Host Hyphae Research Articles

Reduction of Pythium Damping-Off in Soybean by Biocontrol Seed Treatment.

Pythium spp. is one of the major groups of pathogens that cause seedling diseases on soybean, leading to both preemergence and postemergence damping-off and root rot. More than 100 species have been identified within this genus, with Pythium irregulare, P. sylvaticum, P. ultimum var ultimum, and P. torulosum being particularly important for soybean production given their aggressiveness, prevalence, and abundance in production fields. This study investigated the antagonistic activity of potential biological control agents (BCAs) native to the U.S. Midwest against Pythium spp. First, invitro screening identified BCAs that inhibit P. ultimum var. ultimum growth. Scanning electron microscopy demonstrated evidence of mycoparasitism of all potential biocontrol isolates against P. ultimum var. ultimum and P. torulosum, with the formation of appressorium-like structures, short hyphal branches around host hyphae, hook-shaped structures, coiling, and parallel growth of the mycoparasite along the host hyphae. Based on these promising results, selected BCAs were tested under field conditions against six different Pythium spp. Trichoderma afroharzianum 26 used alone and a mix of T. hamatum 16 + T. afroharzianum 19 used as seed treatments protected soybean seedlings from Pythium spp. infection, as BCA-treated plots had on average 15 to 20% greater plant stand and vigor than control plots. Our results also indicate that some of these potential BCAs could be added with a fungicide seed treatment with minimum inhibition occurring, depending on the fungicide active ingredient. This research highlights the need to develop tools incorporating biological control as a facet of soybean seedling disease management programs. The harnessing of native BCAs could be integrated with other management strategies to provide efficient control of seedling diseases.

The antagonistic effect of locally isolated Trichoderma spp. against dry root rot of mungbean

The current investigation deals with the isolation, identification and testing the antagonistic ability of Trichoderma spp. on Macrophomina phaseolina under in vitro and in vivo conditions. The locally isolated biocontrol agents were subjected to morphological and molecular identification and further research was carried out to test all the biocontrol agents. T. harzianum was found to be highly effective in dual confrontation test, restricting the fungal growth by 67.59% as compared to control. Various secondary metabolites of Trichoderma spp. were studied with inverted plate and culture filtrate assay. Mycoparasitism and antibiosis were also examined using light and Scanning Electron Microscopy (SEM) which indicated that all the Trichoderma spp. restricted the growth by coiling around the host hyphae and through secondary metabolites. In vivo experiments showed that all the treatments significantly reduced the disease severity and intensity where T. harzianum was found to be the most effective biocontrol agent (decreased upto 72.88 and 60.24%). Nevertheless, T. harzianum could be used as a potent indigenous biocontrol agent against dry root rot which also helped to reduce its deleterious effect on the host plant.

Talaromyces pinophilus strain AUN-1 as a novel mycoparasite of Botrytis cinerea, the pathogen of onion scape and umbel blights

This study aimed to investigate the mycoparasitism of Botrytis cinerea, the pathogen of scape and umbel blights of onion seed crops, by endophytic Talaromyces pinophilus. The dual culture test showed that the antagonistic potentiality of T. pinophilus against B. cinerea depend on the mycoparasitism that was morphologically detected by the formation of mycelial overgrowth. Moreover, the light micrograph of the mycelia at the contact zone exhibited that the hyphae of T. pinophilus penetrated and grew intracellularly inside the hyphae of B. cinerea. A more illustrative figure of the establishment of coiled hyphae as well as the conformation of the penetration process was assayed by SEM and TEM analyses. SEM micrograph revealed that the hyphae of T. pinophilus grew along hyphae of B. cinerea, attached, coiled around the host hypha and generated pseudoappressorium. A clear disintegration of cell wall of the host hypha was observed at the penetration site. The micrographs of TEM exhibited the ability of T. pinophilus to produce pseudoappressorium, penetrate and then entere a hypha of B. cinerea causing distinct cytoplasmic disorganization. High activities of cell wall degrading enzymes (chitinase, lipase and protease) involved in the mycoparasitism were evaluated by the endophytic T. pinophilus. In conclusion, this study demonstrated that the endophytic T. pinophilus may be a promising biocontrol agent against phytopathogenic fungi instead of chemical fungicides.

Ultrastructural characterization of the host–parasite interface between Allomyces anomalus (Blastocladiomycota) and Rozella allomycis (Cryptomycota)

Rozella allomycis is an obligate endoparasite of the water mold Allomyces and a member of a clade (= Opisthosporidia) sister to the traditional Fungi. Gaining insights into Rozella's development as a phylogenetically pivotal endoparasite can aid our understanding of structural adaptations and evolution of the Opisthosporidia clade, especially within the context of genomic information. The purpose of this study is to characterize the interface between R. allomycis and Allomyces anomalus. Electron microscopy of developing plasmodia of R. allomycis in host hyphae shows that the interface consists of three-membrane layers, interpreted as the parasite's plasma membrane (inner one layer) and a host cisterna (outer two layers). As sporangial and resting spore plasmodia develop, host mitochondria typically cluster at the surface of the parasite and eventually align parallel to the three-membrane layered interface. The parasite's mitochondria have only a few cristae and the mitochondrial matrix is sparse, clearly distinguishing parasite mitochondria from those of the host. Consistent with the expected organellar topology if the parasite plasmodia phagocytize host cytoplasm, phagocytic vacuoles are at first bounded by three-membrane layers with host-type mitochondria lining the inner membrane. Thus, Rozella's nutrition, at least in part, is phagotrophic in contrast to osmotrophic nutrition of traditional fungi.

Mycoparasitic Scytalidium parasiticum as a potential biocontrol agent against Ganoderma boninense basal stem rot in oil palm

ABSTRACTThe objective of this study was to assess the interactions between Scytalidium parasiticum (Sp) and Ganoderma boninense, the causal agent of basal stem rot (BSR) in oil palm (Elaeis guineensis). When compared with Scytalidium ganodermophthorum and Scytalidium sphaerosporum, Sp showed greater inhibition towards all Ganoderma isolates during dual-culture assays. At the interaction zone, coiling of host hyphae, formation of short lateral enlarged contact structures, and production of appressorium-like organs organs were observed in Sp on G. boninense. These were followed by the degradation, shrinkage, and deformation of G. boninense mycelia. Sp reduced mycelial survival and fruiting body regeneration of G. boninense. Sp's non-volatile metabolites suppressed the growth of G. boninense. Our results show that Sp could be a necrotrophic mycoparasite of G. boninense. Nursery experiments revealed that Sp was non-pathogenic to oil palm seedlings, and it could suppress Ganoderma infection and reduce disease severity. Sp increased the height of palms in the positive control with non-Ganoderma-inoculated rubber wood block and Sp inoculum compared to similar control without Sp. Leaf area was greater in the G. boninense G8 inoculated palms when Sp was present compared to without Sp. These results show that Sp might be a potential biocontrol candidate against BSR.

Plant Growth Promotion and Induced Defense Response in Safflower (<I>Carthamus tinctorius</I> L.) by <I>Trichoderma</I>

Nine potential <em>Trichoderma</em> strains were tested for mycoparasitic, defence enzyme activity and root colonizing behaviour against <em>Macrophomina phaseolina</em> and <em>Fusarium oxysporum</em> f. sp. <em>carthami</em> in safflower (<em>Carthmus tinctorius</em> L). Among them three strains viz., <em>T. harzianum</em> Th4d, <em>T. asperellum</em> TaDOR7316 and T. asperellum Tv5 were found to be most effective showing superior antagonistic activity. Hyphal interaction studies revealed that the inhibition was caused by an interaction that took place in close contact with the host hypha, causing lysis, swelling and coiling of mycelia resulting potentially reduced mycelial growth of <em>M. phaseolina</em> and showed lytic enzymes activity to various extent in Th4d, Tv5 and TaDOR 7316. These strains were also able to solubilize inorganic (P). Increased activity of defense related enzymes viz., peroxidase (PO), polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL) activity in these three potential strains pre-treated safflower plants challenged with M. phaseolina was observed. PO, PPO and PAL activity was also increased two-three folds more in all these bioagents. Interaction between the bioagents and the safflower root system showed profuse adhesion of hyphae to the plant roots as well as colonization of the root epidermis and cortex cells but not the vessels at early stages of safflower root system. Levels of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) in the bioagents treated leaves and untreated (control) were determined microscopically. Application of these bioagents under field conditions reduced the incidence of root rot and Fusarium wilt, increased growth and plant biomass to a reasonable extent with better root colonization, which is directly correlated with the resistance of the plant against infection and high seed yield, was observed with bioagents treatment. Thus, it is evident that the hyphal interaction and enzymes play a key role to stimulate the defense mechanism which aid in disease management as well as plant growth promotion of the host plant against pathogen attack.

Plant Growth Promotion and Induced Defense Response in Safflower (<I>Carthamus tinctorius</I> L.) by <I>Trichoderma</I>

Nine potential Trichoderma strains were tested for mycoparasitic, defence enzyme activity and root colonizing behaviour against Macrophomina phaseolina and Fusarium oxysporum f. sp. carthami in safflower ( Carthmus tinctorius L). Among them three strains viz., T. harzianum Th4d, T. asperellum TaDOR7316 and T. asperellum Tv5 were found to be most effective showing superior antagonistic activity. Hyphal interaction studies revealed that the inhibition was caused by an interaction that took place in close contact with the host hypha, causing lysis, swelling and coiling of mycelia resulting potentially reduced mycelial growth of M. phaseolina and showed lytic enzymes activity to various extent in Th4d, Tv5 and TaDOR 7316. These strains were also able to solubilize inorganic (P). Increased activity of defense related enzymes viz., peroxidase (PO), polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL) activity in these three potential strains pre-treated safflower plants challenged with M. phaseolina was observed. PO, PPO and PAL activity was also increased two-three folds more in all these bioagents. Interaction between the bioagents and the safflower root system showed profuse adhesion of hyphae to the plant roots as well as colonization of the root epidermis and cortex cells but not the vessels at early stages of safflower root system. Levels of hydrogen peroxide (H 2 O 2 ) in the bioagents treated leaves and untreated (control) were determined microscopically. Application of these bioagents under field conditions reduced the incidence of root rot and Fusarium wilt, increased growth and plant biomass to a reasonable extent with better root colonization, which is directly correlated with the resistance of the plant against infection and high seed yield, was observed with bioagents treatment. Thus, it is evident that the hyphal interaction and enzymes play a key role to stimulate the defense mechanism which aid in disease management as well as plant growth promotion of the host plant against pathogen attack.

The Cerato-Platanin protein Epl-1 from Trichoderma harzianum is involved in mycoparasitism, plant resistance induction and self cell wall protection.

Trichoderma harzianum species are well known as biocontrol agents against important fungal phytopathogens. Mycoparasitism is one of the strategies used by this fungus in the biocontrol process. In this work, we analyzed the effect of Epl-1 protein, previously described as plant resistance elicitor, in expression modulation of T. harzianum genes involved in mycoparasitism process against phytopathogenic fungi; self cell wall protection and recognition; host hyphae coiling and triggering expression of defense-related genes in beans plants. The results indicated that the absence of Epl-1 protein affects the expression of all mycoparasitism genes analyzed in direct confrontation assays against phytopathogen Sclerotinia sclerotiorum as well as T. harzianum itself; the host mycoparasitic coiling process and expression modulation of plant defense genes showing different pattern compared with wild type strain. These data indicated the involvement T. harzianum Epl-1 in self and host interaction and also recognition of T. harzianum as a symbiotic fungus by the bean plants.

Trichoderma- Foliar Pathogen Interactions

Among the group of ubiquitous soil inhabiting fungi, Trichoderma species are of considerable commercial importance due to their ability to suppress many plant pathogenic fungi. They have been widely studied and employed for management of root and seedling diseases of many crops since decades. In recent years they are also being utilized for minimising diseases of foliar plant parts, especially to supress grey mold caused due to infections of Botrytis cinerea and powdery mildews on a number of crops. They also express potential to control downy mildews and have improved the shelf life of grapes. Out of the eighty-nine species of Trichoderma, a number of species exhibit the bio-control activity. The destruction of host hyphae and protoplasm by the various lytic enzymes and toxic volatile and non-volatile compounds produced by these species are the major mechanism of bio-control; apart from deactivation of pathogen enzymes and induction of systemic resistance in host plants. However, significant differences in antagonistic potential among isolates even from the same species from the same niche is observed; and hence selection of a strain with wide host range and environment adaptability becomes a key issue for sustainable biological control.

The seven-transmembrane receptor Gpr1 governs processes relevant for the antagonistic interaction of Trichoderma atroviride with its host

Mycoparasitic Trichoderma species are applied as biocontrol agents in agriculture to guard plants against fungal diseases. During mycoparasitism, Trichoderma directly interacts with phytopathogenic fungi, preceded by a specific recognition of the host and resulting in its disarming and killing. In various fungal pathogens, including mycoparasites, signalling via heterotrimeric G proteins plays a major role in regulating pathogenicity-related functions. However, the corresponding receptors involved in the recognition of host-derived signals are largely unknown. Functional characterization of Trichoderma atroviride Gpr1 revealed a prominent role of this seven-transmembrane protein of the cAMP-receptor-like family of fungal G-protein-coupled receptors in the antagonistic interaction with the host fungus and governing of mycoparasitism-related processes. Silencing of gpr1 led to an avirulent phenotype accompanied by an inability to attach to host hyphae. Furthermore, gpr1-silenced transformants were unable to respond to the presence of living host fungi with the expression of chitinase- and protease-encoding genes. Addition of exogenous cAMP was able to restore host attachment in gpr1-silenced transformants but could not restore mycoparasitic overgrowth. A search for downstream targets of the signalling pathway(s) involving Gpr1 resulted in the isolation of genes encoding e.g. a member of the cyclin-like superfamily and a small secreted cysteine-rich protein. Although silencing of gpr1 caused defects similar to those of mutants lacking the Tga3 Gα protein, no direct interaction between Gpr1 and Tga3 was observed in a split-ubiquitin two-hybrid assay.

Sphaerodes quadrangularis biotrophic mycoparasitism on Fusarium avenaceum

Sphaerodes quadrangularis García, Stchigel & Guarro was shown to be a facultative contact biotrophic mycoparasite of Fusarium avenaceum (Fr.) Sacc. (teleomorph: Gibberella avenacea Cooke) that established an intimate relationship with its host by producing hook-shaped and clamp-like attachment structures that appeared to derive nutrients and other essential factors from living host cells. The contact cells of the mycoparasite surrounded and envelop host hyphae and, although they do not appear to penetrate the host, the parasitized hyphae were significantly smaller than unparasitized controls. Sphaerodes quadrangularis did not form specialized attachment structures when cultured together with strains representing F. oxysporum and F. graminearum, indicating some degree of host specificity in the parasite.

Pharmacokinetics and Pharmacodynamics of Amphotericin B Deoxycholate, Liposomal Amphotericin B, and Amphotericin B Lipid Complex in an In Vitro Model of Invasive Pulmonary Aspergillosis

The pharmacodynamic and pharmacokinetic (PK-PD) properties of amphotericin B (AmB) formulations against invasive pulmonary aspergillosis (IPA) are not well understood. We used an in vitro model of IPA to further elucidate the PK-PD of amphotericin B deoxycholate (DAmB), liposomal amphotericin B (LAmB) and amphotericin B lipid complex (ABLC). The pharmacokinetics of these formulations for endovascular fluid, endothelial cells, and alveolar cells were estimated. Pharmacodynamic relationships were defined by measuring concentrations of galactomannan in endovascular and alveolar compartments. Confocal microscopy was used to visualize fungal biomass. A mathematical model was used to calculate the area under the concentration-time curve (AUC) in each compartment and estimate the extent of drug penetration. The interaction of LAmB with host cells and hyphae was visualized using sulforhodamine B-labeled liposomes. The MICs for the pure compound and the three formulations were comparable (0.125 to 0.25 mg/liter). For all formulations, concentrations of AmB progressively declined in the endovascular fluid as the drug distributed into the cellular bilayer. Depending on the formulation, the AUCs for AmB were 10 to 300 times higher within the cells than within endovascular fluid. The concentrations producing a 50% maximal effect (EC50) in the endovascular compartment were 0.12, 1.03, and 4.41 mg/liter for DAmB, LAmB, and ABLC, respectively, whereas, the EC50 in the alveolar compartment were 0.17, 7.76, and 39.34 mg/liter, respectively. Confocal microscopy suggested that liposomes interacted directly with hyphae and host cells. The PK-PD relationships of the three most widely used formulations of AmB differ markedly within an in vitro lung model of IPA.

Potential of Trichoderma spp. as biocontrol agents against pathogens causing maydis leaf blight of maize.

Efficacy of bioagents against Helminthosporium maydis was studied in vitro and under field condition at Trihut college of Agriculture, Dholi (Muzaffarpur) in 2005 and 2006. Among the three native antagonist isolates of Trichoderma viride . Trichoderma harzianum and T. virens screened, T. viride inhibited the radial growth of H. maydis to an extent of 60.7% followed by T. harzianum (55.1%) and T. virens (52.6%). Studies on hyphal interaction between antagonists and test fungus revealed disorganization of protoplasmic content and lysis of host hyphae. Soil application of bioagents was more effective in protecting the crop than foliar spray. Soil application of T. viride resulted in minimum disease intensity (2.6) followed by T. harzianum (2.8) and T. virens (3.0). Soil application of T viride also gave maximum yield (18.6q ha -1 ) followed by T. harzianum (17.7q ha -1 ) and T. virens (17.3q ha -1 ), respectively. These results suggest the need to augment soil application of Trichoderma for obtaining effective management of maydis leaf blight.

Verticillium disease of Agaricus bisporus: variations in host contribution to total fungal DNA in relation to symptom heterogeneity

The pathogenic fungus Verticillium fungicola, responsible for dry bubble disease of the common mushroom Agaricus bisporus, causes various symptoms on its host, bubbles (undifferentiated spherical masses), bent and/or split stipes (blowout) and spotty caps. Host DNA quantification by real-time PCR was used to observed relationships between the type of symptom and the relative amount of A. bisporus and V. fungicola in diseased mushrooms. Verticillium fungicola is involved in bubble formation but does not appear to regulate its growth. Quantifications in bubbles and stipe-bubbles (morphology between bubble and sporophore with stipe blowout) showed that the pathogen has no effect on the growth of undifferentiated host hyphae but prevents morphological differentiation if not initiated and stops it when initiated hyphae are affected. Mushrooms with stipe blowout exhibiting both mature and abortive lamellae reveal that V. fungicola has a restricted area of action in host tissues. Despite their visual aspect, healthy looking parts of mushrooms showing spots or stipe blowout were actually contaminated. Discolouration and symptom development are two distinct events. The colour of the tissues was correlated to the percentage of A. bisporus DNA, suggesting that discolouration is not an efficient defensive mechanism, and occurs at the time V. fungicola developed enough to induce tissues necrosis.

Pathogenicity of Escovopsis weberi: The parasite of the attine ant-microbe symbiosis directly consumes the ant-cultivated fungus

Fungi in the genus Escovopsis are known only from the fungus gardens of attine ants. Previous work has established that these anamorphic fungi, allied with the Hypocreales, are specialized and potentially virulent parasites of the ancient mutualism between attine ants and their fungal cultivars. It is unclear whether the primary nutrient source for the pathogen is the mutualist fungal cultivar or the vegetative substrate placed on the gardens by the ants. Here, we determine whether Escovopsis weberi is a parasite of the fungal cultivar, a competitor for the leaf substrate, or both. Bioassays reveal that E. weberi exhibits rapid growth on pure cultivar and negligible growth on sterilized leaf fragments. Light microscopy examination of hyphalhyphal interactions between E. weberi and the ant fungal cultivar indicate that E. weberi, unlike invasive necrotrophs that always penetrate host hyphae, can secrete compounds that break down host mycelium before contact occurs. Thus, E. weberi is a necrotrophic parasite of the fungal cultivar of attine ants.

Trichoderma atroviride G-protein alpha-subunit gene tga1 is involved in mycoparasitic coiling and conidiation.

The soil fungus Trichoderma atroviride, a mycoparasite, responds to a number of external stimuli. In the presence of a fungal host, T. atroviride produces hydrolytic enzymes and coils around the host hyphae. In response to light or nutrient depletion, asexual sporulation is induced. In a biomimetic assay, different lectins induce coiling around nylon fibers; coiling in the absence of lectins can be induced by applying cyclic AMP (cAMP) or the heterotrimeric G-protein activator mastoparan. We isolated a T. atroviride G-protein alpha-subunit (Galpha) gene (tgal) belonging to the fungal subfamily with the highest similarity to the Galpha1 class. Generated transgenic lines that overexpress Galpha show very delayed sporulation and coil at a higher frequency. Furthermore, transgenic lines that express an activated mutant protein with no GTPase activity do not sporulate and coil at a higher frequency. Lines that express an antisense version of the gene are hypersporulating and coil at a much lower frequency in the biomimetic assay. The loss of Tgal in these mutants correlates with the loss of GTPase activity stimulated by the peptide toxin Mas-7. The application of Mas-7 to growing mycelial colonies raises intracellular cAMP levels, suggesting that Tgal can activate adenylyl cyclase. In contrast, cAMP levels and cAMP-dependent protein kinase activity drop when diffusible host signals are encountered and the mycoparasitism-related genes ech42 and prb1 are highly expressed. Mycoparasitic signaling is unlikely to be a linear pathway from host signals to increased cAMP levels. Our results demonstrate that the product of the tga1 gene is involved in both coiling and conidiation.

In vitro evaluation of trichoderma and gliocladium antagonism against the symbiotic fungus of the leaf-cutting ant Atta cephalotes.

The antagonistic activity of Trichoderma and Gliocladium isolates against Attamyces sp., a symbiotic fungus of the leaf-cutting ant Atta cephalotes, was investigated. A. cephalotes cultures this fungus as the primary food source. Most of the Trichodema and Gliocladium isolates tested in vitro (82.6%) inhibited the Attamyces sp. mycelial growth. which was probably due to their colonization ability and competition for nutrients, both of them known mechanisms of some species of these genera. T. lignorum strain T-26 was the strongest inhibitor achieving a colonization of 23%. Microscopical observations indicate that the inhibitory effect was caused by an interaction that took place in close contact with the host hypha, causing wall deformation that led to the collapse of the turgor pressure.

Evidence for Antibiosis and Induced Host Defense Reactions in the Interaction Between Verticillium lecanii and Penicillium digitatum, the Causal Agent of Green Mold

ABSTRACT Chronological events of the intercellular interaction between Verticillium lecanii and the postharvest pathogen Penicillium digitatum were investigated by transmission electron microscopy and gold cytochemistry. Growth inhibition of P. oligandrum as a response to V. lecanii attack correlated with striking host changes including retraction of the plasma membrane and cytoplasm disorganization. Such changes were associated with the deposition on the inner host cell surface of a chitin- and cellulose-enriched material which appeared to be laid down as a structural defense reaction. The accumulation of chitin in the newly formed material correlated with a decrease in the amount of wallbound chitin. However, the deposition of cellulose appeared to correspond to a de novo synthesis, as evidenced by the occurrence of cellulose-containing vesicles which released their content in the space between the invaginated plasma membrane and the host cell wall. Results of the present study provide the first ultrastructural and cytochemical evidence that antagonism, triggered by V. lecanii, is a multifaceted process in which antibiosis, with alteration of the host hyphae prior to contact with the antagonist, appears to be the key process in the antagonism against P. digitatum.

Mycoparasitism of Coemansia species

The mode of hyphal interaction and parasitism of Coemansia spp., mainly C. aciculifera and C. thaxteri, by Gliocladium roseum, Fusarium flocciferum and Verticillium psalliotae, species that occurred with the Coemansia spp. in wheat rhizospheres, was observed in mixed cultures on low nutrient medium. Two types of interaction occurred. G. roseum and F. flocciferum coiled round, penetrated and grew within the host's hyphae. V. psalliotae grew parallel to and along the host's hyphae, coiling only rarely, and sometimes formed thick mycelial layers on the host's sporangiophores. After infection by each fungus, the host's hyphae became narrower, collapsed and then disintegrated. The observations suggest parasitism followed by lysis rather than the involvement of antibiotics.

Ultrastructural and Cytochemical Aspects of the Interaction Between the Mycoparasite Pythium oligandrum and Soilborne Plant Pathogens

ABSTRACT The interaction between the oomycete Pythium oligandrum and various soilborne oomycete and fungal plant pathogens (P. ultimum, P. aphanidermatum, Fusarium oxysporum f. sp. radicis-lycopersici, Verticillium albo-atrum, Rhizoctonia solani, and Phytophthora megasperma) was studied by light and electron microscopy in order to assess the relative contribution of mycoparasitism and antibiosis in the antagonistic process. Scanning electron microscope investigations of the interaction regions showed that structural alterations of all pathogenic fungi and oomycetes (except for Phytophthora megasperma) occurred soon after contact with the antagonist. Light and transmission electron microscope studies of the interaction region between the antagonist and P. ultimum revealed that intimate contact between both partners preceded a sequence of degradation events including aggregation of host cytoplasm and penetration of altered host hyphae. Localization of the host wall cellulose component showed that cellulose was altered at potential penetration sites. A similar scheme of events was observed during the interaction between P. oligandrum and F. oxysporum f. sp. radicis-lycopersici, with the exception that complete loss of host protoplasm was associated with antagonist invasion. The interaction between P. oligandrum and R. solani resulted in an abnormal deposition of a wall-like material at potential penetration sites for the antagonist. However, the antagonist displayed the ability to circumvent this barrier and penetrate host hyphae by locally altering the chitin component of the host hyphal wall. Interestingly, antagonist cells also showed extensive alteration as evidenced by the frequent occurrence of empty hyphal shells. In the case of Phytophthora megasperma, hyphal interactions did not occur, but hyphae of the plant pathogen were damaged severely. At least two distinct mechanisms appear to be involved in the process of oomycete and fungal attack by P. oligandrum: (i) mycoparasitism, mediated by intimate hyphal interactions, and (ii) antibiosis, with alteration of the host hyphae prior to contact with the antagonist. However, the possibility that the antagonistic process may rely on the dual action of antibiotics and hydrolytic enzymes is discussed.