How Microbial Antagonists Are Shaping the Future of Postharvest Disease Management Through Biological Control

When it comes to preserving the nutritional value of fruits and vegetables, it?s not about keeping them fresh for longer periods. It?s also about ensuring that they are free from harmful pathogens and contaminants that can compromise their quality and safety. However, excessive reliance on these chemicals is raising concerns over the impact on human health and the environment. In recent years, researchers and farmers alike have been exploring innovative strategies for preserving the nutritional value of fruits and vegetables while also keeping them safe and free from disease. This article examines the latest innovations in postharvest disease control and preservation through microbial antagonist biocontrol. Introduction Fruits and vegetables are considered perishable crops compared to grains, legumes, and oilseeds. Most have a very high moisture content, are large, have a higher respiration rate, and are soft in texture, all of which favor the growth and development of various microbial diseases between harvest and consumption. But produce is a living organism whose marketability depends on the prevailing temperature, relative humidity, atmospheric composition during and after harvest, and the nature and extent of microbial and insect infestation. Besides they lose quality during storage due to moisture loss, spoilage by pathogens, rodents, loss of stored energy, loss of nutrients and vitamins, physical loss caused by pests and diseases, quality loss associated with physiological disorders, fiber development, greening (potatoes), root growth, sprouting, rooting, shoot growth, and seed germination. Another cause of postharvest losses is the contamination of food by mycotoxins produced by plant tissues in response to fungal attacks. Understanding the culprits behind postharvest decay A variety of pathogens, such as fungi and bacteria, are responsible for causing disease in fruits and vegetables. However, it is well known that the most significant postharvest losses are caused by fungi such as Alternaria, Aspergillus, Botrytis, Colletotrichum, Diplodia, Monilinia, Penicillium, Phomopsis, Rhizopus, Mucor and Sclerotinia, and bacteria like Erwinia and Pseudomonas. Most of these are weak pathogens that can only invade damaged produce. On the other hand, physical damage is the leading cause of postharvest losses and plays a major role in postharvest deterioration. Several types of damage, caused by weather, insects, birds, rodents, and farm equipment, can occur to produce before and after harvest. Fruit bruises are usually the result of falling onto a hard surface before, during or after packing. However, the damage is usually not immediately clear. Bruising may occur later, but may only be visible on the exterior (eg. apple) or may not be visible until after peeling (eg. potato). Overstocking of bulk commodities in warehouses or overfilling of packages (e.g., grapes) may also result in bruising. Vibration damage may occur in underfilled packages, especially during long-distance road transport. Produce damaged in this way is susceptible to attack by various microbes, resulting in gradual spoilage that may affect the entire production. Bihar Agricultural University published the book Postharvest Disinfection of Fruit and Vegetables?in 2018. Its first chapter,?Postharvest Diseases of Fruits and Vegetables and Their Management, beautifully describes the process of infection. A deep dive into pathogen?s journey from outside to?inside Microorganisms infect produce while immature on the plant (preharvest infection) or during harvest, handling, and marketing (postharvest infection). Mechanical damage to produce, such as fingernail scratches and abrasions, rough handling, insect bites, and cut stalks, facilitates postharvest infection. Produce may be infected by direct penetration of the cuticle or by penetration through stomata, lenticules, wounds, or scar tissue. Preharvest Infection? Infection of fruits and vegetables before harvest can occur in several ways: (1) direct penetration of the skin, (2) infection through natural openings on the produce, and (3) infection through the injured area. On the surface of flower parts and on healthy developing fruits, several types of pathogenic fungi are capable of initiating the infection process.? The infection is then arrested and remains dormant until after harvest when the resistance of the host declines and conditions become favorable for the growth of the pathogen, i.e., when the fruit begins to ripen or its tissues senesce. These ?latent? infections are important in the postharvest losses of many tropical and subtropical fruits. Even weak parasitic fungi and bacteria can enter immature fruits and vegetables through natural openings such as stomata, lenticels, and growth cracks. Again, this infection may not develop until the host becomes less resistant to the invading organism, such as when the fruit ripens. Healthy fruits and vegetables can suppress the growth of these organisms for extended periods of time. Postharvest Infection Many fungi that cause significant crop loss are unable to penetrate the intact peel of the crop but enter through any break in the peel. The damage is microscopic but enough for the pathogens present in the crop to grow. Furthermore, the cut stem is a common entry point for microorganisms, and stem end rot is an important form of postharvest spoilage for many fruits and vegetables. Strategies and solutions for combating postharvest pathogens The basic methods for the control of post-harvest diseases in fruits and vegetables include three different types of approaches: (1) preventing infection, (2) eliminating incipient or latent infection, and (3) preventing pathogen spread in host tissue.? These treatments are generally divided into three groups: physical, chemical, and biological. The effectiveness of each depends on the ability of the treatment or agent to reach the pathogen, the level and susceptibility of the infection, and the susceptibility of the host.? The following section provides a brief overview of the use of microbial antagonists as postharvest biocontrol agents. A comprehensive guide to employing microbial antagonists Antagonism is a phenomenon whereby the action of antagonistic organisms suppresses or disrupts the normal growth, development and activity of environmental phytopathogens. Such organisms can control insect pests and pathogens of horticultural crops and are referred to as ?biological control agents?.? Numerous microbial antagonists with antagonistic activity against pre- and postharvest pathogens have been reported. These microorganisms produce pathogen-specific antifungal compounds/metabolites that inhibit growth and metabolism. They also prevent, inhibit or kill fruit pathogen propagules, thereby controlling further potential fruit spoilage during storage.? Microbial antifungals use both direct and indirect inhibitory mechanisms to biologically suppress fungal growth. Today, several taxonomic groups, including bacteria, yeasts and filamentous fungi, are used as antagonistic microbial agents to control fungal pathogens. During the last decades, various microbial species have been isolated, identified and artificially applied as biocontrol agents on several horticultural commodities. Today is known that microbial-mediated postharvest disease suppression is achieved by exploiting naturally occurring epiphytic antagonistic microflora already present on fruit surfaces and exogenous application of selected microbes with antagonistic activity. Compared to the use of naturally occurring epiphytic microflora, the exogenous application of these microbes is an effective method of postharvest disease control. The review, conducted by the Central Institute of Post-Harvest Engineering & Technology in 2019, analyzes the sources of microbial antagonists, the criteria for their selection, their mechanism of action, and their mode of application. Sources of microbial antagonists Most of the microbial antagonists are present on endemic fruit and vegetable surfaces. Among them, many antagonists have been isolated and identified as suitable biocontrol agents for the control of postharvest pathogens. Besides the fruit surface, microbes may come from related or unrelated sources such as phyllosphere, roots, and soil. Microbial antagonists have also been found in unique natural habitats, such as the ocean. However, compared to those isolated from the fruit surface, marine ones have a greater osmotolerance capacity and thus may be more suitable candidates for use under conditions of high abiotic stress. Criteria for the selection of ideal microbial antagonist For use as postharvest biocontrol agents, effective and antagonistic microorganisms should have desirable characteristics. For example, they should be genetically stable, provide disease control at low concentrations, and be compatible with other physical and chemical treatments. The antagonists must have low-cost nutrition for growth, long shelf life, easy distribution, resistance to common fungicides, and be non-virulent to human health and the host fruit. Demand for an antagonist will increase if it is effective against many fungal pathogens on a variety of fruits, has long-term survival under adverse environmental conditions, does not grow at 37?C, and does not cause infection in humans. In other words, similar growth conditions for antagonists and fungal pathogens improve disease suppression. Under conditions favorable to pathogen growth, the microbial antagonist must perform better. The antagonists should then be able to survive, grow and multiply in an environment favorable to the pathogen. Therefore, the microbial antagonists isolated from the same site are suitable for disease management. Antagonists are better able to adapt to specific environmental conditions than pathogens, resulting in better pathogen control. Another criterion for selection for commercial use is the high viable cell count of a microbial antagonist. Mechanisms of actions of microbial antagonists There are many possible mechanisms for the suppression of pathogen infection. However, competition for nutrients and space, antibiosis through antibiotic production, mycoparasitism, production of cell wall lytic enzymes, and induction of host resistance are the most important biocontrol mechanisms exhibited by antagonists (but often more than one mechanism is involved in successful postharvest biocontrol). Application methods of biocontrol agents Pre-harvest application? Latent infections associated with pathogen infestation of fruit in the field often become a major factor in the occurrence of spoilage during transport and storage. Under these conditions, preharvest applications of antagonistic microbes are often more useful in controlling postharvest spoilage. Field application of microbial antagonists can increase the efficiency of biocontrol because the antagonist has enough time to interact with the pathogens. This also allows the antagonist to precolonize the fruit surface before the pathogens arrive, which occurs with latent infections and incipient infections from harvest injuries. Postharvest application? In this approach, microbial antagonists are applied either as sprays or as dips in solution. This approach is usually more successful than preharvest application and often results in a significant reduction in fungal spoilage. Postharvest biocontrol studies have shown that exogenous postharvest application of microbial bioagents is an appropriate and reliable approach to managing fungal diseases during postharvest handling. Afterword Preserving the nutritional value of fruits and vegetables while ensuring their safety from harmful pathogens and contaminants is a complex challenge. The use of chemicals for postharvest disease management has raised concerns over human health and environmental impact. Researchers and farmers have been exploring innovative strategies to achieve these goals while minimizing risks. This article has shed light on microbial antagonist biocontrol as a promising approach for postharvest disease control and preservation. By understanding the culprits behind postharvest decay and the journey of pathogens from outside to inside, it is possible to develop effective strategies to prevent, eliminate and prevent the spread of postharvest pathogens. Biological control is a sustainable solution that uses naturally occurring microbes to combat pathogens, reduce the use of chemicals, and preserve the quality and safety of fruits and vegetables. Author:?Jorge Luis?Alonso G. (with ChatGPT)?Maximizing Agribusiness Profits with Expert Postharvest Storage Strategies | Horticultural Writing Specialist.?This article was written exclusively for the business platform Postharvest. Cover photo: Courtesy of Pixnio.