Background: Fungal endophytes occur ubiquitously in plants and are being increasingly studied for their ability to support plant health and protect the host from diseases. Using endophytes in disease control provides potential advantages compared to other biocontrol agents since they colonise the plant internally and thereby are protected by the host, thus potentially reducing environmental sensitivity. A thorough understanding of their mechanisms is required in their mutualistic association with plants; both to optimise their efficacy and for registration as plant protection products. Aims: To provide a critical review on the mechanisms employed by endophytic fungi in biological disease control. Furthermore, we draw attention to gaps in our knowledge of the complex interactions between plant, pathogen and endophyte and discuss implications for future research. Methods: Review of literature where endophytic colonisation during the specific interaction has been confirmed. Results: Known disease-reducing mechanisms include direct inhibition of pathogen activity by competition, antibiosis and mycoparasitism and indirect inhibition by induced resistance, where the plant’s own defence system is activated to combat the diseases. Relying on in vitro studies of alone can result in misleading conclusions. Conclusions: We need to investigate nature and requirements for establishment of successful plant-endophyte interactions, for development of efficient biocontrol agents.

• Hydrogen peroxide (H₂O₂) is reported to inhibit biotrophic but benefit necrotrophic pathogens. Infection by necrotrophs can result in a massive accumulation of H₂O₂ in hosts. Little is known of how pathogens with both growth types are affected (hemibiotrophs). The hemibiotroph, Septoria tritici, infecting wheat (Triticum aestivum) is inhibited by H₂O₂ during the biotrophic phase, but a large H₂O₂ accumulation occurs in the host during reproduction. • Here, we infiltrated catalase, H₂O₂ or water into wheat during the biotrophic or the necrotrophic phase of S. tritici and studied the effect of infection on host physiology to get an understanding of the survival strategy of the pathogen. • H₂O₂ removal by catalase at both early and late stages made plants more susceptible, whereas H₂O₂ made them more resistant. H₂O₂ is harmful to S. tritici throughout its life cycle, but it can be tolerated. • The late accumulation of H₂O₂ is unlikely to result from down-regulation of photosynthesis, but probably originates from damage to the peroxisomes during the general tissue collapse, which is accompanied by release of soluble sugars in a susceptible cultivar.

Application of 3.6 mm silicon (Si+) to the rose (Rosa hybrida) cultivar Smart increased the concentration of antimicrobial phenolic acids and flavonoids in response to infection by rose powdery mildew (Podosphaera pannosa). Simultaneously, the expression of genes coding for key enzymes in the phenylpropanoid pathway (phenylalanine ammonia lyase, cinnamyl alcohol dehydrogenase, and chalcone synthase) was up-regulated. The increase in phenolic compounds correlated with a 46% reduction in disease severity compared with inoculated leaves without Si application (Si−). Furthermore, Si application without pathogen inoculation induced gene expression and primed the accumulation of several phenolics compared with the uninoculated Si− control. Chlorogenic acid was the phenolic acid detected in the highest concentration, with an increase of more than 80% in Si+ inoculated compared with Si− uninoculated plants. Among the quantified flavonoids, rutin and quercitrin were detected in the highest concentrations, and the rutin concentration increased more than 20-fold in Si+ inoculated compared with Si− uninoculated plants. Both rutin and chlorogenic acid had antimicrobial effects on P. pannosa, evidenced by reduced conidial germination and appressorium formation of the pathogen, both after spray application and infiltration into leaves. The application of rutin and chlorogenic acid reduced powdery mildew severity by 40% to 50%, and observation of an effect after leaf infiltration indicated that these two phenolics can be transported to the epidermal surface. In conclusion, we provide evidence that Si plays an active role in disease reduction in rose by inducing the production of antifungal phenolic metabolites as a response to powdery mildew infection.