Morphological and molecular characterisation of Pythium species causing chilli (Capsicum annuum L.) damping-off

Twenty-five Pythium isolates comprising five species viz., Pythium aphanidermatum, P. deliense, P. graminicola, P. heterothallicum and P. ultimum from different geographical locations of Tamil Nadu (Coimbatore, 4; Cuddalore, 6; Dindigul, 1; Dharmapuri, 1; Erode, 1; Madurai, 1; Namakkal, 7; Thanjavur, 1; Theni, 1; Thirunelveli, 1 and Vellore, 1) isolated from chilli crop were analysed with randomly amplified polymorphic DNA (RAPD) markers. Morphological and molecular characteristics of these different species were correlated with the RAPD. Polymerase chain reaction amplification of total genomic DNA with six random primers generated unique banding patterns depending on the primer and the isolate. The isolate I₁₇ produced identical banding patterns, while other isolates produced dissimilar bands within the particular species, indicating the genetic diversity among the isolates within a species. Morphological characters were also different from each other even in isolate I₁₇ which shared identical bands. Cluster analysis showed minimum and maximum per cent similarities among the tested Pythium species which ranged from 49 to 89%, respectively. RAPD markers were better suited for differentiating isolates within a species rather than species.     

Virus–vector Relationships,Host Range,Detection and Sequence Comparison of Chilli leaf curl virus Associated with an Epidemic of Leaf Curl Disease of Chilli in Jodhpur,India

An epidemic of chilli leaf curl disease was recorded in 2004 in Jodhpur, a major chilli‐growing area in Rajasthan, India. Several isolates were efficiently transmitted by the whitefly (Bemisia tabaci), all of which induced severe leaf curl symptoms in chilli. A single whitefly was capable of transmitting the virus, and eight or more whiteflies per plant resulted in 100% transmission. The minimum acquisition access period (AAP) and inoculation access period (IAP) were 180 and 60 min, respectively. The virus persisted in whiteflies for up to 5 days postacquisition. Of 25 species tested, the virus infected only five (Capsicum annuum, Carica papaya, Solanum lycopersicum, Nicotiana tabacum and N. benthamiana). The virus was identified as Chilli leaf curl virus (ChiLCV), which shared the closest sequence identity (96.1%) with an isolate of ChiLCV from potato in Pakistan and showed sequence diversity up to 12.3% among the ChiLCV isolates reported from India and Pakistan. A betasatellite was identified, which resembled most closely (97.3%) that of Tomato leaf curl Bangladesh betasatellite previously reported from chilli and tomato leaf curl in India. The betasatellite was very different from that reported from chilli leaf curl in Pakistan, indicating that different betasatellites are associated with chilli leaf curl in India and Pakistan. We describe here for the first time the virus–vector relationships and host range of ChiLCV.     

Enhanced activity of introduced biocontrol agents in solarized soils and its implications on the integrated control of tomato damping-off caused by Pythium spp

Soil solarization in combination with introduction of biocontrol agents (BCA) was evaluated as a potential disease management strategy for tomato damping-off caused by Pythium spp. A rifampicin resistant Pseudomonas fluorescens strain (PfT-8) and a carbendazim resistant Trichoderma harzianum strain (ThM-1) were introduced into soil following solarization. Tomato seeds were planted into treated field plots. The influence of soil solarization and application of biocontrol agents on damping-off incidence, plant biomass, rhizosphere population of introduced antagonists, and native Pythium spp. was assessed by two consecutive field trials. Damping-off incidence was significantly reduced in solarized plots compared to control. Soil inoculation of biocontrol agents into solarized plots resulted in the highest suppression of damping-off incidence (PfT-8 up to 92%; ThM-1 up to 83%), and increase in plant biomass (PfT-8 up to 66%; ThM-1 up to 48%) when compared to un-solarized control plots. Rhizosphere population of introduced biocontrol agents gradually increased (PfT-8 up to 102% and ThM-1 up to 84%) in solarized soils when compared to unsolarized control. The population of Pythium spp in rhizosphere soil was reduced up to 55% in solarized plots; whereas, application of BCA to solarized soils reduced the rhizosphere population of Pythium spp. by 86 and 82% in P. fluorescens and T. harzianum applied plots respectively.     

Isolation and Identification of Pythiaceous Fungi from Irrigation Water and their Pathogenicity to Antirrhinum, Tomato and Chamaecyparis lawsoniana

Pythiaceous fungi were isolated from irrigation water using a variety of natural and artificial baits. Isolates were also obtained by plating water samples directly on the surface of selective agar media. The selective medium of Ocana and Tsaq (1966) PlOVP, was modified by substituting rifampicin and ampicillin (10 and 500 μg cm^?3 respectively) for vancomycin to suppress bacterial growth from water samples. The pythiaceous fungi were identified as Pythiitm dissotocitm, P. middletonii, P. mamillatum, P. rostratum, Pythium“group 1”, “group 2” and “group 3” and Phytophtbom gona-podyides. All isolates of P. gonapodyides were the A1 strain and produced oospores when paired with an A2 isolate of P. drechsleri. Isolates were tested for their pathogenicity to Antirrhinum, tomato and Chatmaecyparis lawsoniana cv. Ellwoodii. Pythium middletonii and Pythium“group 1” caused severe pre-emergence damping-off of Antirrhinum seedlings, P. mamillatuni, P. rostratum and Pythium“group 3” were less pathogenic to the same host while P. dissotocum, Pythium“group 2” and Phytophthora gonapodyides were non-pathogenic. Only isolates of Pythium“group 1” were pathogenic to tomato seedlings. None of the fungi was pathogenic to rooted cuttings of Chamaecyparis lawsoniana cv. Ellwoodii.     

Sterols in species of Pythium

Summary Analysis by gas chromatography revealed the presence of small amounts of squalene, but not lanosterol nor ergosterol in Pythium paroecandrum, P. ultimum, P. graminicola, and P. arrhenomonas. However, when acetate-¹⁴C was used as a precursor for sterols, even squalene was not found in P. graminicola. The deficiency in the sterol synthesizing mechanism may therefore be at or before the squalene forming step. Both squalene and ergosterol were present in the mycelium of Rhizoctonia solani, as shown by both gas chromatography and by the incorporation of acetate-¹⁴C into ergosterol. The absence of ergosterol in Pythium and its presence in Rhizoctonia is consistant with the resistance to the antibiotic filipin of Pythium species and the sensitivity of R. solani.     

Control of Pythium damping-off of sugar beet by seed treatment with crop straw powders and a biocontrol agent

Seed treatment with non-sterilized powdered straws from 39 crops was tested for the control of Pythium damping-off of sugar beet. Four straws, including flax, coriander, pea, and lentil were effective in controlling the disease in soil artificially infested with Pythium sp. “group G.” Sterilizing flax and pea straws eliminated the efficacy of these straws. Wheat straw powder coated on sugar beet seeds increased the incidence of Pythium damping-off but this effect was reversed by the co-inoculation of wheat straws with the biocontrol agent Pseudomonas fluorescens 708. Coating sugar beet seeds with P. fluorescens 708 and flax or pea straws also increased the efficiency of the bacterial strain for the control of Pythium damping-off. Pea straws and to a lesser extent lentil straws produced volatile substances that affected mycelial growth of Pythium sp. “group G” on potato dextrose agar in Petri plates when the straws were mixed with water and left to ferment for two days. Fermentation of pea straws led to the accumulation of volatile ammonia, which was produced by the reduction of the large amount of nitrate stored in the straw. Reduction of nitrate and therefore the release of volatile ammonia did not occur in sterilized pea straws. However, fermenting sterile pea straws with bacteria from different genera restored nitrate reduction and the release of volatile ammonia, suggesting that microorganisms associated with pea straws are responsible for the conversion of nitrate into volatile ammonia which in turn control Pythium damping-off disease in sugar beet.     

Induction of systemic resistance by mixtures of fungal and endophytic bacterial isolates against <Emphasis Type="Italic">Pythium aphanidermatum</Emphasis>

The efficacy of eight fungal and ten endophytic bacterial isolates were tested for their ability to inhibit the growth of Pythium aphanidermatum, the causal agent of chilli damping-off. In vitro studies revealed that Trichoderma viride (TVA) and endophytic Pseudomonas fluorescens (EBL 20-PF) showed the highest inhibition of mycelial growth (71.5%; 76.7%) of P. aphanidermatum. Both the antagonists were compatible with each other and they were tested alone and together in vivo for the control of P. aphanidermatum. Besides, the induction of defense-related enzymes such as peroxidase (PO), polyphenoloxidase (PPO), phenylalanine ammonia-lyase (PAL), PR-protein like β-1, 3-glucanase and the accumulation of phenolics in chilli seedlings due to the application of bioagents were also studied. Combined application of talc-based formulation of bio-agents and challenge inoculated with P. aphanidermatum recorded maximum induction of defense-related enzymes, PR-proteins and accumulation of phenolics compared with individual application. This study suggest that the increased induction of defense-related enzymes (four to fivefold) and phenolic content (sixfold) due to the combination treatment of bioagents might have involved in the reduction of damping-off incidence and in turn increased the plant growth and yield of chilli.     

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.     

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.     

Biocontrol of <Emphasis Type="Italic">Pythium</Emphasis> damping-off in cucumber by arbuscular mycorrhiza-associated bacteria from the genus <Emphasis Type="Italic">Paenibacillus</Emphasis>

The Pythium biocontrol features of 17 Paenibacillus strains, all previously isolated from the rhizosphere, hyphosphere or bulk soil from mycorrhizal and non-mycorrhizal cucumber plants, were examined using a cucumber seedling emergence bioassay. Thirteen strains – four strains of Paenibacillus polymyxa, eight strains of P. macerans and one strain of Paenibacillus sp. – significantly increased the percentage of seedling emergence of seeds inoculated with agar plugs of Pythium aphanidermatum FC42. Overall, the efficacy of Pythium biocontrol did not seem to differ between isolates of Paenibacillus originating from either mycorrhizal or non-mycorrhizal systems. No strains significantly reduced the damping-off incidence caused by the aggressive isolate Pythium sp. B5. Two strains of P. macerans not only reduced the incidence of pre-emergence damping-off by 73%, but they also counteracted the plant growth-depressing effect of P. aphanidermatum FC42, so that 68–82% of the emerged seedlings remained healthy 7 days after sowing. Two strains of P. macerans and one strain of P. polymyxa also significantly increased the percentage of seedling emergence following inoculation with approximately 10⁵ zoospores of P. aphanidermatum FC42. There was no significant difference between the dry weight of three selected bacteria-inoculated and -uninoculated plants in the absence of Pythium; however, the dry weight of bacteria-inoculated plants was significantly higher than that of the uninoculated control plants with bacteria in the presence of P. aphanidermatum FC42.     

Rice root-knot nematode (Meloidogyne graminicola) infestation in rice

Rice (Oryza sativa) is an important staple food crop for majority of human population in the world in general and in Asia in particular. However, among various pests and diseases which constitute important constraints in the successful crop production, plant parasitic nematodes play an important role and account for yield losses to the extent of 90%. The major nematode pests associated with rice are Ditylenchus angustus, Aphelenchoides besseyi, Hirschmanniella spp., Heterodera oryzicola and Meloidogyne graminicola. However, rice root-knot nematode (M. graminicola) happens to be the most important pest and is prevalent in major rice producing countries of the world. In India, the distribution of M. graminicola in rice growing areas of different states has been documented in nematode distribution atlas prepared by All India Coordinated Research Project (Nematodes) and published by Directorate of Information and Publications of Agriculture, Indian Council of Agricultural Research, New Delhi, India during 2010. M. graminicola affected rice plants show stunting and chlorosis due to the characteristic terminal swellings/galls on the roots which ultimately result in severe reduction in growth and yield. Number of eco-friendly management technologies against M. graminicola have been developed and demonstrated, including the use of bioagents for minimising the losses due to rice root-knot nematode. This review is focused on collating information to understand the current scenario of rice root-knot nematodes with greater emphasis on its ecological requirements, damage symptoms, biology, morphology, host range and management strategies.     

Development of Multiplex PCR to Detect Five Pythium Species Related to Turfgrass Diseases

The objective of this study was to develop multiplex PCR detection method for five Pythium species associated with turfgrass diseases, Pythium aphanidermatum, Pythium arrhenomanes, Pythium graminicola, Pythium torulosum and Pythium vanterpoolii. Species‐specific primers and two common primers were designed based on the sequences of the internal transcribed spacer region of ribosomal DNA. Another primer set by which all organisms would be amplified in 18S rDNA was used as a positive control. When these total nine primers were applied to the multiplex PCR, all species were individually discriminated in the mixture of five species culture DNA. Furthermore, all five Pythium species were detected in naturally infected plants using the multiplex PCR.     

Characterization of Pythium spp. associated with root rot of tobacco seedlings produced using the float tray system in Zimbabwe

The study was undertaken to identify and characterize Pythium isolates associated with root rot disease of tobacco seedlings as a first step towards developing management strategies for the pathogen. A total of 85 Pythium isolates were collected from diseased tobacco seedlings during 2015–2016 tobacco growing season. The isolates were identified to species level using sequencing of the internal transcribed spacer region. Thereafter, a subset of the isolates was tested for sensitivity to the commonly used fungicides, metalaxyl, azoxystrobin and a combination of fenamidone/propamocarbby growing isolates on Potato Dextrose Agar plates amended with the fungicides. The sequence analysis of the ITS‐rDNA identified Pythium myriotylum as the dominant Pythium species associated with the root rot of tobacco seedlings in Zimbabwe. Pythium aphanidermatum and P. insidiosum were also identified albeit at lower frequencies. Phylogenetic analyses of the ITS region of the P. myriotylum isolates showed little sequence diversity giving rise to one distinct clade. The fungicide sensitivity tests showed that metalaxyl provided the best control of P. myriotylum in vitro, as compared to other fungicides. To the best of our knowledge, this is the first comprehensive study to determine and characterize Pythium species associated with root rot of tobacco in the float seedling production system in Zimbabwe.     

Evaluation of fungicides on Indian isolates of Phytophthora colocasiae causing leaf blight of taro

Leaf blight caused by Phytophthora colocasiae is the most destructive disease affecting taro (Colocasia esculenta) worldwide including India. Fungicides (primarily metalaxyl) remain as an important strategy to manage taro leaf blight in India over decades. It is important to monitor isolate sensitivity to identify build-up of fungicide resistance and thereby modify fungicide usage strategies. P. colocasiae isolates representing four different geographical regions of India were evaluated for their sensitivity to metalaxyl and three other commercially available fungicides viz. Samarth, Biofight and Akoton by poisoned media technique. All the isolates tested were sensitive to metalaxyl, nevertheless there is an increase in the effective concentration compared to the previous reports. Among the other fungicides, Samarth was found to be superior in completely inhibiting mycelial growth at 0.05% followed by Biofight at 1%. Metalaxyl and Akoton^® shared a common inhibitory concentration at 2%. The most effective fungicide determined by the in vitro method was evaluated in vivo for studying the pattern of inhibition before and after the disease development in detached taro leaf. The results of the study revealed that build-up on metalaxyl resistance in P. colocasiae is in its course and caution should be taken while administering against taro leaf blight. Fungicide Samarth could be used as an alternative to metalaxyl for management of taro leaf blight.     

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.     

Molecular characterization of plant growth promoting rhizobacteria that enhance peroxidase and phenylalanine ammonia-lyase activities in chile (<Emphasis Type="Italic">Capsicum annuum</Emphasis> L.) and tomato (<Emphasis Type="Italic">Lycopersicon esculentum</Emphasis> Mill.)

Pythium and Phytophthora species are associated with damping-off diseases in vegetable nurseries and reduce seedling stand and yield. In this study, bacterial isolates were selected on the basis of in vitro antagonism potential to inhibit mycelial growth of damping-off pathogens along with plant growth properties for field assessment in wet and winter seasons. We demonstrate efficacy of bacterial isolates to protect chile and tomato plants under natural vegetable nursery and artificially created pathogen-infested (Pythium and Phytophthora spp.) nursery conditions. After 21 days of sowing, chile and tomato plants were harvested and analysed for peroxidase and phenylalanine ammonia-lyase activities. Pseudomonas sp. strains FQP PB-3, FQA PB-3 and GRP₃ were most effective in increasing shoot length (P > 0.05%) in both artificial and natural field sites. For example, Pseudomonas sp. FQA PB-3 treatment increased shoot length by 40% in the artificial Pythium 4746 infested nursery site in chile plants in the wet season. The bacterial treatments significantly increased the activity of peroxidase and phenylalanine ammonia-lyase in chile and tomato plant tissues, which are well known as indicators of an active lignification process. Thus, we conclude that treatment with potential bacterial plant growth promoting agents help plants against pathogen invasion by modulating plant peroxidase and phenylalanine ammonia-lyase activities.     

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.     

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.     

Antifungal metabolites (monorden, monocillins I, II, III) from Colletotrichum graminicola, a systemic vascular pathogen of maize

Colletotrichum graminicola is a systemic vascular pathogen that causes anthracnose stalk rot and leaf blight of maize. In the course of an effort to explore the potential presence and roles of C. graminicola metabolites in maize, ethyl acetate extracts of solid substrate fermentations of several C. graminicola isolates from Michigan and Illinois were found to be active against Aspergillus flavus and Fusarium verticillioides, both mycotoxin-producing seed-infecting fungal pathogens. Chemical investigations of the extract of one such isolate (NRRL 47511) led to the isolation of known metabolites monorden (also known as radicicol) and monocillins I–III as major components. Monorden and monocillin I displayed in vitro activity against the stalk- and ear-rot pathogen Stenocarpella maydis while only the most abundant metabolite (monorden) showed activity against foliar pathogens Alternaria alternata, Bipolaris zeicola, and Curvularia lunata. Using LC–HRESITOFMS, monorden was detected in steam-sterilized maize stalks and stalk residues inoculated with C. graminicola but not in the necrotic stalk tissues of wound-inoculated plants grown in an environmental chamber. Monorden and monocillin I can bind and inhibit plant Hsp90, a chaperone of R-proteins. It is hypothesized that monorden and monocillins could support the C. graminicola disease cycle by disrupting maize plant defenses and by excluding other fungi from necrotic tissues and crop residues. This is the first report of natural products from C. graminicola, as well as the production of monorden and monocillins by a pathogen of cereals.     

Relationship between temperature optima and secreted protease activities of three Pythium species and pathogenicity toward plant and animal hosts

The in vitro physiological characteristics of three species of Pythium (oomycetes) that utilize different food sources were compared with their ecological activities: P. insidiosum is a pathogen of mammals (including humans), P. graminicola infects the roots of graminaceous hosts, and P. grandisporangium is an enigmatic water mold isolated from mangrove leaves and marine algae. P. insidiosum and P. graminicola showed peak growth rates at 37 °C before complete inhibition of growth at 40 °C; P. grandisporangium grew fastest at 22 °C. Differences between the invasive pressures exerted by the hyphae of these microorganisms were not considered significant in relation to the substrates colonized by these water molds. All three species showed substantial secreted protease activity, producing three or more serine proteases with weights ranging from 24-38 kDa. Fastest growth rates were supported when collagen was supplied as the sole carbon source, and none of the species were able to grow on purified plant cell wall polysaccharides. The growth and nutritional characteristics of P. graminicola and P. grandisporangium bear little obvious relationship to the ecological niches that they inhabit. This highlights the caution necessary in extrapolating from laboratory analyses to the natural environment, and points to the potential importance of ecological opportunity in determining the host range and food source of certain microorganisms.