Phytopathogenic fungi, causing significant economic and production losses, are becoming a serious threat to global food security. Due to an increase in fungal resistance and the hazardous effects of chemical fungicides to human and environmental health, scientists are now engaged to explore alternate non-chemical and ecofriendly management strategies. The use of biocontrol agents and their secondary metabolites (SMs) is one of the potential approaches used today. Trichoderma spp. are well known biocontrol agents used globally. Many Trichoderma species are the most prominent producers of SMs with antimicrobial activity against phytopathogenic fungi. Detailed information about these secondary metabolites, when grouped together, enhances the understanding of their efficient utilization and further exploration of new bioactive compounds for the management of plant pathogenic fungi. The current literature provides the information about SMs of Trichoderma spp. in a different context. In this review, we summarize and group different antifungal SMs of Trichoderma spp. against phytopathogenic fungi along with a comprehensive overview of some aspects related to their chemistry and biosynthesis. Moreover, a brief overview of the biosynthesis pathway, action mechanism, and different approaches for the analysis of SMs and the factors affecting the regulation of SMs in Trichoderma is also discussed.

Losses in crops caused by plant pathogenic bacteria and parasitic nematode are increasing because of a decrease in efficacy of traditional management measures. There is an urgent need to develop nonchemical and ecofriendly based management to control plant diseases. A potential approach of controlling plant disease in the crops is the use of biocontrol agents and their secondary metabolites (SMs). Luckily fungi and especially the genus Trichoderma comprise a great number of fungal strains that are the potential producer of bioactive secondary metabolites. In this study secondary metabolites from ten Trichoderma spp. were evaluated for their antibacterial and nematicidal potential against phytopathogenic bacteria Ralstonia solanacearum, Xanthomonas compestris and plant parasitic nematode Meloidogyne incognita. Five different growth media were evaluated for the production of SMs. It was shown that SMs of different Trichoderma spp. obtained on different growth media were different in the degree of their bioactivity. Comparison of five growth media showed that SMs produced on solid wheat and STP media gave higher antibacterial activity. SMs of T. pseudoharzianum (T113) obtained on solid wheat media were more effective against the studied bacteria followed by SMs from T. asperelloides (T136), T. pseudoharzianum (T129) and T. pseudoharzianum (T160). Scanning electron microscopy (SEM) was further conducted to observe the effect of SMs on bacterial cell morphology. As evident from the SEM, SMs produced severe morphological changes, such as rupturing of the bacterial cell walls, disintegration of cell membrane and cell content leaking out. SMs from T. viridae obtained on liquid STP and solid wheat media showed the highest percent of M. incognita juveniles (J2s) mortality and inhibition in egg hatching of M. incognita. The results of our study suggest that T. pseudoharzianum (T113) and T. viridae could be selected as an effective candidate for SMs source against phytopathogenic bacteria and M. incognita respectively.

Matricaria chamomilla flower extract was used as a biocompatible material for synthesis of zinc oxide nanoparticles (ZnONPs). The synthesized NPs were evaluated for their antibacterial potential in vitro and in vivo against Ralstonia solanacearum that causes devastating bacterial wilt disease in tomato and other crops. Synthesized ZnONPs were further analyzed by UV-Visible spectroscopy, fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The synthesized polydisperse ZnONPs were found to be in the size range of 8.9 to 32.6 nm, and at 18.0 µg ml-1 exhibited maximum in vitro growth inhibition of R. solanacearum. SEM analysis of affected bacterial cells showed morphological deformation such as disruption of cell membrane, cell wall and leakage of cell contents. Results of in vivo studies also showed that application of ZnONPs to the artificially inoculated tomato plants with R. solanacearum significantly enhanced the plant growth by reducing bacterial soil population and disease severity as compared to untreated control. Biosynthesized ZnONPs could be an effective approach to control R. solanacearum.