Peter Jeffries

The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and soil fertility

Abstract. Beneficial plant–microbe interactions in the rhizosphere are primary determinants of plant health and soil fertility. Arbuscular mycorrhizas are the most important microbial symbioses for the majority of plants and, under conditions of P-limitation, influence plant community development, nutrient uptake, water relations and above-ground productivity. They also act as bioprotectants against pathogens and toxic stresses. This review discusses the mechanism by which these benefits are conferred through abiotic and biotic interactions in the rhizosphere. Attention is paid to the conservation of biodiversity in arbuscular mycorrhizal fungi (AMF). Examples are provided in which the ecology of AMF has been taken into account and has had an impact in landscape regeneration, horticulture, alleviation of desertification and in the bioremediation of contaminated soils. It is vital that soil scientists and agriculturalists pay due attention to the management of AMF in any schemes to increase, restore or maintain soil fertility.

4 Arbuscular Mycorrhiza: A Key Component of Sustainable Plant–Soil Ecosystems

In sustainable agriculture arbuscular mycorrhizas play a key role in helping the plant not only to survive but to be productive under adversity. Mycorrhizal formation is an adaptive strategy which provides the plant with an increased ability for nutrient capture and cycling in soils with low nutrient availability and an increased tolerance to environmental stresses. The importance of maintaining mycorrhizal diversity and a functioning soil mycelium is vital to sustaining plant growth. Stimulation of natural mycorrhizal populations or augmentation through the use of mycorrhizal inoculants are important tools in sustainable ecosystem management. This review focuses on how AMF help plants withstand: (1) soil-borne pathogen attack, (2) aggressive agricultural weeds, (3) drought/salinity stress, (4) the presence of toxic pollutants and (5) desertification. The evidence shows that arbuscular mycorrhizas should be considered as an essential natural resource for ensuring sustainable growth and health of plants and fully deserve their title as ‘biological fertilizers’ and ‘bioprotectors’.

Biology and biocontrol potential of Ampelomyces mycoparasites, natural antagonists of powdery mildew fungi

Historically, pycnidial fungi belonging to the genus Ampelomyces were among the first mycoparasites to be studied in detail and were also the first fungi used as biocontrol agents of plant parasitic fungi. The interactions between host plants, powdery mildew fungi and Ampelomyces mycoparasites are one of the most evident cases of tritrophic relationships in nature although their study has received little attention in fungal and plant ecology so far. Ampelomyces mycoparasites have now become one of the most advanced in terms of commercial development of a fungal biocontrol agent, although there is still a need for more development work to produce a product with reliability approaching that of conventional chemical treatments. This review summarizes the taxonomy, genetic diversity, life cycle, mode of action, natural occurrence, host range, biocontrol potential, mass production and commercialization of these mycoparasites and compares the biocontrol ability of Ampelomyces with that of other fungal antagonists of powdery mildews.

Biogeochemical cycling and arbuscular mycorrhizas in the sustainability of plant-soil systems

The fundamental importance of microbial processes for the efficient cycling of mineral nutrients within biosphere has been realized for many years (fig.1), and the involvement of the arbuscular mycorrhizal fungi (AMF) fully recognized (Mosse, 1986; Bethlenfalvay and Lindermann, 1992; Barea and Jeffries, 1994).

Effect of fungicides on three vesicular-arbuscular mycorrhizal fungi associated with winter wheat (Triticum aestivum L.)

SummaryWe studied the effects of field application rates of three fungicides on spore germination and root infection in three species of Glomus. Bavistin (carbendazim) prevented germination of spores of G. monosporum and G. mosseae whereas Calixin (tridemorph) had no effect. Tilt Turbo (propiconazole) inhibited spore germination less than Bavistin. Spores of G. geosporum germinated in the presence of all three fungicides. Foliar applications of Bavistin alone and in a triple-spray programme significantly reduced infection of all three Glomus species in wheat roots grown in conditions of low P availability (2 mg P 1−1). Two fungicides with triazole derivatives, Sportak and Tilt Turbo, also decreased infection and the former significantly reduced spore production of all three Glomus species. Calixin-treated plants, however, showed increased infection levels, resulting in greater yields and greater foliar-P concentrations than control plants. Milgo (ethirimol) was not detrimental to the vesicular-arbuscular mycorrhizal (VAM) symbiosis when applied as a foliar spray.

Isolation of Microbial Antagonists for Biocontrol of Grey Mould Disease of Strawberries

A screening programme is described for the assessment of the potential of biocontrol agents to control grey mould of strawberries caused by Botrytis cinerea. Bacteria were isolated from strawberry fruits, leaves and flowers from a commercial field site and screened for antagonism towards B. cinerea using two in vitro and one in vivo screening techniques. From 559 microorganisms isolated, 108 inhibited pathogen growth on agar plates and 27 of these prevented spore germination on Cellophane membranes. The ability of these 27 isolates to inhibit infection of young strawberry leaves by B. cinerea on whole plants under glass was then tested. Seven isolates reduced grey mould development and were subsequently assessed in a field trial. Two isolates, one of Bacillus pumilus and one of Pseudomonas fluorescens, were as effective or more effective than standard dichlofluanid sprays and may therefore be of potential value as antagonists of B. cinerea.

Phenotypic degeneration occurs during sector formation in Metarhizium anisopliae.

Aims:  The formation of sectors was observed during subculturing of an isolate of the entomopathogenic fungus Metarhizium anisopliae, a fungus used for biological control of insect pests. The aim of the investigation was to establish whether sector formation was accompanied by changes in physiological characters.

Ampelomyces mycoparasites from apple powdery mildew identified as a distinct group based on single-stranded conformation polymorphism analysis of the rDNA ITS region

Pycnidial fungi belonging to the genus Ampelomyces are the most common natural antagonists of powdery mildews worldwide. During a study of the interactions between apple powdery mildew (Podosphaera leucotricha) and Ampelomyces mycoparasites, 52 new Ampelomyces isolates were obtained from P. leucotricha and, in addition, 13 new isolates from other species of the Erysiphaceae in four European countries. Their genetic diversity was screened using single-stranded conformation polymorphism (SSCP) analysis of the internal transcribed spacer (ITS) region of the ribosomal DNA (rDNA). For comparison, 24 isolates obtained from genetic resource collections or other sources were included in this study. Based on the ITS-SSCP patterns, the isolates were placed in eight groups. The isolates belonged to two types based on their growth in culture. The faster-growing and the slower-growing isolates were included in different SSCP groups. A phylogenetic analysis of the ITS sequences of representatives of these groups confirmed the results obtained with the SSCP method, and showed that the faster-growing isolates do not belong to Ampelomyces as suggested by earlier studies. All the isolates from P. leucotricha fell into a distinct SSCP group of genetically homogeneous isolates. This suggests that Ampelomyces mycoparasites which occur in apple powdery mildew are slightly different from the other Ampelomyces groups which contain mycoparasites from various powdery mildew species. This may be because the main growth period of Ampelomyces mycoparasites in apple powdery mildew is isolated in time from that of Ampelomyces isolates that occur in other species of the Erysiphaceae. P. leucotricha starts its life-cycle early in the season, usually in March-April, while most powdery mildews are active in the same environments only late in the year.

Species and population structures of Pisolithus and Scleroderma identified by combined phenotypic and genomic marker analysis

Isolates of Pisolithus and Scleroderma species from different northern temperate and tropical geographical regions were subjected to analyses of pure culture morphology, colony growth rates, isozyme (allozyme) variation and ribosomal DNA (rDNA) restriction fragment length polymorphisms (RFLPs). Cultural characteristics enabled clear species separation of isolates and together with growth rates suggested geographically-linked intraspecific variability in the Pisolithus populations. Combined or method-specific hierarchical cluster analyses of allozyme polymorphisms and RFLPs of the fungal internal transcribed spacer (ITS) and intergenic spacer (IGS) sequences confirmed the Scleroderma species groupings and considerable geographical and host-linked variation in the Pisolithus population. Isolates of Pisolithus from the Philippines were genetically very homogeneous and distinct from less related isolates from Europe, Scandinavia and North America. Based on the ITS—RFLP and isozyme polymorphism data, the isolates investigated probably represent four different ‘species groupings’ which supports similar findings from recent taxonomic and genetic studies of Australian Pisolithus species.

A generic theoretical model for biological control of foliar plant diseases

We have developed a generic modelling framework to understand the dynamics of foliar pathogen and biocontrol agent (BCA) populations in order to predict the likelihood of successful biocontrol in relation to the mechanisms involved. The model considers biocontrol systems for foliar pathogens only and, although it is most applicable to fungal BCA systems, does not address a specific biocontrol system. Four biocontrol mechanisms (competition, antibiosis, mycoparasitism and induced resistance) were included within the model rubric. Because of the wide range of mechanisms involved we use Trichoderma/Botrytis as an exemplar system. Qualitative analysis of the model showed that the rates of a BCA colonising diseased and/or healthy plant tissues and the time that the BCA remains active are two of the more important factors in determining the final outcome of a biocontrol system. Further evaluation of the model indicated that the dynamic path to the steady-state population levels also depends critically on other parameters such as the host-pathogen infection rate. In principle, the model can be extended to include other potential mechanisms, including spatio-temporal heterogeneity, fungicide effects, non-fungal BCA and strategies for BCA application, although with a cost in model tractability and ease of interpretation.