Fungal Root Pathogens Research Articles

Root infection and systemic colonization of maize by Colletotrichum graminicola.

Colletotrichum graminicola is a filamentous ascomycete that causes anthracnose disease of maize. While the fungus can cause devastating foliar leaf blight and stalk rot diseases, little is known about its ability to infect roots. Previously published reports suggest that C. graminicola may infect maize roots and that root infections may contribute to the colonization of aboveground plant tissues, leading to disease. To determine whether C. graminicola can infect maize roots and whether root infections can result in the colonization of aboveground plant tissues, we developed a green fluorescent protein-tagged strain and used it to study the plant root colonization and infection process in vivo. We observed structures produced by other root pathogenic fungi, including runner hyphae, hyphopodia, and microsclerotia. A mosaic pattern of infection resulted from specific epidermal and cortical cells becoming infected by intercellular hyphae while surrounding cells were uninfected, a pattern that is distinctly different from that described for leaves. Interestingly, falcate conidia, normally restricted to acervuli, were also found filling epidermal cells and root hairs. Twenty-eight percent of plants challenged with soilborne inoculum became infected in aboveground plant parts (stem and/or leaves), indicating that root infection can lead to asymptomatic systemic colonization of the plants. Many of the traits observed for C. graminicola have been previously reported for other root-pathogenic fungi, suggesting that these traits are evolutionally conserved in multiple fungal lineages. These observations suggest that root infection may be an important component of the maize anthracnose disease cycle.

Evaluation of Daphne Germplasm for Resistance to Daphne Sudden Death Syndrome Caused by the Soil-borne Pathogen Thielaviopsis basicola

Many daphne cultivars are susceptible to fungal root pathogens and require frequent fungicide applications during production. To identify taxon differences to disease susceptibility, we evaluated 32 Daphne species and cultivars for resistance to the soil-borne pathogen, Thielaviopsis basicola (Berk. and Broome) Ferr., by both in vitro- and in vivo-based methods. Disease-free plant roots were inoculated with the pathogen through topical application of a spore suspension and observed weekly for disease development/progression. Significant variation for disease severity among the taxa evaluated was determined using a plant disease index. Plant reactions ranged from highly resistant, e.g., D. tangutica and D. retusa, to highly susceptible, e.g., D. cneorum. In addition, a high correlation was found between the in vitro and in vivo techniques for the seven selected species, indicating that they are comparable. However, the in vitro assay provided results in significantly less time than the in vivo assay.

Effect of chitosan on hyphal growth and spore germination of plant pathogenic and biocontrol fungi

To investigate the toxic effect of chitosan on important root pathogenic and biocontrol fungi (nematophagous, entomopathogenic and mycoparasitic). We have used standard bioassays to investigate the effect of chitosan on colony growth and developed bioassays to test spore germination. The results showed that the root pathogenic and mycoparasitic fungi tested were more sensitive to chitosan than nematophagous and entomopathogenic fungi. Chitosanases (and perhaps related enzymes) are involved in the resistance to chitosan. Two fungi, one sensitive to chitosan, Fusarium oxysporum f. sp. radicis-lycopersici, and one less sensitive, Pochonia chlamydosporia, were selected for ultrastructural investigations. Transmission electron microscopy revealed differences in the ultrastructural alterations caused by chitosan in the spores of the plant pathogenic fungus and in those of the nematophagous fungus. Confocal laser microscopy showed that Rhodamine-labelled chitosan enters rapidly into conidia of both fungi, in an energy-dependent process. Nematophagous and entomopathogenic fungi are rather resistant to the toxic effect of chitosan. Resistance of nematophagous and entomopathogenic fungi to chitosan could be associated with their high extracellular chitosanolytic activity. Furthermore, ultrastructural damage is much more severe in the chitosan sensitive fungus. The results of this paper suggest that biocontrol fungi tested could be combined with chitosan for biological control of plant pathogens and pests.

Population performance of collembolans feeding on soil fungi from different ecological niches

The potential reproductive value of arbuscular mycorrhizal fungi ( Glomus intraradices and Glomus invermaium), root pathogenic fungi ( Rhizoctonia solani and Fusarium culmorum) and saprotrophic fungi ( Penicillium hordei and Trichoderma harzianum) were examined for the collembolans Folsomia candida Willem and Folsomia fimetaria L. Dried baker's yeast ( Saccharomyces cerevisiae) was used as a reference standard food in laboratory cultures. Collembolan performance was determined as final size, fecundity and population growth rate after when fed the fungal food sources for 31 days. The mycorrhizal fungi gave the least growth and fecundity compared with the other fungi, but G. intraradices gave good fecundity for F. candida. In terms of growth, Baker's yeast was a high-quality food for both adults and juveniles of both species, but it was a poorer food in terms of fecundity of F. candida. Preference of the fungi in all possible pairwise combinations showed that although F. fimetaria did not perform well on Glomus spp. and F. candida did not grow well on Glomus spp. their preference for these fungi did not reflect this. The highest fecundity was seen with the root pathogen F. culmorum. Different quality indicators such as the C:N ratio of the fungal food sources as well as other biological parameters are discussed in relation to their reproductive value and Collembola preferential feeding.

Suppression of root pathogenic fungus, Fusarium oxysporum, by the culture filtrates of ectomycorrhizal(ECM) fungi

Suppression of root pathogenic fungus, Fusarium oxysporum, by the culture filtrates of ectomycorrhizal(ECM) fungi

What are the mechanisms and specificity of mycorrhization helper bacteria?

What are the mechanisms and specificity of mycorrhization helper bacteria?

The role of dsbA in colonization of the wheat rhizosphere by Pseudomonas fluorescens Q8r1-96

Certain well-conserved genes in fluorescent Pseudomonas spp. are involved in pathogenic interactions between the bacteria and evolutionarily diverse hosts including plants, insects and vertebrate animals. One such gene, dsbA, encodes a periplasmic disulfide-bond-forming enzyme implicated in the biogenesis of exported proteins and cell surface structures. This study focused on the role of dsbA in Pseudomonas fluorescens Q8r1-96, a biological control strain that produces the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) and is known for its exceptional ability to colonize the roots of wheat and pea. The deduced DsbA protein from Q8r1-96 is similar to other predicted thiol : disulfide interchange proteins and contains a conserved DsbA catalytic site, a pattern associated with the thioredoxin family active site, and a signal peptide and cleavage site. A dsbA mutant of Q8r1-96 exhibited decreased motility and fluorescence, and altered colony morphology; however, it produced more 2,4-DAPG and total phloroglucinol-related compounds and was more inhibitory in vitro to the fungal root pathogen Gaeumannomyces graminis var. tritici than was the parental strain. When introduced separately into a natural soil, Q8r1-96 and the dsbA mutant did not differ in their ability to colonize the rhizosphere of wheat in greenhouse experiments lasting 12 weeks. However, when the two strains were co-inoculated, the parental strain consistently out-competed the dsbA mutant. It was concluded that dsbA does not contribute to the exceptional rhizosphere competence of Q8r1-96, although the dsbA mutation reduces competitiveness when the mutant competes with the parental strain in the same niche in the rhizosphere. The results also suggest that exoenzymes and multimeric cell surface structures are unlikely to have a critical role in root colonization by this strain.

The association of necrotrophic fungal pathogens and plant parasitic nematodes with the loss of productivity of annual medic-based pastures in Australia and options for their management

Annual medics (annual Medicago spp.) are often an integral component of dry-land farming and cropping systems worldwide, including significant areas of the winter rainfall regions of southern Australia. Necrotrophic fungal pathogens frequently dominate such areas because of the ease of survival of these trash-and soilborne pathogens on infested residues over the relatively dry summer period. Necrotrophic fungal pathogens, in general, are favoured by the nutrient impoverished soils across many parts of these regions, which provide little microbial buffering against these pathogens. This review first, outlines the major and most widespread diseases caused by necrotrophic fungal pathogens and plant parasitic nematodes; second, defines the association of these pathogens with the loss of productivity of annual medic-based pastures; and finally, investigates the spectrum of control options for their management. Important necrotrophic foliar fungal pathogens include Phoma medicaginis, Colletotrichum trifolii, Leptosphaerulina trifolii, Pseudopeziza medicaginis, Stemphylium botryosum, S. vesicarium and Stagonospora meliloti. Necrotrophic foliar fungal pathogens, for example Phoma medicaginis, are also known to stimulate production of phyto-oestrogenic compounds to high levels that can adversely affect ovulation rates in sheep. There are also necrotrophic fungal root pathogens in Australia that have been associated with significant decline in productivity and pose such a serious threat to annual medic pastures such that reseeding is required. In particular, Rhizoctonia solani and various Fusarium species such as F. avenaceum, F. acuminatum, F. culmorum, F. graminearum and F. lateritium, and various Pythium species, in particular P. irregulare, P. ultimum and P. spinosum, are of concern. Another important soilborne and root-attacking necrotrophic pathogen is Phytophthora clandestina. The association of Fusarium spp. with annual medic roots, crowns and burrs in Australia is cause for additional concern as some of them are responsible for the production of deleterious mycotoxins. Plant parasitic nematodes are important as a potential yield limiting factor in annual medics. Approaches to disease management include strategies that have been utilised to varying degrees for the control of necrotrophic foliar and root pathogens in annual medic pastures. This paper reviews the current literature on the topic and provides an assessment of options available for their management. In particular, host resistance offers the most cost-effective, longterm control, particularly as some useful resistance to several of these pathogens has been identified. One or more cultural control strategies, including grazing, fertiliser application, rotations and seed health, offer further opportunities for restricting losses in annual medics from diseases caused by necrotrophic fungi and/or plant parasitic nematodes, especially if applied as an integrated management strategy with only minor reliance on fungicides. As many of these foliar and soilborne fungal pathogens and parasitic nematodes can also threaten crops grown in rotation, these pathogens pose a wider threat to the farming systems employed across southern Australia and reduce the potential for providing a ‘disease break’. In addition, the role played by these pathogens is likely far wider and of greater adverse impact than previously considered as a consequence of their potential for causing mycotoxin and/or phyto-oestrogen production. The full array of losses, not just herbage and seed yields, needs to be considered for the more important fungal and nematode pathogens. Genetic resistance to individual pathogens provides the best option for management. The success and outcome with sourcing resistance in other annual pasture legumes such as Trifolium spp. highlights the value of seeking out new sources of host resistance from the Mediterranean centre of origin, even if the particular diseases of interest do not occur there, in the same way that has been shown for herbicide resistances and for Kabatiella on subterranean clover. pasture legumes such as Trifolium spp. highlights the value of seeking out new sources of host resistance from the Mediterranean centre of origin, even if the particular diseases of interest do not occur there, in the same way that has been shown for herbicide resistances and for Kabatiella on subterranean clover.

Three small RNAs jointly ensure secondary metabolism and biocontrol in Pseudomonas fluorescens CHA0

In many Gram-negative bacteria, the GacS/GacA two-component system positively controls the expression of extracellular products or storage compounds. In the plant-beneficial rhizosphere bacterium Pseudomonas fluorescens CHA0, the GacS/GacA system is essential for the production of antibiotic compounds and hence for biological control of root-pathogenic fungi. The small (119-nt) RNA RsmX discovered in this study, together with RsmY and RsmZ, forms a triad of GacA-dependent small RNAs, which sequester the RNA-binding proteins RsmA and RsmE and thereby antagonize translational repression exerted by these proteins in strain CHA0. This small RNA triad was found to be both necessary and sufficient for posttranscriptional derepression of biocontrol factors and for protection of cucumber from Pythium ultimum. The same three small RNAs also positively regulated swarming motility and the synthesis of a quorum-sensing signal, which is unrelated to N-acyl-homoserine lactones, and which autoinduces the Gac/Rsm cascade. Expression of RsmX and RsmY increased in parallel throughout cell growth, whereas RsmZ was produced during the late growth phase. This differential expression is assumed to facilitate fine tuning of GacS/A-controlled cell population density-dependent regulation in P. fluorescens.

Fruiting body development of a root pathogenic fungus, rhizina undulata, after forest fire in eastern coastal pine forests of Korea

Since late July, 2000, fruiting bodies of Rhizina undulata, an ascomycetous fungus which causes root rot on coniferous trees, began to develop after a forest fire in the eastern coastal pine forests of Korea. In a survey conducted in August, they were abundantly developed in the fire‐damaged pine forests. The occurrence frequency of the fruiting bodies seemed to be correlated with the degree of damage by the forest fire. That is, in severely damaged or completely burned areas, fruiting bodies were found around trees with abundant frequency, whereas moderate or slightly damaged areas showed a relatively low occurrence frequency. In addition, pine seedlings transplanted just after the fire outbreak were infected and killed by this fungus, producing typical symptoms on the infected roots. These phenomena present a warning that Rhizina root rot may cause severe damage on still‐alive pine trees after being burnt in fire‐damaged forests. This disease also may attack pine seedlings which are transplanted to restore the symbiotic relationships with the mycelium of pine mushroom, Tricholoma matsutake, just after a forest fire.

Antimicrobial proteins from cowpea root exudates: inhibitory activity against Fusarium oxysporum and purification of a chitinase-like protein

Plants exude a variety of substances through their external surfaces and from germinating seeds, some of which have an inhibitory action against plant pathogens. The aim of this study was the investigation and characterization of defense proteins present in exudates from roots of cowpea seedlings (Vigna unguiculata (L.) Walp.). Root exudates were collected from seedlings that were grown hydroponically in three different media, including, 100 mM sodium acetate buffer pH 4.5, water pH 6.0 and 100 mM sodium phosphate buffer pH 7.5. The proteins from these exudates were analyzed by SDS–PAGE and SDS–Tricine–PAGE and the presence of antimicrobial proteins in the exudates was investigated by immunological and enzymatic assays. Results showed that roots from cowpea seedlings contained β-1,3-Glucanases, chitinases and lipid transfer proteins (LTPs), all of which may potentially function as plant defense proteins. Immunolocalization of one of these proteins, chitinase, revealed its presence in the xylem cell wall vessel elements. These exudates also demonstrated an inhibitory effect on the growth of the fungus, Fusarium oxysporum, in vitro. The results suggest that plant roots may exude a variety of proteins that may function to repress the growth of root pathogenic fungi.

Non-target effects of bacterial biological control agents on soil Protozoa

Biological control agents (BCAs) have gained increasing interest as an alternative to chemical pesticides in agriculture. Before widespread environmental use, risk assessment of effects on target and non-target organisms are needed. However, the knowledge about the effect of BCAs on non-target soil Protozoa is insufficient to support thorough risk assessment. In this study we report on the effects of Pseudomonas fluorescens DR54 that is a potential BCA active against root pathogenic fungi. We present evidence of negative effects of P. fluorescens DR54 on growth of the amoebae Hartmanella vermiformis and Acanthamoeba sp. cultures and natural assemblages of soil protozoans. The observed effects were larger than those of the P. fluorescens type strain DSM50090 and Enterobacter aerogenes SC and were tentatively attributed to viscosinamide, which is an antimicrobial compound with surfactant properties produced by P. fluorescens DR54.

Non-target effects of bacterial biological control agents suppressing root pathogenic fungi

Microorganisms have knowingly been used during the last century to control plant diseases. During the last decades, research and application of biological control agents (BCAs) as a pest control strategy have gained increasing attention. This review focuses specifically on non-target effects of bacterial BCAs that are used to suppress root pathogenic fungi. It attempts to critically evaluate the strengths and weaknesses of non-target effect studies published to date and relate them to the success of the BCA in fungal pathogen control. Significant non-target effects of BCAs have indeed been observed, but these are generally small in scale and limited to a growth season, and have not been proven to affect soil health. We discuss these studies and point out what we believe are notable deficiencies. Among the modes of disease suppression by BCAs, antibiotic production is believed to be of major importance. But assurances that in situ antibiotic production actually occurs in environmental samples are lacking in the non-target effect studies. Also the effectiveness of the BCA on the target pathogen, the absence of appropriate controls for inoculation effects, and the presence of pathogenic fungi are missing in most studies. In future non-target effect studies we recommend focusing on proven effective BCAs and clearly distinguishing effects of antimicrobial compounds from effects of general microbial activity.

Evaluation of endophytic actinobacteria as antagonists of Gaeumannomyces graminis var. tritici in wheat

Endophytic actinobacteria isolated from healthy cereal plants were assessed for their ability to control fungal root pathogens of cereal crops both in vitro and in planta. Thirty eight strains belonging to the genera Streptomyces, Microbispora, Micromonospora, and Nocardioidies were assayed for their ability to produce antifungal compounds in vitro against Gaeumannomyces graminis var. tritici (Ggt), the causal agent of take-all disease in wheat, Rhizoctonia solani and Pythium spp. Spores of these strains were applied as coatings to wheat seed, with five replicates (25 plants), and assayed for the control of take-all disease in planta in steamed soil. The biocontrol activity of the 17 most active actinobacterial strains was tested further in a field soil naturally infested with take-all and Rhizoctonia. Sixty-four percent of this group of microorganisms exhibited antifungal activity in vitro, which is not unexpected as actinobacteria are recognized as prolific producers of bioactive secondary metabolites. Seventeen of the actinobacteria displayed statistically significant activity in planta against Ggt in the steamed soil bioassay. The active endophytes included a number of Streptomyces, as well as Microbispora and Nocardioides spp. and were also able to control the development of disease symptoms in treated plants exposed to Ggt and Rhizoctonia in the field soil. The results of this study indicate that endophytic actinobacteria may provide an advantage as biological control agents for use in the field, where others have failed, due to their ability to colonize the internal tissues of the host plant.

Interactions of Newly Isolated Orchid Mycorrhizal Fungi with Korean Cymbidium kanran Hybrid ‘Chungsu’

Two fungal isolates obtained from roots of Cymbidium goeriingii in Jeju island were confirmed to be symbiotic with orchid plantlets, and were compared with other orchid mycorrhizal (OM) fungi previously isolated. The two isolates differed in their peloton structures formed in the roots of Cymbidium kanran hybrid ‘Chungsu’ and in responses of orchid plant. These two isolates differed from the additionally tested OM fungi in some features, and from root damaging species of Rhizoctonia and Fusarium as based on cluster analysis after PCR-RAPD with the primers, Bioneer-28 and OPO-2. With this simple and fast technique, it was possible to distinguish OM fungi from the plant root pathogenic fungi based on calculation of their polymorphic bands. This technique can therefore be helpful to distinguish the OM fungi from the root pathogens. Particularly, the new isolates are considered as new resource of symbiotic fungi for horticultural industries.

Restriction Analyses of PCR Amplified Partial SSU Ribosomal DNA to Arbuscular Mycorrhizal Fungi from Other Fungi Colonizing Plant Roots

Roots of Glycine max and Miscanthus sinensis and soil samples were collected from various field sites at Goesan, Chungbuk in Korea. Microscopic observations of the roots indicated high colonization rates of both arbuscular mycorrhizal fungi (AMF) and other fungi. The partial small subunit of ribosomal DNA genes were amplified with the genomic DNA extracted from their roots by nested polymerase chain reaction (PCR) with universal primer NS1 and fungal specific primers AM1. Restriction fragment length polymorphism (RFLP) was analyzed using the combinations of three restriction enzymes, HinfI, AluI and AsuC21. Nucleotides sequence analysis revealed that ten sequences from Miscanthus sinensis and one sequence from Glycine max were close to those of arbuscular mycorrhizal fungi. Also, 33% of total clones amplified with NS31-AM1 primers from M. sinensis and 97% from G. max were close to Fusarium oxysporum or other pathogenic fungi, and they were successfully distinguished from AMF. Results suggested that these techniques could help to distinguish arbuscular mycorrhizal fungi from root pathogenic fungi in the plant roots. Especially, DNA amplified by these primers showed distinct polymorphisms between AMF and plant pathogenic species of Fusarium when digested with AsuC21.

Management practices to improve soil health and reduce the effects of detrimental soil biota associated with yield decline of sugarcane in Queensland, Australia

Yield decline (YD) of sugarcane is a widespread problem throughout the Australian sugar industry. It is defined as “the loss of productive capacity of sugarcane-growing soil under long-term monoculture”. Factors contributing to YD are the monoculture itself, excessive tillage of the soil at planting and severe soil compaction resulting from the use of heavy machinery during the harvesting operation. Collectively, these crop management practices have led to the development of sugarcane-growing soils that are low in organic C and cation exchange capacity, have a high bulk density and have a low microbial biomass. This in turn is associated with a build up of populations of detrimental soil organisms, which affect the growth and health of the sugarcane root system. Significant yield increases have been demonstrated following pasteurization or fumigation of the soil or treatment of the soil with fungicides or nematicides. Several detrimental soil organisms associated with YD have been identified, including a fungal root pathogen ( Pachymetra chaunorhiza) and the lesion nematode ( Pratylenchus zeae). Experimental evidence, however, suggests there are many other unidentified detrimental soil organisms associated with YD. In order to circumvent YD, major changes to the cane cropping system need to be considered. Different rotation breaks (sown pasture, alternate crops, bare fallow) were evaluated for their impact on soil health and the composition of the community of organisms in soil previously under cane monoculture. Despite the breaks having different effects on populations of beneficial soil biota, all breaks reduced populations of known detrimental soil biota and significantly increased the yield of the following cane crop. A single legume-based break crop appeared to be sufficient to capture the majority of these benefits. Other possible management options including the use of organic amendments and minimum tillage techniques are discussed.

Real-Time Quantitative RT-PCR Assay of Gene Expression in Plant Roots during Fungal Pathogenesis

Real-time quantitative RT-PCR is becoming the preferred method for high-sensitivity, rapid-throughput RNA transcript quantification. However, due to the significant developmental costs of dedicated fluorogenic probes, a real-time assay that is simple to establish, comparatively inexpensive, and readily adaptable would be advantageous for the detailed analysis of large sets of expressed sequences. We have devised a flexible real-time quantitative RT-PCR assay that employs a nonspecific DNA binding dye for product detection and uses a relative quantification formula to account for differences in PCR amplification efficiency between the target and reference products. The latter permits the use of an exogenous reference transcript and therefore avoids the normal requirement for the construction of a recombinant RNA reference transcript or extensive characterization of housekeeping gene expression. In an investigation of class II chitinase expression in two varieties of Bermuda grass (Cynodon spp.), following infection with the fungal root pathogen Ophiosphaerella narmari, this assay identified 16- and 28-fold peaks in gene expression at 24 and 96 h after inoculation, respectively.

Root herbivores, pathogenic fungi, and competition between Centaurea maculosa and Festuca idahoensis

We used a common garden experiment to evaluate the isolated and combined effects of a biocontrol agent, the insect (Agapeta zoegana, Lepidoptera), and a native North American fungal pathogen (Sclerotinia sclerotiorum) on competition between the noxious weed Centaurea maculosa and the native Festuca idahoensis. In 0.5-m2 plots with 24 plants per plot, competition between Centaurea and Festuca was highly asymmetrical, with Centaurea strongly reducing the final biomass and reproduction of Festuca, and Festuca having no effect, or possibly a positive effect, on Centaurea. The direct effects of the biological control agents differed entirely. All Centaurea individuals died in plots receiving Sclerotinia, but Agapeta did not significantly reduce the growth of Centaurea, and apparently stimulated a compensatory reproductive response in the weed. Individual Centaurea plants that had been damaged by Agapeta produced more flowerheads, and the number of Centaurea plants with Agapeta root damage in a plot was positively correlated with total Centaurea biomass. These differences in the direct effects of the consumers were reflected in their indirect effects. In plots where Sclerotinia killed Centaurea (strong direct effects) Festuca growth and reproduction was equal to that in Festuca plots without Centaurea and the reproductive output of Festuca increased substantially (strong indirect effects). However, in the absence of Sclerotinia, the application of Agapeta did not significantly decrease Centaurea biomass (weak direct effects) and actually stimulated small, but significant decreases in Festuca reproduction and trends towards lower Festuca biomass (weak and opposite indirect effects –Agapeta does not eat Festuca). If the direct effects of biocontrol agents on Centaurea are weak, as suggested by our results, natives are unlikely to be released from the competitive effects of Centaurea, and natives may suffer from Centaurea's compensatory response to herbivory.

Phosphorus uptake of an arbuscular mycorrhizal fungus is not effected by the biocontrol bacterium Burkholderia cepacia

The biocontrol bacterium Burkholderia cepacia is known to suppress a broad range of root pathogenic fungi, while its impact on other beneficial non-target organisms such as arbuscular mycorrhizal (AM) fungi is unknown. Direct interactions between five B. cepacia strains and the AM fungus, Glomus intraradices (BEG87) were studied in root-free soil compartments separated from a rooting compartment by a fine nylon-mesh. B. cepacia had no effect on AM fungal biomass and energy reserves measured using the signature fatty acid 16:1ω5 from phospholipid fatty acids (PLFAs) and neutral lipid fatty acids (NLFAs), respectively. Hyphal P transport was also unaffected by the biocontrol bacterium, which either stimulated, reduced or had no effect on length of the external mycelium of G. intraradices. The cyclic PLFAs cy17:0 and cy19:0 were suggested to be useful markers for estimation of biomass of B. cepacia. The presence of mycelium of G. intraradices reduced the biomass of three out of five B. cepacia strains as indicated by a reduction in PLFAs cy17:0 and cy19:0, while other bacterial PLFAs were unaffected by mycelium of G. intraradices. On the other hand, two out of five B. cepacia strains reduced the amount of branched PLFAs suggesting a reduction in the population of Gram-positive bacteria in these cases. In conclusion, the B. cepacia seems to have no impact on neither mycorrhiza formation nor on the functioning of the AM fungus G. intraradices in terms of P transport, whereas our results suggest that mycorrhiza might have adverse effects on B. cepacia.