Relationships between Oak powdery mildew incidence and severity and commensal fungi

Oak (Quercus robur) powdery mildew is a common and damaging fungal disease. In a local survey at Reading, UK, oak powdery mildew was common on trees of all height classes but was most common on trees of 3-9m. A variety of other fungal species were commonly found growing in association with oak powdery mildew colonies. The abundance of such fungi was estimated through stratified sample surveys for 2.5 years. The taxa most commonly associated with oak powdery mildew were Acremonium sp., Trichoderma sp., Ampelomyces/Phoma sp. and Leptosphaerulina australis. Nearly 90% of mildew colonies were associated with L. australis, which is not generally considered as a mycoparasite or antagonist, in contrast with the other three fungi. Abundance varied between June and October surveys. Acremonium sp. abundance was greater in summer samplings whereas L. australis and Trichoderma sp. abundances were greater in autumn samplings. Ampelomyces/Phoma sp. was never observed in the absence of powdery mildew. Relationships between the mildew-associated fungi and oak powdery mildew appeared curved and differed significantly between sampling years. L. australis was positively correlated with the other three associated fungi studied when powdery mildew was also present. The variety and high population densities of the mildew associated fungi suggest that they may be important in determining the final density of oak mildew and the damage caused by it.

Antagonistic activities of selected fungal isolates against Armillaria mellea

The antagonistic activities of six selected fungal isolates against Armilloria mellea were studied on two different concentrations of three media, on fungicides-amended malt extract agar (MEA) medium, and in glasshouse pots filled with John Innes No.2 compost and natural field soil. Trichoderma hamatum isolate Tham1 was found the most effective in reducing Armillaria growths on both the low and high concentrations of malt extract, potato dextrose and V-8 juice in MEA, potato dextrose agar (PDA) and V-8 juice agar (VJA), respectively, followed by T. harzianum isolate Th2 and T. viride isolate Tv3. Neither dose rate (200 or 2000 mg l(-1)) of fenpropidin allowed any growth of Armillaria on MEA, while that of the antagonists was also completely inhibited or greatly restricted. However, both dose rates of fosetyl-A1 allowed the growth of Armillaria and almost all the antagonists. Data on colony diameters of Armillaria showed Tham1 as the most effective antagonist along with Th2, Th23 and Tv3. Tham1 was also found the most effective in protecting hazel billets from colonization by Armillaria, followed by Th2 and Th23. Compared with 7.1 colonized billets in the inoculated controls, only 1.3, 2.6 and 2.7 billets (out of ten) were colonized, respectively, when protected with these antagonists. The results indicate that the Trichoderma isolates are able to maintain their antagonistic effects on A. mellea under a variety of nutritional, chemical and edaphic regimes. More investigations are needed to develop a system of control for the disease with these potential antagonists.

Cultural techniques for improvement in biocontrol potential of fungal antagonists for biological control of Armillaria root rot of strawberry plants under glasshouse conditions

Several in vitro and in vivo experiments were conducted to develop an effective technique for culturing potential fungal antagonists (isolates of Trichoderma harzianum, Dactylium dendroides, Chaetomium olivaceum and one unidentified fungus) selected for activity against Armillaria mellea. The antagonists were inoculated onto (1) live spawn of the oyster mu shroom (Pleurotus ostreatus), (2) extra-moistened or sucrose-enriched mushroom composts containing living or autoclaved mycelia of P. ostreatus or Agaricus bisporus (button mushroom), (3) pasteurized compost with or without A. bisporus mycelium, wheat bran, wheat germ and (4) spent mushroom composts with living mycelia of A. bisporus, P. ostreatus or Lentinus edodes (the Shiitake mushroom). In one experiment, a representative antagonist (isolate Th2 of T. harzianum) was grown together with the A. bisporus mycelium, while in another one, the antagonist was first grown on wheat germ or wheat bran and then on mushroom compost with living mycelium of A. bisporus. Some of the carrier substrates were then added to the roots of potted strawberry plants in the glasshouse to evaluate their effectiveness against the disease. The antagonists failed to grow on the spawn of P. ostreatus even after reinoculations and prolonged incubation. Providing extra moisture or sucrose enrichment also did not improve the growth of Th2 on mushroom composts in the presence of living mycelia of A. bisporus or P. ostreatus. The antagonist, however, grew rapidly and extensively on mushroom compost with autoclaved mycelia, and also on wheat germ and wheat bran. Colonization of the substrates by the antagonist was positively correlated with its effectiveness in the glasshouse studies. Whereas only 33.3% of the inoculated control plants survived in one experiment monitored for 560 days, 100% survival was achieved when Th2 was applied on wheat germ or wheat bran. Growth of the antagonist alone on pasteurized or sterilized compost (without A. bisporus mycelia) and simultaneous growth of the antagonist and mushroom on pasteurized compost did not improve survival over the inoculated controls, but growth over mushroom compost with the living mycelium resulted in 50% survival rate. C. olivaceum isolate Co was the most effective, resulting in overall survival rate of 83.3% compared with only 8.3% for the inoculated and 100% for the uninoculated (healthy) controls. This antagonist gave the highest survival rate of 100% on spent mushroom compost with L. edodes. T harzianum isolate Th23, with 75% survival rate, was the most effective on spent mushroom compost with P. ostreatus, while D. dendroides isolate SP resulted in equal survival rates of 50% on all the three mushroom composts.

Comparisons between the in vitro and in vivo efficacies of potential fungal antagonists of Armillaria mellea

Seventeen fungal isolates were tested in vitro as potential antagonists of two isolates of the root rot pathogen, Armillaria mellea. Some of the isolates were also added on mushroom composts with living mycelia to the roots of Armillaria-inoculated potted strawberry plants in the glasshouse to find out if they had the same degree of efficacy against the disease. Dactylium dendroides isolate SP was the most effective in reducing mycelial growth of A. mellea isolate 1 (Am1), followed by Trichoderma harzianum isolate Th2 and T. viride isolate Tv4. Th2, Th22, Tv3 and SP grew extensively over Am1 colonies, disintegrating the rhizomorphs. Isolate Tham1 of T hamatum was the most effective in reducing mycelial growth of A. mellea isolate 2 (Am2), followed by Tv3. Th12, Th22, Tv1, Tv3 and SP inhibited the initiation and growth of rhizomorphs of Am2. Regeneration tests showed that both Am1 and Am2 attacked by Trichoderma isolates and SP were no longer viable. Th23 and SP were almost as effective in vivo as in vitro. But isolate Co of Chaetomium olivaceum, which was ineffective in vitro, was found effective in vivo. Conversely, Th2, which exhibited good antagonistic activity in vitro, performed poorly in vivo. These results show that the in vitro and in vivo efficacies of potential antagonists may not necessarily be closely correlated. Hence, there is a danger that potentially effective isolates may be discarded if decisions are made only on the basis of preliminary screening tests carried out under laboratory conditions.

Genotype and temperature influence pea aphid resistance to a fungal entomopathogen

The influence of temperature on life history traits of four Acyrthosiphon pisum clones was investigated, together with their resistance to one genotype of the fungal entomopathogen Erynia neoaphidis . There was no difference among aphid clones in development rate, but they did differ in fecundity. Both development rate and fecundity were influenced by temperature, but all clones showed similar responses to the changes in temperature (i.e. the interaction term was nonsignificant). However, there were significant differences among clones in susceptibility to the pathogen, and this was influenced by temperature. Furthermore, the clones differed in how temperature influenced susceptibility, with susceptibility rankings changing with temperature. Two clones showed changes in susceptibility which mirrored changes in the in vitro vegetative growth rate of E. neoaphidis at different temperatures, whereas two other clones differed considerably from this expected response. Such interactions between genotype and temperature may help maintain heritable variation in aphid susceptibility to fungal pathogen attack and have implications for our understanding of disease dynamics in natural populations. This study also highlights the difficulties of drawing conclusions about the efficacy of a biological control agent when only a restricted range of pest genotypes or environmental conditions are considered.

Human microbiota in health and disease

Each human body plays host to a microbial population which is both numerically vast (at around 1014 microbial cells) and phenomenally diverse (over 1,000 species). The majority of the microbial species in the gut have not been cultured but the application of culture-independent approaches for high throughput diversity and functionality analysis has allowed characterisation of the diverse microbial phylotypes present in health and disease. Studies in monozygotic twins, showing that these retain highly similar microbiota decades after birth and initial colonisation, are strongly indicative that diversity of the microbiome is host-specific and affected by the genotype. Microbial diversity in the human body is reflected in both richness and evenness. Diversity increases steeply from birth reaching its highest point in early adulthood, before declining in older age. However, in healthy subjects there appears to be a core of microbial phylotypes which remains relatively stable over time. Studies of individuals from diverse geopraphies suggest that clusters of intestinal bacterial groups tend to occur together, constituting ‘enterotypes’. So variation in intestinal microbiota is stratified rather than continuous and there may be a limited number of host/microbial states which respond differently to environmental influences. Exploration of enterotypes and functional groups may provide biomarkers for disease and insights into the potential for new treatments based on manipulation of the microbiome. In health, the microbiota interact with host defences and exist in harmonious homeostasis which can then be disturbed by invading organisms or when ‘carpet bombing’ by antibiotics occurs. In a portion of individuals with infections, the disease will resolve itself without the need for antibiotics and microbial homeostasis with the host’s defences is restored. The administration of probiotics (live microorganisms which when administered in adequate amounts confer a health benefit on the host) represents an artificial way to enhance or stimulate these natural processes. The study of innate mechanisms of antimicrobial defence on the skin, including the production of numerous antimicrobial peptides (AMPs), has shown an important role for skin commensal organisms. These organisms may produce AMPs, and also amplify the innate immune responses to pathogens by activating signalling pathways and processing host produced AMPs. Research continues into how to enhance and manipulate the role of commensal organisms on the skin. The challenges of skin infection (including diseases caused by multiply resistant organisms) and infestations remain considerable. The potential to re-colonise the skin to replace or reduce pathogens, and exploring the relationship between microbiota elsewhere and skin diseases are among a growing list of research targets. Lactobacillus species are among the best known ‘beneficial’ bacterial members of the human microbiota. Of the approximately 120 species known, about 15 are known to occur in the human vagina. These organisms have multiple properties, including the production of lactic acid, hydrogen peroxide and bacteriocins, which render the vagina inhospitable to potential pathogens. Depletion of the of the normal Lactobacillus population and overgrowth of vaginal anaerobes, accompanied by the loss of normal vaginal acidity can lead to bacterial vaginosis – the commonest cause of abnormal vaginal discharge in women. Some vaginal anaerobes are associated with the formation of vaginal biofilms which serve to act as a reservoir of organisms which persists after standard antibiotic therapy of bacterial vaginosis and may help to account for the characteristically high relapse rate in the condition. Administration of Lactobacillus species both vaginally and orally have shown beneficial effects in the treatment of bacterial vaginosis and such treatments have an excellent overall safety record. Candida albicans is a frequent coloniser of human skin and mucosal membranes, and is a normal part of the microbiota in the mouth, gut and vagina. Nevertheless Candida albicans is the most common fungal pathogen worldwide and is a leading cause of serious and often fatal nosocomial infections. What turns this organism from a commensal to a pathogen is a combination of increasing virulence in the organism and predisposing host factors that compromise immunity. There has been considerable research into the use of probiotic Lactobacillus spp. in vaginal candidiasis. Studies in reconstituted human epithelium and monolayer cell cultures have shown that L. rhamnosus GG can protect mucosa from damage caused by Candida albicans, and enhance the immune responses of mucosal surfaces. Such findings offer the promise that the use of such probiotic bacteria could provide new options for antifungal therapy. Studies of changes of the human intestinal microbiota in health and disease are complicated by its size and diversity. The Alimentary Pharmabiotic Centre in Cork (Republic of Ireland) has the mission to ‘mine microbes for mankind’ and its work illustrates the potential benefits of understanding the gut microbiota. Work undertaken at the centre includes: mapping changes in the microbiota with age; studies of the interaction between the microbiota and the gut; potential interactions between the gut microbiota and the central nervous system; the potential for probiotics to act as anti-infectives including through the production of bacteriocins; and the characterisation of interactions between gut microbiota and bile acids which have important roles as signalling molecules and in immunity. The important disease entity where the role of the gut microbiota appears to be central is the Irritable Bowel Syndrome (IBS). IBS patients show evidence of immune activation, impaired gut barrier function and abnormal gut microbiota. Studies with probiotics have shown that these organisms can exert anti-inflammatory effects in inflammatory bowel disease and may strengthen the gut barrier in IBS of the diarrhoea-predominant type. Formal randomised trials of probiotics in IBS show mixed results with limited benefit for some but not all. Studies confirm that administered probiotics can survive and temporarily colonise the gut. They can also stimulate the numbers of other lactic acid bacilli in the gut, and reduce the numbers of pathogens. However consuming live organisms is not the only way to influence gut microbiota. Dietary prebiotics are selectively fermented ingredients that can change the composition and/or activity of the gastrointestinal microbiota in beneficial ways. Dietary components that reach the colon, and are available to influence the microbiota include poorly digestible carbohydrates, such as non-starch polysaccharides, resistant starch, non-digestible oligosaccharides (NDOs) and polyphenols. Mixtures of probiotic and prebiotic ingredients that can selectively stimulate growth or activity of health promoting bacteria have been termed ‘synbiotics’. All of these approaches can influence gut microbial ecology, mainly to increase bifidobacteria and lactobacilli, but metagenomic approaches may reveal wider effects. Characterising how these changes produce physiological benefits may enable broader use of these tactics in health and disease in the future. The current status of probiotic products commercially available worldwide is less than ideal. Prevalent problems include misidentification of ingredient organisms and poor viability of probiotic microorganisms leading to inadequate shelf life. On occasions these problems mean that some commercially available products cannot be considered to meet the definition of a probiotic product. Given the potential benefits of manipulating the human microbiota for beneficial effects, there is a clear need for improved regulation of probiotics. The potential importance of the human microbiota cannot be overstated. ‘We feed our microbes, they talk to us and we benefit. We just have to understand and then exploit this.’ (Willem de Vos).

The endophytic fungus Piriformospora indica protects wheat from Fusarium crown rot disease in simulated UK autumn conditions

The root endophytic fungus Piriformospora indica (Sebacinacea) forms mutualistic symbioses with a broad range of host plants, increasing their biomass production and resistance to fungal pathogens. We evaluated the effect of P. indica on Fusarium crown rot disease of wheat, under in vitro and glasshouse conditions. Interaction of P. indica and Fusarium isolates under axenic culture conditions indicated no direct antagonistic activity of P. indica against Fusarium isolates. Seedlings of wheat were inoculated with P. indica and pathogenic Fusarium culmorum or F. graminearum and grown in sterilised soil-free medium or in a non-sterilised mix of soil and sand. Fusarium alone reduced emergence and led to visible browning and reduced root growth. Roots of seedlings in pots inoculated with both Fusarium isolates and P. indica were free of visible symptoms; seed emergence and root biomass were equivalent to the uninoculated. DNA was quantified by real-time polymerase chain reaction (qPCR). The ratio of Fusarium DNA to wheat DNA rose rapidly in the plants inoculated with Fusarium alone; isolates and species were not significantly different. P. indica inoculation reduced the ratio of Fusarium to host DNA in the root systems. The reduction increased with time. The ratio of P. indica to wheat DNA initially rose but then declined in root systems without Fusarium. With Fusarium, the ratio rose throughout the experiment. The absolute amount of Fusarium DNA in root systems increased in the absence of P. indica but was static in plants co-inoculated with P. indica.

Piriformospora indica reduces Fusarium head blight disease severity and mycotoxin DON contamination in wheat under UK weather conditions

Piriformospora indica (Sebacinaceae) is a cultivable root endophytic fungus. It colonises the roots of a wide range of host plants. In many settings colonisation promotes host growth, increases yield and protects the host from fungal diseases. We evaluated the effect of P. indica on Fusarium head blight (FHB) disease of winter (cv. Battalion) and spring (cv. Paragon, Mulika, Zircon, Granary, KWS Willow and KWS Kilburn) wheat and consequent contamination by the mycotoxin deoxynivalenol (DON) under UK weather conditions. Interactions of P. indica with an arbuscular mycorrhizal fungus (Funneliformis mosseae), fungicide application (Aviator Xpro) and low and high fertiliser levels were considered. P. indica application reduced FHB disease severity and incidence by 70%. It decreased mycotoxin DON concentration of winter and spring wheat samples by 70% and 80% respectively. P. indica also increased above ground biomass, 1000 grain weight and total grain weight. P. indica reduced disease severity and increased yield in both high and low fertiliser levels. The effect of P. indica was compatible with F. mosseae and foliar fungicide application. P. indica did not have any effects on plant tissue nutrients. These results suggest that P. indica might be useful in biological control of Fusarium diseases of wheat.