Pythium Group Research Articles

Probability Models Based on Soil Properties for Predicting Presence-Absence of Pythium in Soybean Roots.

Associations between soil properties and Pythium groups on soybean roots were investigated in 83 commercial soybean fields in North Dakota. A data set containing 2877 isolates of Pythium which included 26 known spp. and 1 unknown spp. and 13 soil properties from each field were analyzed. A Pearson correlation analysis was performed with all soil properties to observe any significant correlation between properties. Hierarchical clustering, indicator spp., and multi-response permutation procedures were used to identify groups of Pythium. Logistic regression analysis using stepwise selection was employed to calculate probability models for presence of groups based on soil properties. Three major Pythium groups were identified and three soil properties were associated with these groups. Group 1, characterized by P. ultimum, was associated with zinc levels; as zinc increased, the probability of group 1 being present increased (α=0.05). Pythium group 2, characterized by Pythium kashmirense and an unknown Pythium sp., was associated with cation exchange capacity (CEC) (α<0.05); as CEC increased, these spp. increased. Group 3, characterized by Pythium heterothallicum and Pythium irregulare, were associated with CEC and calcium carbonate exchange (CCE); as CCE increased and CEC decreased, these spp. increased (α=0.05). The regression models may have value in predicting pathogenic Pythium spp. in soybean fields in North Dakota and adjacent states.

Pythium and Phytopythium species associated with hydroponically grown crops around the City of São Paulo, Brazil

A survey of Pythium and Phytopythium species was conducted in hydroponics systems located in the metropolitan region of Sao Paulo and nearby cities, SP, Brazil. Different samples including water from artesian well, nutrient solution, substrates used for seedlings production and roots of hydroponically grown crops were collected and baited for Pythium and Phytopythium recovery. It was possible to isolate Pythium species from the four types of samples while Phytopythium species were isolated only from the substrates. The species were identified according to their morphological features and phylogenetic placement in a combined analysis of the ITS rDNA and COI mDNA regions within eight different taxa: Pythium kashmirense, P. middletonii, P. oopapillum, P. periplocum, P. segnitium, Pythium group F, Phytopythium helicoides and Phy. vexans. The isolates identified as Pythium group F were the most frequent species. While Pythium species were distributed in clades B, D and E, Phytopythium were distributed in clades 2 and 3. This study brings important information about the diversity of these species in hydroponic systems in Brazil and provides sequences of species rarely reported such as P. kashmirense, P. oopapillum and P. segnitium.

Pythium kandovanense sp. nov., a fungus-like eukaryotic micro-organism (Stramenopila, Pythiales) isolated from snow-covered ryegrass leaves.

Pythiumkandovanense sp. nov. (ex-type culture CCTU 1813T = OPU 1626T = CBS 139567T) is a novel oomycete species isolated from Lolium perenne with snow rot symptoms in a natural grassland in East-Azarbaijan province, Iran. Phylogenetic analyses based on sequence data from internal transcribed spacer (ITS)-rDNA, coxI and coxII mitochondrial genes clustered our isolates in Pythium group E as a unique, well supported clade. Pythium kandovanense sp. nov. is phylogenetically and morphologically distinct from the other closely related species in this clade, namely Pythium rostratifingens and Pythium rostratum. Pythium kandovanense sp. nov. can be distinguished from these two species by its cylindrical sporangia and lower temperatures for optimum and maximum growth rate. The development of zoospores released through a shorter discharge tube is an additional morphological feature which can be used to differentiate Pythium kandovanense sp. nov. from Pythium rostratifingens. Laboratory inoculation tests demonstrated the pathogenicity of Pythium kandovanense sp. nov. to L. perenne under wet cold (0-3 °C) conditions.

Structure and organization of the rDNA intergenic spacer region in Pythium ultimum

Nucleotide sequences of the rDNA intergenic spacer (IGS) region in Pythium ultimum were determined in 16 clones obtained from three isolates differing in production of sexual organs. Several sequences with different lengths were detected in each isolate, showing heterogeneity in the IGS region. In addition, several tandem repeat regions were detected in all the clones. The sequences, length, and number of each copy largely varied among repeat regions. Length heterogeneity arose from the complex combination of the number of copy within the repeat regions. Furthermore, the nucleotide sequence of each copy and the number of repetition varied not only between isolates but also between clones from an isolate. Based on the sequence similarity and the number of copies in repeat regions, specific patterns different between homothallic P. ultimum and the Pythium group HS (hyphal swellings) were recognized in a few regions. These results suggest that these two groups have slight genetic differences in the IGS region, although the differences in most of the repeat regions were not enough to identify each group.

Etiology and epidemiology of Pythium root rot in hydroponic crops: current knowledge and perspectives

The etiology and epidemiology of Pythium root rot in hydroponically-grown crops are reviewed with emphasis on knowledge and concepts considered important for managing the disease in commercial greenhouses. Pythium root rot continually threatens the productivity of numerous kinds of crops in hydroponic systems around the world including cucumber, tomato, sweet pepper, spinach, lettuce, nasturtium, arugula, rose, and chrysanthemum. Principal causal agents include Pythium aphanidermatum, Pythium dissotocum, members of Pythium group F, and Pythium ultimum var. ultimum. Perspectives are given of sources of initial inoculum of Pythium spp. in hydroponic systems, of infection and colonization of roots by the pathogens, symptom development and inoculum production in host roots, and inoculum dispersal in nutrient solutions. Recent findings that a specific elicitor produced by P. aphanidermatum may trigger necrosis (browning) of the roots and the transition from biotrophic to necrotrophic infection are considered. Effects on root rot epidemics of host factors (disease susceptibility, phenological growth stage, root exudates and phenolic substances), the root environment (rooting media, concentrations of dissolved oxygen and phenolic substances in the nutrient solution, microbial communities and temperature) and human interferences (cropping practices and control measures) are reviewed. Recent findings on predisposition of roots to Pythium attack by environmental stress factors are highlighted. The commonly minor impact on epidemics of measures to disinfest nutrient solution as it recirculates outside the crop is contrasted with the impact of treatments that suppress Pythium in the roots and root zone of the crop. New discoveries that infection of roots by P. aphanidermatum markedly slows the increase in leaf area and whole-plant carbon gain without significant effect on the efficiency of photosynthesis per unit area of leaf are noted. The platform of knowledge and understanding of the etiology and epidemiology of root rot, and its effects on the physiology of the whole plant, are discussed in relation to new research directions and development of better practices to manage the disease in hydroponic crops. Focus is on methods and technologies for tracking Pythium and root rot, and on developing, integrating, and optimizing treatments to suppress the pathogen in the root zone and progress of root rot.

Microbial community responses associated with the development of oomycete plant pathogens on tomato roots in soilless growing systems

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. 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. In these systems, the microbial communities were well-established 6 weeks after set-up and were resistant to biological and physical perturbation. 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.

Molecular characterization of Pythium group F isolates by ribosomal-and intermicrosatellite-DNA regions analysis

Pythium group F is a ubiquitous, though minor, pathogen in several soilless and soil cultures; investigations were carried out to analyze different regions of the DNA and better understand the nature of this group. Fourty-two isolates were obtained from a variety of plants (cucumber, lettuce, tomato) grown in soil or soilless cultures collected in various countries (Canada, Denmark, France, Norway, Sweden and United Kingdom). All Pythium group F isolates displayed amplified ITS1-5,8S-ITS2 ribosomal DNA region (rDNA) of similar length, whereas polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) revealed that, among the seven enzymes used, polymorphism was only identified with Hin6I. After cloning of ITS1-5,8S-ITS2 rDNA region from Pythium group F isolates that displayed restriction polymorphism patterns with Hin6I, comparisons of sequence and restriction mapping data showed a slight variation consisting in a single base change. Inter Simple Sequence Repeat (ISSR)-PCR method was also used to obtain data related to the entire genome and not only to a single DNA region. It identified repeated motifs in the genome of Pythium group F isolates. Two primers (CAC)5 and (CCA)5 detected polymorphism, and isolates were classified among 11 molecular clusters. The genetic diversity of this group was not correlated with the geographical locations or the host plants from which the isolates originated. Polymorphism of Pythium group F isolates pointed out by ISSR is discussed

Fungi associated with roots of cucumber grown in different greenhouse root substrates

Hydroponic soilless culture systems are commonly used in the greenhouse industry to mitigate soilborne root pathogens, but root diseases continue to be a major problem. A thorough knowledge of the root zone ecology of greenhouse crops is needed for the successful biological control of root diseases. The objectives of this study were to characterize the fungi associated with symptomless cucumber roots grown using different greenhouse substrates and to determine the effects of these fungi on plant growth. A total of 1250 fungal isolates were collected from cucumber roots grown in soil, rockwool, sawdust, or nutrient film and identified to genus and species. There was a greater density of fungal colonies on roots grown in soil than on roots grown in other substrates. Penicillium (87.2% of all isolates), Trichoderma (4.6%), and Pythium (3.0%) were the most common genera isolated from the cucumber roots. The two most common species were Penicillium oxalicum Currie &amp; Thom (69.9%) and Penicillium janthinellum Biourge(13.4%). Pythium group F, Pythium group G, Pythium aphanidermatum (Edson) Fitzp., and Pythium irregulare Buisman were also isolated from healthy cucumber roots and all except Pythium group F were pathogenic to germinating cucumber seedlings. Treatment of cucumber seeds with Penicillium spp. resulted in significantly greater shoot length, root length, and seedling fresh weight of the resulting cucumber seedlings than did treatment with Trichoderma spp. One isolate of Penicillium janthinellum and one of Aspergillus sp. significantly improved survival of cucumber seedlings growing in a Pythium aphanidermatum-infested medium.Key words: hydroponics, Penicillium, Trichoderma, Pythium, Cucumis sativus.

Identification and pathogenicity of Pythium species causing damping-off in sugarbeet in northwest Iran

Seventy-eight Pythium isolates were obtained from diseased sugarbeet seedlings that were planted in soil samples collected from different sugarbeet fields in northwest Iran. P. ultimum var. ultimum was the dominant species, representing 52.6% of isolates, and P. aphanidermatum, P. deliens, P. oligandrum, P. ostracodes, Pythium group HS and Pythium group T were the other species identified. Five other isolates could not be identified and were designated as Pythium sp. In the greenhouse pathogenicity test, P. ostracodes and two isolates of P. oligandrum and Pythium sp. were non-pathogenic and four isolates belonging to P. aphanidermatum, P. ultimum var. ultinmum and Pythium group HS were very highly pathogenic. The remaining 24 isolates were weakly, moderately or highly pathogenic. This is the first report of isolation of Pythium group T and P. ostracodes from sugarbeet and the pathogenicity of the Pythium group T on sugarbeet. It is also the first report of P. paroecandrum and Pythium group HS being isolated from sugarbeet and their pathogenicity on sugarbeet in Iran. Pythium group T is a new record for Iran.

The characterization of Pythium group G occurring on kenaf in South Africa

Kenaf provides a source of high quality cellulose fibers and its cultivation is being investigated in South Africa with a view to commercial production. In March 2001, kenaf plants in experimental plots near Bloemfontein, South Africa, developed large, black sunken lesions (10–20 cm long) at the base of the stem, and severe root rot. The causal agent was subsequently identified as Pythium group G and various studies were undertaken to characterize the pathogen and the disease it causes on kenaf. The relative susceptibilities of the kenaf cultivars available for commercial cultivation were determined by means of artificial inoculations in the glasshouse. Mean lesion lengths ranging from 43.4 mm to 58.3 mm with an overall mean of 52.7 mm, were obtained seven days after inoculation of potted plants grown in the glasshouse. Significant differences (P < 0.05) were found between the mean lesion lengths of the 10 inoculated cultivars. Everglades 41 had the longest mean lesion length (58.3mm) while Everglades 71 had the shortest (43.1 mm). The ability of Pythium group G to cause damping- off on kenaf was tested by artificially inoculating seedlings in the glasshouse. Mean damping-off of seedlings ranged from 35–60% and no significant (P < 0.05) difference in susceptibility was observed between cultivars. The optimum temperature range for growth of Pythium group G was 20–30°C. Six fungicides were screened in order to determine their inhibitory effect on the in vitro radial growth of an isolate of Pythium group G. Dichlorophen and mancozeb/metalaxyl were the most inhibitory. This study has important implications for the cultivation of kenaf in South Africa.

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.

Occurrence of Pythium spp. and Phytophthora spp. in Norwegian Greenhouses and their Pathogenicity on Cucumber Seedlings

AbstractSamples of tomato, lettuce and cucumber submitted for diagnosis to the Plant Protection Centre at the Norwegian Crop Research Institute and samples of soil, water and cucumber collected from greenhouses employing hydroponic cultures were examined for the occurrence of Pythium spp. and Phytophthora spp. Two species of Phytophthora and 16 species of Pythium were identified. Phytophthora cryptogea was found on tomato and lettuce. Phytophthora nicotianae was found on tomato fruit. Phytophthora was not found on cucumbers. Pythium irregulare and Pythium group F were the two most commonly found Pythium species in hydroponically cultivated cucumbers. A pathogenicity test with 56 isolates was performed on cucumber seedlings. The most aggressive species were Pythium aphanidermatum, P. irregulare, Pythium paroecandrum and Pythium ultimum.

Comparison of serological, culture, and bait methods for detection of Pythium and Phytophthora zoospores in water

Two immunodiagnostic detection assay procedures were compared with two conventional assays for their sensitivity in detecting propagules of Pythium ultimum var. sporangiiferum, Pythium Group F, Phytophthora cactorum and P. cryptogea in dilution series in sterile distilled water. The most sensitive assay for all four species was the zoospore trapping immunoassay (ZTI). Conventional membrane filtration‐dilution plating gave similar results to ZTI with the two Phytophthora spp., but was less sensitive in Pythium detection. Immunodiagnostic dipstick assays and conventional bait tests showed similar sensitivities in the dilution series, and were generally about two orders of magnitude less sensitive than ZTI. The four techniques were also compared for their detection efficacy with water samples collected from horticultural nurseries and in in situ tests of infected root zones of Chamaecyparis, tomato and Chrysanthemum. In these comparisons, ZTI was again the most sensitive test for water samples, although membrane filtration‐dilution plating proved to be a more consistent test. Dipstick and baiting assays were the best techniques for in situ testing, and dipsticks provided epidemiologically valuable, quantitative data on pathogen propagule numbers.

Pythium spp. Infecting Hydroponically Grown Lettuce in South Africa.

Root rot and wilting of lettuce (Lactuca sativa L. var. capitata L.) commonly occur in hydroponic systems in South Africa. Plants showing stunted growth, wilt, and root rot were collected at two commercial hydroponic systems in Gauteng Province, and root segments were plated on a Pythium selective medium (1). P. irregulare and three species groups without oogonia (F, HS, and T) were identified (2). Pythium groups F, HS, and T, and P. irregulare were represented by 63, 6, 5, and 26% of the isolates, repectively. Koch's postulates were confirmed in two greenhouse experiments by inoculating 4-week-old lettuce cv. Lutetia seedlings grown in aerated nutrient solution at 21°C. Two V8 juice agar cultures of an isolate of each species or species group were blended in 500 ml of sterile water, and 166 ml of suspension was added to the nutrient solution in each of three 5-liter receptacles containing four lettuce seedlings. Control plants received sterile V8 juice agar suspension. After 4 weeks, all Pythium isolates caused root rot and reduced fresh plant weight significantly, although no wilting occurred. Symptoms were more severe with P. irregulare and Pythium group HS than with groups F and T. Compared to the control, P. irregulare reduced fresh root and shoot weight of plants by 51 and 38%, Pythium group HS by 41 and 33%, Pythium group T by 29 and 26%, and Pythium group F by 30 and 24%, respectively.

First Report of Basal Stem Rot Caused by Pythium Group G on Kenaf in South Africa.

Kenaf (Hibiscus cannabinus L. Malvaceae) presents a source of high-quality cellulose fibers and is being investigated in South Africa for commercial production. In March 2001, 12 5-month-old kenaf plants grown from seed in experimental plots near Bloemfontein, South Africa, displayed large, black, sunken lesions (10 to 20 cm long) at the base of the stem, and severe root rot. A study was undertaken to characterize the pathogen, and to determine the relative susceptibilities of five kenaf genotypes being considered for commercial cultivation. Isolations from diseased tissue on malt extract agar consistently yielded a fungus identified as Pythium group G (1). Four-month-old kenaf plants were artificially inoculated in the field by inserting wooden toothpick tips colonized by the pathogen approximately 25 cm above soil level into the stems of 10 plants of each of five genotypes. Inoculation points were wrapped using Parafilm. The fungus was highly virulent to all five kenaf genotypes in two experiments, with mean cambial lesion lengths of 117, 119, 120, 122, and 139 mm at 7 days after inoculation for Tainung-2, Cuba 108, SF-459, El Salvador, and Everglades 41, respectively. Lesions ranged from 44 to 164 mm, with an overall mean of 124 mm for all five genotypes. No lesions developed in control plants. Although Everglades had the longest lesions, there were no significant differences (P < 0.05) among genotypes. Koch's postulates were completed by reisolating the fungus from all inoculated plants. To our knowledge, there are no published reports of Pythium group G causing stem or root rot of kenaf.

Production of indole-3-acetic acid and tryptophol by Pythium ultimum and Pythium group F: Possible role in pathogenesis

Pythium group F is a minor pathogen which induces symptomless infections that occur frequently and results in yield losses in tomato soilless cultures. To elucidate the mode of action of this microorganism, the influence of culture filtrates of Pythium group F on tomato growth was investigated and compared to that of the pathogen Pythium ultimum. Depending on metabolite production by the fungus, marked differences were observed in plant response. Pythium group F crude culture filtrates or low molecular weight fractions (< 500) caused swelling behind the root tip and reduced root growth; the cohesion and adherence of cells within the cortical area were also affected. These symptoms were similar to those observed on plants treated with indole-3-acetic acid. By contrast, P. ultimum filtrates caused a marked distortion of cell shape accompanied with folding of host cell walls, particularly in the cortical area. These symptoms were characteristic of the activity of toxic compound(s) on host cells. Chemical analysis of the filtrates demonstrated that indole-3-acetic acid and tryptophol were produced by Pythium group F and P. ultimum. However, Pythium group F took up and metabolized more indole-3-acetic acid precursors, especially tryptophan, a key amino acid in the pathways leading to indole-3-acetic acid synthesis. The fact that Pythium group F and P. ultimum transformed tryptamine and indole-3-acetaldehyde into indole-3-acetic acid and tryptophol confirms the existence of a tryptamine pathway within both fungi. These results support the hypothesis that auxins facilitate Pythium group F infections. On the other hand, toxin(s) and hydrolytic enzymes are likely involved in P. ultimum pathogenesis.

Ultrastructural and Cytochemical Investigation of Asymptomatic Infection by Pythium spp.

ABSTRACT The influence exerted by Pythium group F (a minor pathogen ubiquitous in soilless cultures) and P. uncinulatum (a nonpathogenic species) colonization on tomato roots was investigated. In both interactions, infected roots did not exhibit obvious symptoms; however, major physiological changes occurred within the host tissues colonized by Pythium group F compared to P. uncinulatum. According to our cytological observations, Pythium group F colonization involved a series of events: first, development and growth of the fungus in the epidermis and outer cortex tissues, which was associated with marked host cell disorganization and even breakdown. In colonized roots, symptoms were not easily discernible because alterations were restricted to the epidermis and outer cortex tissues. Second, pathogen ingress in the inner cortex and stele tissues was associated with massive induction of host defense reactions and alteration of invading hyphae. In a complex interaction that involved major metabolic changes in root cells, an array of defense-related reactions was produced, as exemplified by the formation of wall appositions and plugging of host cells with osmiophilic, electron-dense, granular, or fibrillar materials. P. uncinulatum growth was restricted to the epidermis and outer cortex tissues and associated with relatively minor damage to host cells. P. uncinulatum colonization of root tissues did not result in defense events similar to those observed in Pythium group F-infected inner cortex and stele tissues. Moreover, most of the invading P. uncinulatum hyphae were moribund. The results of the current study indicated that symptomless infections can be insignificant in terms of pathology for the plant (i.e., P. uncinulatum) or can be potentially dangerous (i.e., Pythium group F). Our results clearly showed a striking difference between nonpathogenic and minor pathogenic Pythium spp. attacks. The description of the Pythium group F infection process suggests that this interaction is unique, because the fungus behaves as a necrotrophic fungus in the epidermis and outer cortex tissues, whereas it is a potential inducer of plant defense reactions in the inner cortex and stele tissues. To our knowledge, cytological events similar to those observed with Pythium group F have not been described until now, even though descriptions of these events could help elucidate several aspects of the relationships between plants and minor pathogens.

Detection of Pythium ultimum Using Polymerase Chain Reaction with Species-Specific Primers.

This study was conducted to sequence the rDNA internal transcribed spacer (ITS) region of Pythium ultimum and Pythium group HS, design species-specific primers for polymerase chain reaction (PCR), and detect P. ultimum from diseased seedlings using PCR. The sequence of the ITS region of P. ultimum was identical with that of Pythium group HS. The results support the reports that the HS group is an asexual strain of P. ultimum. Using PCR, the primer pair K1+K3, designed on portions of the sequence of the ITS region, amplified isolates of P. ultimum and the HS group but not isolates of 20 other Pythium species. DNA extracts from damped-off seedlings were not amplified, but a 10-fold dilution of the extracts with Tris-EDTA (TE) buffer diluted the inhibitors and allowed PCR amplification. The primer pair used detected P. ultimum from a single diseased seedling.

Isolation, identification and pathogenicity of Phytophthora species from declining oak stands

SummaryWithin the scope of a research project on the condition of roots of declining oaks (Quercus robur, Quercus petraea), samples of fine roots and surrounding soil, specimens of stripe cankers on the stem base, and samples of stream water were examined for the presence of Phytophthora species using both baiting methods and selective agar media. At 27 sites in Germany (Bavaria, Rheinland‐Pfalz, Schleswig‐Holstein), Switzerland, Hungary, Italy and Slovenia the following species were isolated (mainly from soil): Phytophthora citricola, Phytophthora cactorum, Phytophthora cambivora, Phytophthora gon‐apodyides, Phytophthora undulata, a species with affinity to Phytophthora drechsleri, and two additional species with close affinity to the Phytophthora cactorum group. Moreover, Pythium group P, Pythium anandrum, Pythium chamaehyphon, and many other Pythium species that have not yet been identified could be recovered. In a soil infestation test most isolates induced dieback of long root tips and necrotic lesions in the root cortex and at the root collar of Quercus robur seedlings. All Phytophthora species tested and Pythium group P caused cortical necrosis after stem inoculation of young Quercus robur trees. It could be shown in vitro that Phytophthora gonapodyides and Pythium group P were able to produce a wilting toxin. Nitrogen input and climatic changes are discussed as predisposing factors for root damage observed in the field.

Self-Sterile Isolates of Pythium Mate with Self-Fertile Isolates of Pythium ultimum

Self-sterile isolates of Pythium (type S or Pythium group HS) that possessed cultural growth characteristics and morphologies similar to P. ultimum var. ultimum produced antheridia and mated successfully when paired with self-fertile (type O or homothallic) isolates of this species and P. ultimum var. sporangiiferum. The self-sterile isolates produced antheridia that fused with oogonia produced by self-fertile isolates of P. irregulare but the oogonia aborted. The self-sterile isolates were identified as male forms of P. ultimum, since they exclusively formed antheridia when paired with self-ferile isolates of this species. Progenies from crosses between male and self-fertile isolates displayed traits of recombination in culture, indicating normal sexual reproduction occurred during mating.