What Is a Biocide? Classification, Mechanisms and Applications
Surveys show that more than 80,000 species of plant-pathogenic microorganisms (fungi, bacteria, rickettsia, mycoplasma, viruses, algae, etc.) exist worldwide. Plant diseases cause enormous losses to agriculture, reducing global crop yields by about 500 Mt per year on average. History has repeatedly seen famines - even mass starvation - caused by epidemics of a single plant disease. Using biocides (fungicides/bactericides) is an economical and effective way to control plant diseases. On this basis, industry experts have carried out extensive research and improvement.
Multi-Site Protective Agents
Such agents act on multiple targets and can disrupt many steps in the important metabolic pathways of pathogenic microorganisms, so pathogens cannot develop high-level resistance through mutation of a single gene. Representative compound classes include dithiocarbamates (mancozeb, propineb, etc.), which mainly inhibit the sulfhydryl enzymes involved in many biochemical reactions in fungal cells (such as pyruvate dehydrogenase); phthalimides (captan, folpet), whose mechanism is not yet fully understood but is presumed to relate to regulation of the respiratory chain; and chlorinated nitrobenzenes (quintozene), used for soil disinfection against Rhizoctonia and other soil-borne diseases. As mainstay crop-protection chemicals, multi-site protectants are often combined with specific insecticides or fungicides to slow the emergence of resistance, though their non-selective toxicity may pose ecological and safety risks.
Benzimidazole Fungicides and Resistance Management
Compounds such as carbendazim, thiophanate-methyl and benomyl belong to the benzimidazole fungicides. Their mechanism is specific binding to the beta subunit of fungal tubulin, blocking microtubule polymerization, disturbing the formation of the mitotic spindle and halting nuclear division. Because of their excellent systemic activity, broad-spectrum activity and low cost, they have been widely used in world agriculture and forestry. However, because their target is highly specific, some pathogens (Botrytis cinerea strains causing gray mold, powdery mildew pathogens, etc.) can accumulate effective resistance through point mutations, producing stable and reliable resistant phenotypes that sharply reduce or completely nullify drug efficacy. This has become a classic example showing that the traditional fungicide design philosophy must change, driving modern resistance management strategies and the development of new high-efficiency, low-risk pesticides.
Plant Immune Activators
These substances, known as plant immune activators, are characterized by having no direct germicidal capability or only limited toxicity, but being recognizable by plants. Through certain signal transduction pathways they activate the plant's internal defense system, inducing systemic acquired resistance or systemic resistance and improving the plant's ability to resist subsequent pathogen attack. By origin and nature they fall into two classes. The first is chemically synthesized inducers, mainly benzothiadiazole (BTH), tiadiniil and isotianil, which work by mimicking natural biological signals. The second comes from natural or biological sources: chitosan (a chitin derivative), salicylic acid and its analogs, methyl jasmonate, and various oligosaccharides (lentinan, chitin oligosaccharides, etc.). These products control disease effectively with long-lasting action and clear environmental and ecological-safety advantages, making them an important direction for green agriculture. Because the induced effect is influenced by the plant's growth stage, genotype and environmental factors (temperature, light), they generally need to be applied early or in advance for the best protection.
New and Functional Formulations
To overcome the shortcomings of traditional pesticide formulations and meet increasingly strict environmental regulations and the demands of modern agriculture, new environmentally friendly, functional formulations are becoming the main direction of R&D. Water-based formulations dominate thanks to their clear advantages, mainly suspension concentrates (even dispersion of the technical material in water with polymeric suspending agents), emulsions in water (oil-water systems stabilized with surfactants) and microemulsions (thermodynamically stable and transparent). Water-dispersible granules combine suspension concentrates with wettable powders, offering rapid disintegration, dust-free handling and precise application. Functional formulations mainly improve efficacy, extend action and reduce ecological risk: polymeric carriers encapsulate target compounds into micron-scale particles for sustained release, while nano-formulations, with their special physicochemical properties, improve solubility and enable targeted delivery, and intelligent responsive systems can further improve pesticide performance and advance dose-reduction strategies. Dedicated formulations for particular crops or environmental conditions keep emerging, including seed-treatment suspensions and low-volume spray liquids, all with good prospects.
Application Methods
This model is based on traditional equipment and processes with standardized operating characteristics. Spraying is the main application method; depending on dosage per unit area and droplet distribution, it can be classified into conventional-rate spraying, low-volume spraying and ultra-low-volume application. Dusting has gradually exited the market because it easily causes drift and environmental pollution. Seed-coating technology (seed dressing or film coating) is widely used to control seed-borne, soil-borne and seedling diseases by forming a protective film on the seed surface. Soil treatment mainly includes drenching, furrow application and broadcasting of granules, used mainly against root diseases. Fumigation is used mainly for full disinfection of greenhouses or seedbeds, but because it is complex and high-risk it is generally carried out by professionals. In determining a specific application plan, factors such as disease type, chemical properties, crop condition and environmental conditions should all be considered.
Precision Application
Precision application technology is a key means of raising pesticide utilization and achieving dose reduction, and has become a major development direction in modern plant protection. The main research directions are variable-rate application based on information sensing, development of high-efficiency spraying equipment, and application innovation for special scenarios (such as aerial application). Precision variable application relies on numerous sensing technologies and intelligent algorithms, integrating machine vision, multispectral imaging and meteorological monitoring, with the help of geographic information systems, to produce differentiated prescription maps and thereby tailor pesticide use to specific crops or pests and diseases. High-efficiency spraying mainly overcomes the waste caused by traditional uniform spraying - for example using electrostatic effects to enhance droplet adhesion or recycling systems to reduce residual pollution. For operations in complex environments, air-assisted sprayers and plant-protection drones also offer great advantages, greatly improving work efficiency while lowering operational difficulty and technical requirements. Targeted application is a frontier concept with preliminary application validation in the control of specific crops or diseases, providing an important theoretical basis for future green and sustainable agriculture.
Wood Preservation
Wood decay, mold and biological attack greatly affect wood durability; effective preservative treatment is the main means of improving wood's resistance to microbial attack. Industry currently relies mainly on pressure impregnation, using alternating vacuum and pressure gradients to make the preservative fully penetrate the wood cell walls. As environmental regulations tighten, traditional heavy-metal preservatives (arsenic, chromium) are being phased out, and the development of new environmentally friendly alternatives has become the research focus. Copper-based ACQ composites, CA-C systems, borates and azole organic biocides have become the main product types. Rubberwood, a typical easily decayed wood with high starch and cellulose content, is prone to fungal infection and quality decline. To address this, R&D teams have developed highly efficient and environmentally friendly boron-based treatments, using high-pressure processes to overcome the biological limitations of rubberwood and enabling its efficient recycling, providing key technical support for sustainable management of the world's wood resources.
Medical Disinfectants
In healthcare, to meet the microbial-kill performance required in different scenarios, biocides are divided into three classes: high-level disinfectants/sterilants, intermediate-level disinfectants and low-level disinfectants. High-level disinfectants and sterilants have broad-spectrum activity and can kill bacterial spores and all other microorganisms; typical representatives are glutaraldehyde, peracetic acid and chlorine dioxide, commonly used to disinfect endoscopes and precision instruments. Intermediate-level disinfectants inhibit most pathogens well - they kill vegetative bacteria, mycobacteria, some viruses and fungi but are weak against spores; common examples are chlorine compounds (sodium hypochlorite), alcohols (ethanol or isopropanol), and phenols and their compound preparations (iodophors), used mostly for cleaning and disinfecting everyday environmental surfaces. Low-level disinfectants remove only some non-resistant bacteria and common lipophilic viruses or fungi - namely quaternary ammonium compounds (benzalkonium bromide, etc.). Strictly following disinfectant concentration standards, scientifically determining contact time and operating by procedure is the foundation of hospital infection control.
Preservatives in Cosmetics and Personal Care
In the manufacture and storage of cosmetics and personal care products (shampoos, body washes, lotions, cleansers, etc.), wet wipes and household liquid cleaners, broad-spectrum functional preservatives must be scientifically selected and properly added to ensure product safety and quality and prevent microbial contamination that could harm human health. An ideal preservative should have good chemical stability, good formulation compatibility, and no skin irritation or low sensitization. The main preservatives on the market today include paraben compounds based on p-hydroxybenzoic acid esters, DMDM hydantoin and imidazolidinyl urea that release formaldehyde slowly for antibacterial effect, and phenoxyethanol, together with mixed high-performance antimicrobial systems combining these. Consumer preference for "preservative-free", "gentle" and "natural" claims is driving the development of preservative-free or novel alternative products as an industry trend. Under the cosmetics safety regulations of various countries and regions, market access review follows each jurisdiction's own laws and regulations. Cosmetics safety technical specifications are followed by domestic regulators; the EU has built a fairly systematic and complete framework under Regulation EC 1223/2009 and other provisions, and continues to update the relevant assessment guidelines and scientific advice.
Protective Agents
When a chemical is applied to the plant or its environment before the pathogen contacts or penetrates the plant, inhibiting spore germination or killing germinating spores and thereby protecting the plant, the effect is called a protective effect, and such agents are protective agents. Examples include Bordeaux mixture, zineb, copper sulfate, copper soap, mancozeb and chlorothalonil.
Non-Systemic Fungicides
Non-systemic (contact) fungicides are not absorbed or translocated within the plant. Most products in this class do not easily induce resistance in pathogens and are economical, but most have only a protective effect and cannot control diseases deep inside plant tissues. Examples include zinc sulfate, copper sulfate, dodine, chlorothalonil, copper soap, surfactants, synergists, lime-sulfur mixture, wood ash, Bordeaux mixture, mancozeb and thiram. Fungicides can also be classified by use method, such as seed-treatment agents, soil disinfectants and spray agents.
Mechanisms of Action
Interfering with the respiration of fungi and inhibiting energy production.
Interfering with the biosynthesis of essential life substances such as proteins, nucleic acids and sterols.
Protective fungicides are mostly broad-spectrum products with relatively low killing power. Systemic fungicides are generally more potent but narrower in spectrum, and some have specific selective toxicity against particular pathogens. Because systemic agents have a relatively single site of action inside the fungus, resistance easily arises through genetic mutation. To avoid or delay resistance, appropriate protectants and systemic agents are usually mixed or alternated, combining their strengths for better control. Depending on the characteristics of the disease, seed treatment, foliar spraying and soil treatment should be chosen as appropriate.
Toxicity and Efficacy Metrics
The insecticidal or germicidal potency of an agent is commonly expressed as the "median lethal dose" - the dose required to kill half (50%) of a biological population (median lethal dose, mg/kg), abbreviated LD50. If concentration expresses the dose, it is the "median lethal concentration", abbreviated LC50. For fungicides, ED50 or EC50 is used - the dose or concentration that inhibits spore germination by 50%.
To measure this, a suspension of pathogen spores or mycelium is mixed with agar medium. After the medium solidifies, sterile filter-paper discs (about 6 mm in diameter) dipped in solutions of different concentrations are placed on the agar surface. After incubation at a constant temperature for a set time, the diffusion of the solution inhibits fungal growth, forming an "inhibition zone". The size of the inhibition zone is measured to compare the potency of fungicides.
Regulation and Standards
Market access for pesticides is strict across countries and regions, and biocide products are managed under the principles of "prevention first and risk control as the foundation". Under China's Pesticide Management Regulations, a new fungicide must pass a series of evaluation procedures before entering the market. The applicant submits a scientific, systematic data package to the national agricultural and rural authorities, including basic substance information (chemical structure, physicochemical properties, quality standards); safety analyses covering acute toxicity, chronic effects, genotoxicity and other test results; efficacy trials against target organisms; environmental metabolism studies; and ecological compatibility assessment. Only after comprehensive review by national professional institutions, and only if the product shows clear control efficacy within the approved uses while effectively protecting user health and ecosystem stability, can it qualify for a formal registration certificate. This approval process is generally time-consuming, technically demanding and costly.
Maximum residue limits (MRLs) are established by the Codex Alimentarius Commission and government bodies (China's National Health Commission and Ministry of Agriculture and Rural Affairs, the US FDA, the European Commission, etc.). Relying on standardized test data, national dietary structure analyses and toxicological evaluations (i.e., the acceptable daily intake, ADI), these standards provide key technical support for the safe food supply worldwide.
The risk assessment framework is the main approach of modern pesticide management, using the internationally accepted four-step process: hazard identification, dose-response assessment, exposure assessment and risk characterization. This system can accurately assess the severity of a chemical's impact on human health and provides a scientific reference for subsequent regulation and supervision decisions.
Good agricultural practice (GAP) is an internationally accepted guideline for improving the quality and safety of agricultural products. It aims to guide growers in the correct use of scientific plant-protection techniques and in recording detailed information such as pesticide names and quantities, reducing pollution and harm from unnecessary pesticide use at the source and promoting sustainable production.
Resistance management measures: special management recommendations for crop pest and disease resistance advocate rotating or mixing different classes of chemicals to slow the evolution of resistance in target organisms, and promote the development and application of ecologically friendly control technologies.
