In Vivo Antifungal Activity of South African Medicinal Plant Extracts against Fusarium pathogens and Their Phytotoxicity Evaluation

Phytochemicals as alternative fungicides for controlling plant diseases: A comprehensive review of their efficacy, commercial representatives, advantages, challenges for adoption, and possible solutions

Fungal infections are responsible for about 70-80% of the losses in agricultural production brought on by microbial diseases. Synthetic fungicides have been employed to manage plant diseases caused by phytopathogenic fungi but their use has been criticized due to unfavorable side effects. As alternative strategies, botanical fungicides have caught the interest of many researchers in recent years. There are numerous experimental studies on the fungicidal activities of phytochemicals against phytopathogenic fungi, but there is not a thorough review article that summarizes these experimental studies. The purpose of this review is therefore to consolidate data from in vitro and in vivo studies on the antifungal activity of phytochemicals reported by various researchers. This paper describes antifungal activities of plant extracts and compounds against phytopathogenic fungi, approved botanical fungicides, their benefits, obstacles and mitigation strategies. Relevant sources were collected using online data bases such as Google Scholar, PubMed and Science Direct, and comprehensively reviewed for preparation of this manuscript. This review revealed that phytochemicals are effective to manage plant diseases caused by phytopathogenic fungi. Botanical fungicides are endowed with benefits such as resistance inhibition, being ecofriendly, effective, selective, and more affordable compared to synthetic fungicides. However, there are only small number of approved botanical fungicides due to the many challenges that hinder their adoption and utilization for a wider scale production. Farmers' reluctance, lack of standardized formulation techniques, strict legislation, rapid degradation, and other factors hinder their adoption and utilization. The ways to address these challenges include increasing awareness among farmers, conducting more research to identify potential plants with fungicidal properties, standardizing extraction and formulation techniques, implementing the idea of plant breeding to increase bioactive agents, identifying favorable environments for site-specific plant species production, discovering synthetic analogues of the active ingredient to maintain quality standards, establishing reasonable regulation procedures and price points for a quicker market introduction. To put all these into practice, we recommend collaboration of regulatory agencies and researchers from a variety of fields.

Lantadene A and boswellic acid isolated from the leaves of Lantana camara L. have the potential to control phytopathogenic Fusarium species

Phytopathogenic Fusarium species are restricting factors causing diseases and yield loss in crop production. As part of exploration for pesticides from medicinal plants, this study aimed to isolate and characterize bioactive compounds from Lantana camara L. and evaluate their efficiency against Fusarium phytopathogens. Phytochemical investigation of ethyl acetate leaf extract led to separation of lantadene A (22-angeloyloxy-9-hydroxy-3-oxo-olean-12-en-28-oic acid) and boswellic acid (11-keto-β-boswellic acid). The chemical structures of the aforementioned compounds were confirmed using physical properties, spectroscopic analysis, and published data. Lantadene A exhibited significant antifungal activity against F. subglutinans, F. proliferatum, F. solani, F. graminearum, and F. semitectum with minimum inhibitory concentration (MIC) less than or equal to 0.63 mg/mL. Boswellic acid exhibited strong activity (MIC = 0.63 mg/mL) against F. subglutinans and F. semitectum. In terms of their toxicity towards Raw 264.7 cells, lantadene A and boswellic acid recorded half-maximal inhibitory concentration values of 84.2 μg/mL and 186.6 μg/mL, respectively. Both lantadene A and boswellic acid had no phytotoxic effect against seed germination and seedling root length. Lantadene A and boswellic acid have strong potential to be further investigated as lead natural fungicides (biopesticides) to control Fusarium crop diseases.

Recent Research on Fusarium Mycotoxins in Maize-A Review

Maize (Zea mays L.) is one of the most susceptible crops to pathogenic fungal infections, and in particular to the Fusarium species. Secondary metabolites of Fusarium spp.-mycotoxins are not only phytotoxic, but also harmful to humans and animals. They can cause acute or chronic diseases with various toxic effects. The European Union member states apply standards and legal regulations on the permissible levels of mycotoxins in food and feed. This review summarises the most recent knowledge on the occurrence of toxic secondary metabolites of Fusarium in maize, taking into account modified forms of mycotoxins, the progress in research related to the health effects of consuming food or feed contaminated with mycotoxins, and also the development of biological methods for limiting and/or eliminating the presence of the same in the food chain and in compound feed.

Natural Products from Medicinal Plants against Phytopathogenic Fusarium Species: Current Research Endeavours, Challenges and Prospects

Many Fusarium species are pathogenic, causing crop diseases during crop production and spoilage of agricultural products in both commercial and smallholder farming. Fusarium attack often results into food contamination, yield loss and increases in food insecurity and food prices. Synthetic fungicides have been used as a control strategy for the management of crop diseases caused by Fusarium pathogens. The negative effects associated with application of many synthetic pesticides has necessitated the need to search for alternative control strategies that are affordable and environmentally safe. Research on medicinal plants as control agents for Fusarium pathogens has received attention since plants are readily available and they contain wide variety of secondary metabolites that are biodegradable. The activities of solvent extracts, essential oils and compounds from medicinal plants have been tested against Fusarium phytopathogenic species. A summary of recent information on antifungal activity of plants against Fusarium species is valuable for the development of biopesticides. This paper reviews the antifungal research conducted on medicinal plants against Fusarium pathogens, over a 10-year period, from January 2012 to May 2021. We also highlight the challenges and opportunities of using natural products from medicinal plants in crop protection. Several databases (Science Direct and Web of Science) were used to obtain information on botanical products used to control Fusarium diseases on crops. Keywords search used included natural products, antifungal, Fusarium, crops diseases, phytopathogenic, natural compounds and essential oil.

Current Perspectives of Biocontrol Agents for Management of Fusarium verticillioides and Its Fumonisin in Cereals-A Review

Fusarium verticillioides is the most predominant fungal phytopathogen of cereals and it is posing great concern from a global perspective. The fungus is mainly associated with maize, rice, sorghum, wheat, sugarcane, banana, and asparagus and causes cob, stalk, ear, root, crown, top, and foot rot. F. verticillioides produces fumonisins as the major secondary metabolite along with trace levels of beauvericin, fusaric acid, fusarin C, gibberiliformin, and moniliformin. Being a potential carcinogen, fumonisins continue to receive major attention as they are common contaminants in cereals and its processed food products. The importance of elimination of F. verticillioides growth and its associated fumonisin from cereals cannot be overemphasized considering the significant health hazards associated with its consumption. Physical and chemical approaches have been shown to reduce fumonisin B1 concentrations among feeds and food products but have proved to be ineffective during the production process. Hence, biological control methods using microorganisms, plant extracts, antioxidants, essential oils, phenolic compounds, and other advanced technologies such as growing disease-resistant crops by applying genetic engineering, have become an effective alternative for managing F. verticillioides and its toxin. The different methods, challenges, and concerns regarding the biocontrol of F. verticillioides and production of fumonisin B1 have been addressed in the present review.

Antifungal compound, methyl hippurate from Bacillus velezensis CE 100 and its inhibitory effect on growth of Botrytis cinerea

Botrytis cinerea, the causal agent of gray mold is one of the major devastating fungal pathogens that occurs in strawberry cultivation and leads to massive losses. Due to the rapid emergence of resistant strains in recent years, an ecofriendly disease management strategy needs to be developed to control this aggressive pathogen. Bacillus velezensis CE 100 exhibited strong antagonistic activity with 53.05% against B. cinerea by dual culture method. In the present study, 50% of culture filtrate supplemented into PDA medium absolutely inhibited mycelial growth of B. cinerea whereas the highest concentration (960 mg/L) of different crude extracts including ethyl acetate, chloroform, and n-butanol crude extracts of B. velezensis CE 100, strongly inhibited mycelial growth of B. cinerea with the highest inhibition of 79.26%, 70.21% and 69.59% respectively, resulting in severe damage to hyphal structures with bulging and swellings. Hence, the antifungal compound responsible was progressively separated from ethyl acetate crude extract using medium pressure liquid chromatography. The purified compound was identified as methyl hippurate by nuclear magnetic resonance and mass spectrometry. The inhibitory effect of methyl hippurate on both spore germination and mycelial growth of B. cinerea was revealed by its dose-dependent pattern. The spore germination rate was completely restricted at a concentration of 3 mg/mL of methyl hippurate whereas no mycelial growth was observed in agar medium supplemented with 4 mg/mL and 6 mg/mL of methyl hippurate by poisoned food method. Microscopic imaging revealed that the morphologies of spores were severely altered by long-time exposure to methyl hippurate at concentrations of 1 mg/mL, 2 mg/mL and 3 mg/mL and hyphae of B. cinerea were severely deformed by exposure to methyl hippurate at concentrations of 2 mg/mL, 4 mg/mL and 6 mg/mL. No significant inhibition on tomato seed germination was observed in treatments with methyl hippurate (2 mg/mL) for both 6 h and 12 h soaking period as compared to the controls. Based on these results, B. velezensis CE 100 could be considered a potential agent for development of environmentally friendly disease control strategies as a consequence of the synergetic interactions of diverse crude metabolites and methyl hippurate.

Antifungal Activity of Isolated Compounds from the Leaves of Combretum erythrophyllum (Burch.) Sond. and Withania somnifera (L.) Dunal against Fusarium Pathogens

Crop diseases caused by Fusarium pathogens, among other microorganisms, threaten crop production in both commercial and smallholder farming. There are increasing concerns about the use of conventional synthetic fungicides due to fungal resistance and the associated negative effects of these chemicals on human health, livestock and the environment. This leads to the search for alternative fungicides from nature, especially from plants. The objectives of this study were to characterize isolated compounds from Combretum erythrophyllum (Burch.) Sond. and Withania somnifera (L.) Dunal leaf extracts, evaluate their antifungal activity against Fusarium pathogens, their phytotoxicity on maize seed germination and their cytotoxicity effect on Raw 264.7 macrophage cells. The investigation led to the isolation of antifungal compounds characterized as 5-hydroxy-7,4'-dimethoxyflavone, maslinic acid (21-hydroxy-3-oxo-olean-12-en-28-oic acid) and withaferin A (4β,27-dihydroxy-1-oxo-5β,6β-epoxywitha-2-24-dienolide). The structural elucidation of the isolated compounds was established using nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy (MS) and, in comparison, with the available published data. These compounds showed good antifungal activity with minimum inhibitory concentrations (MIC) less than 1.0 mg/mL against one or more of the tested Fusarium pathogens (F. oxysporum, F. verticilloides, F. subglutinans, F. proliferatum, F. solani, F. graminearum, F. chlamydosporum and F. semitectum). The findings from this study indicate that medicinal plants are a good source of natural antifungals. Furthermore, the isolated antifungal compounds did not show any phytotoxic effects on maize seed germination. The toxicity of the compounds A (5-hydroxy-7,4'-dimethoxyflavone) and AI (4β,27-dihydroxy-1-oxo-5β,6β-epoxywitha-2-24-dienolide) was dose-dependent, while compound B (21-hydroxy-3-oxo-olean-12-en-28-oic acid) showed no toxicity effect against Raw 264.7 macrophage cells.