Cell wall structure of Pythium debaryanum.
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Cell Wall↗
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An efficient in vitro test to determine carrot genotypes resistance to cavity spot.
As part of a carrot breeding programme, our objective was to develop a test for determining genotypes resistance against two Pythium species responsible for cavity spot. Ten different media were tested for carrot callus culture, Pythium culture and dual cultures. Carrot callus growth was greater on Murashige and Skoog medium plus 1.25 microM 2.4D plus 2.5 microM BA plus 2.5 microM NAA. Pythium violae was more sensitive to medium composition than Pythium sulcatum. When carrot callus were inoculated by Pythium violae or sulcatum on Murashige and Skoog mineral medium, callus growth of Bolero, an resistant cultivar in the field, was highly inhibited whereas Nanco, a susceptible cultivar was not. Such a medium should be used for screening genotypes resistance to Pythium. A Murashige and Skoog medium supplemented with 1.25 microM 2.4D, 2.5 microM BA and 2.5 microM NAA, where callus growth was highly reduced whatever the genotype used, should be useful for selection of cell lines tolerant to Pythium.
Coculture Techniques↗
Effects of fungal root pathogens on the population dynamics of biocontrol strains of fluorescent pseudomonads in the wheat rhizosphere.
The influences of Gaeumannomyces graminis var. tritici (which causes take-all of wheat), Rhizoctonia solani AG-8 (which causes rhizoctonia root rot of wheat), Pythium irregulare, P. aristosporum, and P. ultimum var. sporangiiferum (which cause pythium root rot of wheat) on the population dynamics of Pseudomonas fluorescens 2-79 and Q72a-80 (bicontrol strains active against take-all and pythium root rot of wheat, respectively) in the wheat rhizosphere were examined. Root infection by either G. graminis var. tritici or R. solani resulted in populations of both bacterial strains that were equal to or significantly larger than their respective populations maintained on roots in the absence of these pathogens. In contrast, the population of strain 2-79 was significantly smaller on roots in the presence of any of the three Pythium species than on noninfected roots and was often below the limits of detection (50 CFU/cm of root) on Pythium-infected roots after 40 days of plant growth. In the presence of either P. aristosporum or P. ultimum var. sporangiiferum, the decline in the population of Q72a-80 was similar to that observed on noninfected roots; however, the population of this strain declined more rapidly on roots infected by P. irregulare than on noninfected roots. Application of metalaxyl (which is selectively inhibitory to Pythium spp.) to soil naturally infestated with Pythium spp. resulted in significantly larger rhizosphere populations of the introduced bacteria over time than on plants grown in the same soil without metalaxyl. It is apparent that root infections by fungal pathogens may either enhance or depress the population of fluorescent pseudomonads introduced for their control, with different strains of pseudomonads reacting differentially to different genera and species of the root pathogens.
Journal Article↗
Zoospore homing and infection events: effects of the biocontrol bacterium Burkholderia cepacia AMMDR1 on two oomycete pathogens of pea (Pisum sativum L.).
Burkholderia cepacia AMMDR1 is a biocontrol agent that protects pea and sweet corn seeds from Pythium damping-off in field experiments. The goal of this work was to understand the effect of B. cepacia AMMDR1 on Pythium aphanidermatum and Aphanomyces euteiches zoospore homing events and on infection of pea seeds or roots. In vitro, B. cepacia AMMDR1 caused zoospore lysis, prevented cyst germination, and inhibited germ tube growth of both oomycetes. B. cepacia AMMDR1 also reduced the attractiveness of seed exudates to Pythium zoospores to nondetectable levels. However, when present at high levels on seeds, B. cepacia AMMDR1 had little net effect on zoospore attraction, probably because it also enhanced seed exudation. Seed-applied B. cepacia AMMDR1 dramatically reduced the incidence of infection by Pythium zoospores in situ compared with an antibiosis-deficient Tn5 mutant strain. This mutant strain also decreased Pythium infection incidence to some extent, but only when the pathogen inoculum potential was low. B. cepacia AMMDR1 did not affect attraction of Aphanomyces zoospores or Aphanomyces root rot incidence. These results suggest that B. cepacia AMMDR1 controls P. aphanidermatum largely through antibiosis, but competition for zoospore-attracting compounds can contribute to the effect. Differences in suppression of Aphanomyces and Pythium are discussed in relation to differences in the ecology of the two pathogens.
Burkholderia cepacia↗
Ethylene insensitivity impairs resistance to soilborne pathogens in tobacco and Arabidopsis thaliana.
Transgenic ethylene-insensitive tobacco (Tetr) plants spontaneously develop symptoms of wilting and stem necrosis when grown in nonautoclaved soil. Fusarium oxysporum, F. solani, Thielaviopsis basicola, Rhizopus stolonifer, and two Pythium spp. were isolated from these diseased Tetr plants and demonstrated to be causal agents of the disease symptoms. Pathogenicity of the two Pythium isolates and four additional Pythium spp. was tested on ethylene-insensitive tobacco and Arabidopsis seedlings. In both plant species, ethylene insensitivity enhanced susceptibility to the Pythium spp., as evidenced by both a higher disease index and a higher percentage of diseased plants. Based on the use of a DNA probe specific for Pythium spp., Tetr plants exhibited more pathogen growth in stem and leaf tissue than similarly diseased control plants. These results demonstrate that ethylene signaling is required for resistance to different root pathogens and contributes to limiting growth and systemic spread of the pathogen.
Arabidopsis↗
A role for jasmonate in pathogen defense of Arabidopsis.
To investigate the role of jasmonate in the defense of plants against fungal pathogens, we have studied a mutant of Arabidopsis, fad3-2 fad7-2 fad8, that cannot accumulate jasmonate. Mutant plants were extremely susceptible to root rot caused by the fungal root pathogen Pythium mastophorum (Drechs.), even though neighboring wild-type plants were largely unaffected by this fungus. Application of exogenous methyl jasmonate substantially protected mutant plants, reducing the incidence of disease to a level close to that of wild-type controls. A similar treatment with methyl jasmonate did not protect the jasmonate-insensitive mutant coi1 from infection, showing that protective action of applied jasmonate against P. mastophorum was mediated by the induction of plant defense mechanisms rather than by a direct antifungal action. Transcripts of three jasmonate-responsive defense genes are induced by Pythium challenge in the wild-type but not in the jasmonate-deficient mutant. Pythium species are ubiquitous in soil and root habitats world-wide, but most (including P. mastophorum) are considered to be minor pathogens. Our results indicate that jasmonate is essential for plant defense against Pythium and, because of the high exposure of plant roots to Pythium inoculum in soil, may well be fundamental to survival of plants in nature. Our results further indicate that the fad3-2 fad7-2 fad8 mutant is an appropriate genetic model for studying the role of this important signaling molecule in pathogen defense.
Journal Article↗
Studies of the mycoparasitism in rhizosphere of emerging sugar-beet.
Pythium oligandrum Drechsler was found as a very weak pathogen of emerging sugar-beet by means of the inoculation of the sterilized soil. The hyperparasitic ability of Pythium oligandrum Drechsler to relative species, viz. Pythium ultimum Trow and Pythium debaryanum (Hesse) were proved in rhizosphere of emerging sugar-beet. The rhizosphere soil inoculation by P. ultimum resulted in the sugar-beet emergence about 3% while the sugar-beet emergence achieved 110% if P. ultimum and P. oligandrum were inoculated simultaneously. The sugar-beet emergence due to the dual inoculation of rhizosphere soil by P. ultimum and P. debaryanum was about 3%, whereas due to P. ultimum, P. debaryanum, and P. oligandrum 77%.
Antibiosis↗
Evaluation of soil microorganisms with inhibitory activity against Rhizoctonia solani causal agent of the damping-off of canola.
Pre- and post-emergence damping-off of canola seedlings caused by Rhizoctonia solani is a serious disease in Western Canada. Other fungi such as Fusarium spp. and Pythium spp. are also related to seedling damping-off. To-day, the search of soil bacteria is becoming a tool to use microorganisms as potential biocontrol agents for several plant diseases. The purpose of this research was to detect bacteria to biologically control R. solani, Pythium spp., and Fusarium spp. Soil samples were collected throughout Alberta during 1987 to isolate bacteria. Canola seedlings were also used to obtain bacteria from the same samples. Plant pathogenic fungi were tested to detect the antagonistic activity of the isolates. Tests were made with coated canola seeds, amendments and fresh of freeze-dried cells. Three hundred forty-one bacterial cultures were isolated. Only 16 inhibited fungal growth: 7 showed the same effects against R. solani and 9 showed uneven effects. Some isolates showed a weak action to Pythium spp. and Fusarium spp. Three isolates showed inhibitory effect on R. solani and Pythium spp. Isolate F1 improved by about 50% the germination of canola seeds in inoculated pots when compared with the inoculated control. Coated seeds had low germination and emergence was below the inoculated control. The emergence of canola seedlings was very much improved when isolate 147 was delivered as an amendment in inoculated pots. Identification showed that 3 bacterial belonged to Bacillus spp., 4 to green fluorescent Pseudomonas spp. and 2 were Streptomyces spp.
Analysis of Variance↗
Microbial community responses associated with the development of oomycete plant pathogens on tomato roots in soilless growing systems.
AIMS: To determine the spread of different oomycete pathogens in hydroponic, soilless tomato growing systems and their impact on established microbial communities, as baseline studies prior to future introduction of microbial inoculants for disease suppression. METHODS AND RESULTS: The oomycete pathogens, Pythium group F, Pythium aphanidermatum and Phytophthora cryptogea, were introduced into small-scale recirculating tomato growing systems containing rockwool 6 weeks after set-up when roots were well-established. Two weeks later, half of the systems were switched over to run-to-waste. Pythium aphanidermatum spreads the fastest, Pythium group F the slowest and Ph. cryptogea was intermediate in its spread. The switch to run-to-waste had no effect on pathogen recovery. Microbial communities, monitored by dilution plating, were well-established at the first sampling, 6 weeks after set-up and although differences in community levels were found between experiments, changes during any one experiment were small, generally less than 1 log10 CFU g(-1) for bacteria. Pathogen introduction increased microbial community levels in roots but the switch to run-to-waste had no effect. Analysis of bacterial communities through amplification of a fragment of the 16S rRNA gene and DGGE profiling showed that different communities were established within each pathogen experiment and that different communities were established on roots, rockwool and in nutrient solutions. However, no significant changes in microbial profiles were found over time in any experiment. CONCLUSIONS: In these systems, the microbial communities were well-established 6 weeks after set-up and were resistant to biological and physical perturbation. SIGNIFICANCE AND IMPACT OF THE STUDY: The implication for microbial inoculation of such systems for disease suppression is that the micro-organisms would either have to be introduced very early during the set-up of the system or be able to replace an established but variable community.
DNA, Fungal↗
Distribution of zoosporic fungi in the mud of major Egyptian lakes.
Twenty-six identified and three unidentified species belonging to eight genera of zoosporic fungi were recovered from seventy-six mud samples of Egyptian lakes using sesame seeds as baits. Nineteen samples were collected along the borders of four major lakes. Pythium, Phytophthora and Allomyces, yielded the highest spectra of zoosporic species, while Aqualinderella, Blastocladia and Dictyuchus, had one species. Pythium, Aphanomyces, Allomyces, Aqualinderella, Phytophthora and Saprolegnia were the dominant zoosporic genera while Blastocladia and Dictyuchus were rare. The most prevalent species of zoosporic fungi were Aqualinderella fermentans, Aphanomyces laevis and unidentified Pythium species while the other isolated species in this study were rare. Menzalah was the richest lake with zoosporic species followed by Borolous and Karoon. Some zoosporic fungal genera were recovered from the mud of the four tested lakes (participator genera) and others from three lakes or from only two lakes. Physicochemical characteristics of the mud samples (pH, total soluble salts and organic matter) revealed no correlation with the fungal occurrence and distribution in these lakes.
Egypt↗
Investigations into the biology of three 'phycomycotic' agents pathogenic for horses in Australia.
Although 'phycomycosis' is a common disease of horses in northern Australia little is known about the causative fungi. In this paper the laboratory methods for diagnosis are described. These revealed 38 cases caused by Pythium sp. (Hyphomyces destruens), 6 cases caused by Basidiobolus haptosporus and 2 caused by Conidiobolus coronatus. Laboratory studies on the chemotatic behaviour of zoospores of Pythium sp. showed that they were strongly attracted to both animal hairs and plant tissue. Because of this behaviour a simple baiting method using human hair was used to trap the fungus from water samples taken from different locations near Townsville, Queensland. B. haptosporus was isolated by indirect culture from the faeces of 5 bearded dragon lizards (Amphibolurus barbatus) captured near a small creek near Brisbane. By combining previously published information with data obtained in the present study, ecological life-cycles for Pythium sp., B. haptosporus and C. coronatus are proposed. They include mechanisms whereby horses may become infected.
Animals↗
Pythiaceous fungi associated with the decomposition of Nymphoides peltata.
Selective isolation procedures were carried out on decaying leaves of Nymphoides peltata (Gmel.) O. Kuntze in order to study the pythiaceous fungi (Peronosporales) associated with the decomposition processes. In addition to Pythium marsipium Drechsler and P. pleroticum T. Ito, a number of Pythium species were isolated with filamentous non-swollen sporangia. Under special culture conditions (in water under a day/night rhythm) sexual reproductive structures of P. diclinum Tokunaga and P. apleroticum Tokunaga were also obtained. However, several isolates designated as Pythium 'F', remained sterile. Within this group three types could be distinguished according to their temperature-growth relationships. As the species recorded have been found very rarely and only one was represented in the CBS collection, detailed descriptions with short discussions are presented.
Fungi↗
Wavelike distributions of infections by an introduced and naturally occurring root pathogen along wheat roots.
Previously, we showed that copiotrophic and oligotrophic bacteria fluctuate as moving waves along roots. These waves probably originate as a result of growth and death cycles at any location where a moving nutrient source passed. In this study, we placed sclerotia of Rhizoctonia solani AG8 along growing roots of wheat and showed that the proportions of root sections from which R. solani was isolated fluctuated with distance from the root tip. Similarly, proportions of root sections from which naturally occurring Pythium spp. were isolated fluctuated with distance from the root tip. Fourier analysis showed that these fluctuations constituted significant waves. Cross-correlation analyses demonstrated that there were negative correlations between R. solani infections and colony forming units of copiotrophic bacteria at the time of inoculation at the same locations on the root (lag = 0 cm), indicating that infection by R. solani could have been inhibited by these bacteria. There was a positive correlation between Pythium infections and copiotrophic bacteria at a lag of 6 cm along the roots. It therefore appears that Pythium infection took place shortly after the initial peak in copiotrophic bacteria following the passage of the root tip.
Colony Count, Microbial↗
Elicitor-mediated induction of tryptophan decarboxylase and strictosidine synthase activities in cell suspension cultures of Catharanthus roseus.
Treatment of one cell line (No. 615) of Catharanthus roseus c.v. Little Delicata with an elicitor preparation of autoclaved and homogenized Pythium aphanidermatum culture resulted in rapid accumulation of indole alkaloids. Alkaloid formation was preceded by rapid transient increases in the extractable activities of the enzymes tryptophan decarboxylase and strictosidine synthase. The induction of these two enzyme activities occurred when cells were transferred to alkaloid production medium or treatment with fungal elicitors. Treatment of this cell line with translational or transcriptional inhibitors prevented the Pythium-induced increases of enzyme activity as well as alkaloid accumulation. When cells were transferred to alkaloid production medium the induction of strictosidine synthase activity preceded that of tryptophan decarboxylase by many hours even when cells were also treated with Pythium elicitor. Results suggested that tryptophan decarboxylase induction proceeds only when endogenous tryptamine levels were decreased by two-third. The internal cellular level of tryptamine, therefore, could regulate expression of tryptophan decarboxylase, whereas induction of strictosidine synthase or of another enzyme in the biosynthetic pathway could control channeling of tryptamine into alkaloids. The results demonstrate that fungal elicitors can be used to facilitate studies of the factors which regulate expression of indole alkaloid pathway enzymes and their ultimate pathway products.
Alkaloids↗
Tropical disease in the immunocompromised host: melioidosis and pythiosis.
Melioidosis is an infection of humans and animals caused by a gram-negative motile bacillus, Pseudomonas pseudomallei. Forty-nine patients with melioidosis complicating diabetes mellitus, collagen vascular disorders, leukemia/lymphoma, and other hematologic malignancies are described. Twenty-nine of these patients had disseminated/septicemic infection, two developed toxic shock syndrome, and one with AIDS experienced recrudescent melioidosis. Patients with disseminated melioidosis often have a variety of defects in cellular immunity both in vitro and in vivo. In humans with recrudescent melioidosis, cellular immunity can be transferred by a transfer factor and by levamisole, a cellular immunopotentiating agent. The results of the treatment of our patients with disseminated/septicemic melioidosis with antimicrobial agents in combination have been successful. In recent years, four cases of fungal arteritis due to Pythium species and one case of keratitis due to Pythium were seen. Almost all patients with fungal arteritis had thalassemia; all presented with pain in the lower extremities and gangrenous lesions of the toes. Pythium species, an aquatic Phycomycetes, was identified in these cases as a human pathogen on the basis of clinical features, pathologic findings, and--of greatest importance--the isolation of the etiologic fungi. These five cases with remarkably similar presentations exhibited certain similarities with and differences from cases of mucormycosis, entomophthoromycosis, and peniciliosis.
Adolescent↗
Specificity of cytoplasmic and cell-wall antigens from four species of Phytophthora.
Cytoplasmic and cell-wall antigens and antisera were prepared from four Phytophthora species, and cell-wall antigens were prepared from two Pythium species. Immunodiffusion of the Pythium and Phytophthora cell-wall antigens showed that the two Pythium species did not cross-react with the Phytophthora cell-wall antisera. Immunodiffusion analysis of both cell-wall and cytoplasmic antigens of Phytophthora revealed some degree of specificity between species but not between A1 and A2 mating types in Phytophthora cinnamomi. Species specificity was improved by using indirect fluorescent antibody techniques and by the use of cross-absorbed sera. Agglutination and quantitative precipitation techniques did not significantly improve specificity. It was possible to distinguish serologically between Phytophthora cinnamomi and Phytophthora cambivora and the Phytophthora cryptogea-Phytophthora drechsleri group. The absence of consistent serological variation between P. cryptogea and P dreschsleri is consistent with the suggestion (Bumbieris, 1974) that P. cryptogea and P. drechsleri should be considered as one species.
Agglutination Tests↗
Human subcutaneous pythiosis.
Two cases of subcutaneous infection caused by the primitive aquatic hyphal organism Pythium are described. Pythium is an important pathogen of horses in the U.S.A. and Australia. Cases of human subcutaneous pythiosis have been cited in the literature, but clinical and histopathological features have not been described previously. Both cases occurred in young immunocompetent males in the periorbital region and showed rapid growth, clinically mimicking a tumor and requiring operative biopsy. In both cases there was a history of exposure to either swampy water or horses. The tissue reaction was distinctive, closely resembling that seen in equine pythiosis, comprising well-defined granular eosinophilic islands bordered by macrophages, multinucleate giant cells, fibrosis and numerous eosinophils. Hyphae were well demonstrated with the Grocott stain but only poorly with the PAS method. Identity of the organisms was confirmed with an immunoperoxidase technique employing a polyclonal antiserum to Pythium. Both patients responded well to amphotericin B.
Adolescent↗
Solar UV-B radiation inhibits the growth of Antarctic terrestrial fungi.
We tested the effects of solar radiation, and UV-B in particular, on the growth of Antarctic terrestrial fungi. The growth responses to solar radiation of five fungi, Geomyces pannorum, Phoma herbarum, Pythium sp., Verticillium sp., and Mortierella parvispora, each isolated from Antarctic terrestrial habitats, were examined on an agar medium in the natural Antarctic environment. A 3-h exposure to solar radiation of >287 nm reduced the hyphal extension rates of all species relative to controls kept in the dark. Pythium sp. cultures exposed to solar radiation for 1.5 h on five consecutive days were most sensitive to radiation of >287 nm, but radiation of >313 nm also inhibited growth to a lesser extent. Radiation of >400 nm had no effect on hyphal growth relative to controls kept in the dark. Short-wave solar UV-B radiation of between 287 and 305 nm inhibited the growth of Pythium sp. hyphae on and below the surface of the agar medium after 24 h, but radiation of > or =345 nm only reduced the growth of surface hyphae. Similar detrimental effects of UV-B on surface and, to a lesser extent, submerged hyphae of all five fungi were shown in the laboratory by using artificial UV-B from fluorescent lamps. A comparison of growth responses to solar radiation and temperature showed that the species that were most resistant to UV radiation grew fastest at higher temperatures. These data suggest that solar UV-B reduces the growth of fungi on the soil surface in the Antarctic terrestrial environment.
Antarctic Regions↗