Fungicides and Antifungal Compounds: General Concepts and Major Chemical Classes

General Principles of Chemical Control and Agrochemical Components

  • Definition: Chemical control refers to the management of pests or pathogens using chemical substances, termed agrochemicals. These agents must fulfill specific basic criteria to be considered viable for agricultural use.

  • Criteria for Effectiveness:

    • The agent must be effective against the target pathogen at concentrations that will not induce phytotoxicity or harm the host plant.

    • It should exhibit low risks to human health and animal safety.

    • It must have minimal negative impacts on the normal microflora residing on the plant surface or within the soil environment.

    • It should ideally possess a low risk for the development of pathogen resistance.

    • It must be compatible with other chemical agents used in the same management program.

  • Main Components of Agrochemicals:

    • Active Ingredient (AI): The specific chemical compound that provides the biological effect against the pathogen.

    • Adjuvant (or Inert Ingredient): Substances added to the pesticide product to enhance its performance or handling, though they lack direct pesticidal activity.

  • Functions of Adjuvants:

    • Improve Product Activity: Increasing the biological efficacy of the active ingredient.

    • Improve Ease of Application: Facilitating the physical process of applying the chemical to the target.

  • Specific Types of Adjuvants:

    • Surfactants (surface-active agents).

    • Solvents.

    • Emulsifiers.

    • Defoaming agents.

    • Stickers (to improve rainfastness and adherence).

Agrochemical Formulations and Selection

  • Dry Formulations:

    • Dusts (D).

    • Granules (G).

    • Pellets (P).

    • Wettable Powders (W).

    • Soluble Powders (SP).

    • Water Dispersible Granules (WDG).

  • Liquid Formulations:

    • Solutions (S).

    • Emulsions (E).

    • Emulsifiable Concentrates (EC).

    • Aerosols (A).

    • Fumigants (F).

  • Selection Criteria for Chemical Control Agents:

    • Know the Problem: Accurate identification of the pathogen or pest is the first step.

    • Justified Use Only: Application should only occur when economically or biologically necessary in the context of Integrated Pest Management (IPM).

    • Resistance Management: Consideration of the potential for the pathogen to develop resistance must be a priority.

    • Hazard Assessment: Evaluation of potential hazards to humans and animals resulting from direct application or residual traces.

    • Target Plant Organs: Identifying the specific parts of the plant where the agrochemical needs to be applied.

    • Pesticide Label: Reading and deciphering the label is critical for safe and effective use.

Pesticide Health Hazards and Safety Precautions

  • Definitions of Hazard and Risk:

    • Pesticide Hazard=Exposure level+Toxicity\text{Pesticide Hazard} = \text{Exposure level} + \text{Toxicity}

    • Pesticide Health Risk=Exposure level+Toxicity+Probability\text{Pesticide Health Risk} = \text{Exposure level} + \text{Toxicity} + \text{Probability}

  • Exposure Routes: This describes how the chemical enters the body.

    • Dermal: Contact with covered or uncovered skin. This is a prominent and frequent route of exposure.

    • Inhalation (Respiratory): Breathing in air carrying pesticide particles or vapors.

    • Eyes (Ocular): Direct splash of the chemical or transfer through contact with contaminated hands.

    • Oral (Ingestion): Accidental ingestion into the mouth or contact with contaminated lips.

  • Risk Reduction: The primary method to reduce risk is the use of Personal Protective Equipment (PPE).

Classification of Fungicides by Role and Mobility

  • Protectant/Preventive Fungicides:

    • Timing: Applied to healthy plants before infection occurs.

    • Mobility: These cannot be absorbed or transported by plant tissues; they remain on the surface.

    • Advantages: Effective against a broad range of fungi.

    • Disadvantages: Require multiple applications because they are easily washed off or diluted; importantly, new growth occurring after application is not protected.

  • Systemic/Curative & Eradicant Fungicides:

    • Timing: Can be applied before infection (Preventive) or after infection has occurred (Curative or Eradicant).

    • Mobility: Typically absorbed and transported to various extents by plant tissues.

    • Advantages: Capability to control and eradicate infections that are already established within the plant.

    • Disadvantages: Usually consist of uni-site fungicides, which makes them highly prone to the development of pathogen resistance.

  • Categorization by Mobility Patterns:

    • Local Systemic (Translaminar): Fungicide moves through the leaf blade from one side to the other but does not travel widely through the vascular system.

    • Xylem-mobile Systemic: Fungicides that move upward within the plant via the transpiration stream in the xylem.

    • Amphi-mobile Systemic: Fungicides capable of moving both upward in the xylem and downward in the phloem.

Modes of Action (MOA) and the FRAC Classification

  • Definition of MOA: The specific biosynthetic pathway or cellular function that a chemical compound inhibits to prevent fungal growth or survival.

  • General MOA Classifications:

    • Uni-site Fungicides: Affect only one specific target site or biochemical step.

    • Multi-site Fungicides: Affect multiple biochemical processes simultaneously.

    • Specific biochemical targets include cell wall biosynthesis, protein synthesis, nucleic acid replication/transcription, and metabolic processes.

  • The FRAC Code:

    • Assigned by the Fungicide Resistance Action Committee (FRAC).

    • Purpose: To group active ingredients that demonstrate potential for cross-resistance.

    • The code (a number and/or letter combination) reflects the specific target site (the exact step in a biochemical pathway) inhibited by the fungicide.

    • Rule of Thumb: Fungicides with the same FRAC code share the same target site and are at high risk for cross-resistance.

Detailed Biochemical Modes of Action for Common Fungicides

  • Mitochondrial Respiration Inhibitors:

    • Succinate-dehydrogenase inhibitors (SDHIs) (FRAC 7): These inhibit the activity of mitochondrial Complex II (Succinate Dehydrogenase) in the electron transfer chain, thereby blocking respiration.

    • Quinone outside Inhibitors (QoIs) or Strobilurins (FRAC 11): These inhibit respiration by binding at the quinol outer binding site (QoQ_o site) of cytochrome b (cytb\text{cytb}) at Complex III of the electron transfer chain.

  • Cytoskeleton Inhibitors:

    • Benzimidazoles (MBCs) (FRAC 1): Highly specific inhibitors of microtubule assembly. They work by binding to the α−\alpha- and β−\beta- tubulin subunits.

    • Aryl-phenyl-ketones (FRAC 50).

  • Sterol Biosynthesis Inhibitors (SBIs):

    • Demethylation Inhibitors (DMIs/Azoles) (FRAC 3): Target ergosterol biosynthesis by inhibiting the cytochrome P450P450 sterol 14α−demethylase14\alpha-demethylase, which is encoded by the CYP51CYP51 (ERG11ERG11) gene.

    • Toxicity Mechanism of DMIs: Not caused solely by the depletion of ergosterol in the membranes, but also by the resulting accumulation of toxic 14−methyl14-methyl sterols induced by the azole treatment.

    • Morpholines (FRAC 5).

  • Nucleic Acid Metabolism Inhibitors:

    • Phenylamides (PA) (FRAC 4): Highly specific inhibitors of ribosomal RNA (rRNA\text{rRNA}) biosynthesis, specifically targeting the RNA polymerization process (RNA polymerase complex I).

  • Signal Transduction Inhibitors:

    • Quinolines (FRAC 13).

    • Dicarboximides (FRAC 2).

    • Phenylpyrroles (FRAC 12).

  • Cell Wall Synthesis:

    • Polymyxin D (FRAC 19).