Fungicides are chemical or biological agents designed to control fungal pathogens, which cause crop loss and plant damage worldwide. Fungi feed on living plants or dead organic matter, leading to infections such as rusts, mildews, and blights. Understanding how these products work is necessary for protecting gardens and agricultural yields. This guide explains the mechanisms, classifications, and best practices for safe application.
How Fungicides Prevent and Treat Disease
Fungicides interfere with the life processes of the target fungus, preventing spores from germinating or disrupting established growth. Mechanisms include inhibiting cellular respiration, which stops energy production and starves the pathogen. Another method disrupts the fungal cell membrane by inhibiting sterol production.
The two main strategies are preventative and curative. Preventative fungicides are applied before infection, acting as a protective barrier that stops fungal spores from germinating or penetrating the plant tissue. These products are the most effective way to manage disease.
Curative (eradicant) fungicides stop established infections, generally only in their early stages. They are absorbed into the plant, stopping pathogen growth after spore germination. They cannot repair damaged tissue and are most effective only within the first 24 to 72 hours following initial infection.
Categories of Fungicides
Fungicides are classified based on how they interact with the plant: contact or systemic. Contact fungicides (protectant fungicides) remain on the outside of the plant tissue, forming a surface barrier against incoming spores. Complete coverage is necessary for effectiveness since they do not penetrate the plant.
Systemic fungicides are absorbed into the plant’s tissues and move through the vascular system, offering internal protection. Some are locally systemic, moving short distances or redistributing (translaminar movement). Systemic products resist being washed off by rain and can protect new growth.
Biological fungicides (biofungicides) are derived from beneficial microbes, such as Bacillus subtilis, or naturally derived compounds. They work by colonizing the plant surface to out-compete or attack pathogens, or by inducing natural defense mechanisms. Biofungicides are a lower-risk option for use on edible plants and in home gardens.
Proper Application Techniques
Effective fungicide use requires strictly following the product label, a legal document containing instructions for application, dosage, and safety. The label specifies correct mixing rates and dilution, ensuring the concentration is effective without harming the plant. Applying too low a concentration can lead to resistance development.
Complete coverage is necessary for both contact and systemic products. Ensure the undersides of leaves are treated, as this is where many fungal spores germinate. Application timing is also important, often requiring application before disease is present or at a specific vulnerable growth stage. Applying when temperatures are cooler and wind is low maximizes effectiveness and minimizes drift.
Fungicide rotation is necessary for long-term disease management to prevent resistance. Rotation involves alternating products with different Modes of Action (MOA), identified by the FRAC code on the label. This strategy prolongs the effectiveness of available treatments.
User and Environmental Safety
Personal Protective Equipment (PPE) is necessary to minimize exposure during mixing and application. The label specifies required PPE, including chemical-resistant gloves, long-sleeved shirts, long pants, eye protection, and sometimes a respirator. Users must wash their hands thoroughly after handling.
Environmental safety practices prevent chemicals from contaminating surrounding areas, including preventing runoff into soil, groundwater, or surface water sources. For edible plants, the Pre-Harvest Interval (PHI) listed on the label must be observed; this is the minimum number of days between the last application and harvest.
Proper disposal of excess pesticide and containers is the final step. Empty containers should be triple-rinsed according to label instructions before recycling or disposal. Contaminated PPE and any excess product must be disposed of as household hazardous waste, following local and federal regulations.