderma harzianum and Gliocladium vitens for the Biological Control of Post-emergence Damping-off and Root Rot of Cucumbers Caused by Pythium ultimum

The antagonist strains Gliocladium virens G2 and Trichoderma harzianum T3 originally isolated from Pythium suppressive peat, and two benomyl-resistant strains of T. harzianum, T12B and T95, were evaluated as biological control agents of damping-off and root rot of cucumbers in sphagnum peat caused by Pythium ultimum. All strains were equally effective when applied as 1 % peat-bran preparations, whereas the effectiveness of disease control was reduced at higher concentrations of the antagonists. The two wild-type strains were also found to be effective when applied as conidial suspensions, and in this case no reduction in disease control was seen at higher concentrations. G. virens G2 and T. harzianum T12B showed antibiotic activity against P. ultimum in in vitro tests; however there were no signs of mycoparasitism of P. ultimum by any of the antagonist strains.     

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.     

Antagonist Microorganisms with the Ability to Control <Emphasis Type="Italic">Pythium</Emphasis> Damping-off of Tomato Seeds in Rockwool

A total of 237 microorganisms were isolated from five different greenhouse tomato growing media. Of those, 40 microorganisms reduced the in vitro mycelial growth of both Pythium aphanidermatum and Pythium ultimum. The ability of these microorganisms to control damping-off was then tested in rockwool. As a result, Pseudomonas corrugata strains 1 and 3, Pseudomonas fluorescens subgroup F and G strains 1, 2, 3, 4 and 5, Pseudomonas marginalis, Pseudomonas putida subgroup B strain 1, Pseudomonas syringae strain 1 and Pseudomonas viridiflava significantly reduced damping-off caused by P. ultimum or P. aphanidermatum. Pseudomonas marginalis was the only microorganism that significantly reduced damping-off caused by both pathogens.     

Potential for integrated biological and chemical control of damping-off disease caused by Pythium ultimum in tomato

Pythium ultimum (Trow) is one of the main causes of damping-off disease in many parts of the world. Control of the disease depends mainly on application of chemical fungicides. However, soil treatments with fungicides are not always feasible due to economical and ecological reasons. Soil-borne, non-pathogenic bacteria of the genus Pseudomonas fluorescens with the ability to antagonise fungal phytopathogens, represent a realistic alternative to chemical fungicides and show great promise with respect to protect plant roots from fungal-induced diseases. In an attempt to find an integrated control system of damping-off disease in tomato, fungicides including azoxystrobin, metalaxyl-M and pyraclostrobin were applied alone and in combination with P. fluorescens isolate CW2. The fungicides were tested in in vitro for their antagonistic potential against P. ultimum and for compatibility with CW2. It was found that the fungicides were fungitoxic to P. ultimum, but did not inhibit the growth of the P. fluorescens. The efficacy of the fungicides alone and in combination with CW2 was also tested in greenhouse experiments against damping-off disease on tomato. Two concentrations (5 and 10?μg?ml^?1) were applied. Damping-off incidence of tomato seedlings in Humosoil^?:sand mixture infested with P. ultimum was reduced following seed treatment with the fungicides. However, the degree of control obtained varied significantly depending on the fungicide used. Combined seed treatment with P. fluorescens and the fungicides resulted in a significant improvement in disease control and improved plant growth as indicated by shoot and root dry weights. Metalaxyl-M treatment applied alone or in combination with P. fluorescens, significantly protected tomato seedlings against damping-off. Strobilurin fungicides stimulated plant growth compared to metalaxyl-M. Combined treatment of tomato seeds with strobilurin fungicides and CW2 showed a moderate to good disease control and an increase in shoot and root dry weights.     

Impact of auxin-compounds produced by the antagonistic fungus Pythium oligandrum or the minor pathogen Pythium group F on plant growth

Plant growth promotion induced by the antagonistic fungus, Pythium oligandrum, is the result of a complex interaction which includes an indirect effect through control of pathogens in the rhizosphere and/or a direct one mediated by plant-induced resistance. The present study shows an increased plant growth associated with direct interaction between P. oligandrum and roots, which is mediated by a fungus-produced auxin compound, tryptamine (TNH₂). In vitro experiments provided evidence that P. oligandrum metabolised specifically indole derivatives, such as tryptophan and indole-3-acetaldehyde, to produce THN₂ through the tryptamine pathway. When P. oligandrum grew in sterile root exudates, it also produced an auxin-like compound. Additional experiments on P. oligandrum–root interaction showed that, in amended nutrient solution of plants, the antagonist metabolised Trp into TNH₂ and that root absorption of this newly formed auxin-compound in appropriate concentrations was associated with enhancement of plant growth. This phenomenon was observed only when nutrient solution was amended with low tryptophan (Trp) concentrations, i.e. 0.05 and 0.1 mM; higher concentration (0.5 and 1 mM Trp) induced abnormal root development. Similar experiments were performed with Pythium group F, a minor pathogen known for its ability to produce auxin-compounds through the tryptamine pathway. In this case, irregular root development was always noticed with all Trp concentrations added to the nutrient solution of plants. Moreover, Pythium group F colonization of roots was associated with leakage of auxin-compounds in the nutrient solution. Our results, therefore, highlight that the production of similar auxin-compounds by two Pythium species has contrary effects on plant development.