06
Sep
Biofungicides Show Promise in Agriculture and Land Management, Study Finds
(Beyond Pesticides, September 6, 2024) A literature review in the Internal Journal of Molecular Sciences provides promising insights into biofungicides as a “sustainable and economically viable alternative†to synthetic fungicides in expanding organic agriculture. The authors note that organic “… is the most sustainable response to current crises of all kinds, as it can better anticipate and prepare for crises and create long-term equity and resilience in food systems.†The authors point out that fungal infections in crops are estimated to account for 20-40% of failures annually, and understanding how to control such agricultural diseases will be crucial to meeting the needs of a growing global population. Organic farmers and land managers note that biological tools can be integrated into practices that work with the ecosystem, rather than be utilized as “substitute†products or controls with practices that ignore soil health and beneficial organisms that enhance biodiversity and provide ecosystem services (see here and here).
Conducted by researchers in Mexico, the review examines data on biosynthesis (how plants create their own fungicide, known as secondary metabolites or SMs); the mechanisms of action of secondary metabolites against phytopathogenic (plant-killing) fungi; extraction techniques and biofungicide formulations; the biological activity of plant extracts on phytopathogenic fungi; and an overview of the current regulation and use of biofungicides in agriculture.
According to the authors, many plants can synthesize one or more of several types of secondary metabolites (SMs) that create very specific conditions. SMs can be categorized into groups based on their chemical structure and biological activity: terpenes (including volatile compounds, sterols, and carotenoids), polysaccharides, phenolic compounds, phytoalexins (sulfur-containing compounds), alkaloids (nitrogen-containing compounds), flavonoids, and hydrocarbons. These compounds are often specific to a plant genus or family and often produced in small amounts. Given their natural origins, they are environmentally friendly with a short environmental lifespan that does not pollute soil and water. It is believed that SMs can even create a microclimate by slowing respiration and thereby protecting the plant during unfavorable conditions.
Because the targeted biofungicides come from natural rather than synthetic ingredients, the authors cite research indicating limited persistence with a reduced half-life in the environment when used. The article notes that while not always harmless to other plants and organisms, plant-based chemical compounds can impact nontarget organisms, but are less toxic than synthetic pesticides. Depending on the plant source and the concentrations used, biofungicides have little influence on the growth, survival, development, and reproduction of other organisms. The review continues and notes that biofungicides exhibit “no residual hazards, and minimize “the pollution of soil, water, and the atmosphere… and finally, they promote a reduction in health problems in farmers, such as chronic degenerative diseases of the skin and respiratory tract associated with the use of synthetic pesticides.†(See here for authors’ citations). Health and environmental advocates note that synthetic fungicides are not only associated with adverse effects on health and the environment but also precipitate resistant fungi that threaten health on a global scale. (See Beyond Pesticides’ reporting on antimicrobial and fungal threats here, here, and here).
Researchers identify three SM primary types: terpenes and terpenoids, alkaloids, and phenolic compounds that are produced via four different mechanisms: the shikimic acid pathway, the malonic acid pathway, the mevalonic acid pathway, and the non-mevalonate (MEP) pathway.
Terpenes
Research shows that due to their lipophilic nature, terpenes can work against fungi by infiltrating the fungal cell membrane and destabilizing the cell. Terpenes can also destroy mitochondria, the part of the cell responsible for energy production, and induce cell death by cellular respiration and oxidative phosphorylation or inhibition of electron transport in the mitochondrial electron transport chain.
Phenolic Compounds
Researchers hypothesize that phenolic compounds can depolarize [neutralize] cellular and mitochondrial membranes of fungi, impairing the ion gradients and eventually causing cell death. Phenolic compounds can also inhibit key enzymes necessary for proper functioning of the biological system. It has also been demonstrated that phenolic compounds can modulate gene expression, which can alter growth, development, and reproduction of fungi.
Alkaloids
Alkaloids have been known to impair fungal gene replication and transcription by infiltrating the fungus’ DNA. They can also disrupt ion gradients in cell membranes by creating ion channels, leading to cell death. Alkaloids can also attach to proteins in the fungal organism, which prevents them from interacting with a plant’s receptors and prevents colonization and infection by the fungus.
The research also highlights that certain biofungicides operate similarly to synthetic fungicides due to a chemical structure akin to fungi’s naturally occurring nucleic acids, which prevent the proper performance of fungi biological functions, such as acylalanines which inhibit ribosomal RNA synthesis. Researchers identify extraction methods using specific solvents via conventional and unconventional techniques. Primarily, extraction is done using solvents and either heating and/or mixing. The type of solvent used depends on the type of compound to be extracted and largely determines the efficiency of the extraction. According to researchers, the polarity of the desired product is the most important factor determining which solvent should be used for extraction. Conventional extraction methods include the Soxhlet extraction, maceration, and hydrodistillation, while unconventional methods include ultrasound-assisted extraction, pulse electric field extraction, enzyme-assisted extraction, microwave-assisted extraction, pressurized liquid extraction, and supercritical fluid extraction.
The authors highlight several techniques that avoid using solvents (hydrodistillation) or reduce the use of solvents (enzyme-assisted extraction and pressurized liquid extraction) as more environmentally responsible and call for additional considerations when selecting solvents: “environmental safety, human toxicity, and financial viability.†Extract-based biofungicides in agricultural systems offer significant advantages to farmers, including enhanced food security, reduced presence of phytopathogens, improved (crop) product quality, and the potential for higher market prices for organic products. Some examples the authors cite of significant phytopathogenic fungi and effective biofungicides include:
- Monilinia fructicola, which infects crops such as peaches, apricots, plums, almonds, apples, and pears. In one study, scientists used purified polyphenolic extract of orange peel while another study used aqueous extracts of rapeseed and Indian mustard to successfully inhibit the growth of Monilinia fructicola by an average of 95% and 31%, respectively.
- The authors called Colletotrichum spp. one of the “most important group of phytopathogenic fungi in the world,†infecting crops such as strawberries, mangoes, avocados, corn, sugarcane, and sorghum and causing sunken necrotic lesions, crown and stem rot, and seedling blight. One study cited found that the extract of Brazilian red propolis inhibited 42% of the growth of Collectotrichum musae within in vitro tests.
- Alternaria alternata, a widespread fungus, is known to cause significant postharvest losses by forming black spots on various fruits and vegetables during cold storage and the marketing period. It affects crops like mangoes, cherry tomatoes, apples, mandarins, kiwifruits, and melons.
- Studies by ​Hernández et al. reveal that polyphenolic extracts from orange peel have potent antifungal activity, completely inhibiting the growth and spore germination of Monilinia fructicola, Botrytis cinerea, and Alternaria alternata. This effect is mainly due to the presence of flavonoids (naringin, hesperidin, and neohesperidin) and phenolic acids (ferulic acid and p-coumaric acid) in the peel.
Despite the promising antifungal properties of these plant extracts, there is no standardized concentration range to classify their effectiveness. Establishing a consensus on concentration ranges is recommended to better categorize these extracts as active, moderately active, slightly active, or harmless, thereby facilitating their application in agricultural settings.
This review demonstrates an evolving understanding of biofungicides—what they are, how they work, and how to acquire them. While the replacement of synthetic fungicides with biofungicides would enable worldwide food production with no or minimal fungicide residue, the authors highlight obstacles still in place preventing broader development.
Currently, the authors note, biofungicides are more expensive and do not yet achieve the level of pest control of traditional synthetic fungicides. There are also challenges related to handling, applying, and producing these natural products that are readily degraded by air, light, and temperature extremes.  In addition, the authors highlight the biopesticide registration processes as a major impediment to a broader scale of use. The cost associated with developing a product, bringing it to market, and navigating national regulation processes can vary globally as biopesticide registration processes rely “too heavily on the criteria used for chemical fungicides and require information that is not as readily available for biofungicides,†noting that expensive toxicological and environmental risk assessments are required, leaving large companies better able to afford the registration process. In the U.S., a biofungicide may be permitted under the Organic Foods Production Act and reviewed by the National Organic Standards Board and the National Organic Program at the U.S. Department of Agriculture as a soil amendment.
The growing demand for organic food contributes, the study argues, to a 15% growth rate for biofungicides annually. Given the detrimental effects of synthetic fungicides on the environment and human health, the authors call for and predict stricter government regulations on synthetic fungicides. This shift could result in increased demand for plant-based alternatives. Plant extracts offer a compelling solution as they are effective, biodegradable, and pose a feasible compared to synthetic chemicals. Transitioning to plant-based formulations aligns well with a forward-thinking approach to food and agricultural policy. Consequently, the production of biofungicides should become standard practice, and clear regulatory frameworks for their commercialization will contribute to the elimination of petrochemical pesticides.  To stay informed and participate in the NOSB review process, sign up for news alerts from Beyond Pesticides here. See also Beyond Pesticides’ Organic Agriculture page.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Sources:
Bio-pesticides for agriculture and environment sustainability, International Journal of Molecular Sciences, special issue, Molecular Studies on Plant and Plant In Vitro Systems Secondary Metabolism, June 2024 https://doi.org/10.3390/ijms25136879Â
“Biopesticides,†with Broad Definition, Challenged as Unsustainable, Daily News, Beyond Pesticides, August 13, 2021
