Learning objectives
- Classify control measures into five categories with examples
- Explain the economic threshold and scouting
- Explain fungicide modes of action and resistance management
1. The five categories of control
Answer every control question with these headings and you cover the syllabus.
- Cultural — crop rotation, resistant varieties, certified seed, spacing, sanitation, timing of planting
- Physical / mechanical — roguing infected plants, solarisation, hot-water seed treatment, pruning
- Biological — Trichoderma, Bacillus subtilis, predatory insects and natural enemies
- Chemical — protectant and systemic fungicides, seed dressings, bactericides
- Legislative — quarantine, certification schemes, restrictions on plant movement
2. Economic threshold
The economic threshold is the pest or disease level at which the cost of control equals the value of the loss prevented. Spraying below the threshold wastes money and encourages resistance; spraying above it is already too late. Scouting on a fixed pattern (W or X across the field) supplies the data.
3. Fungicides and resistance
Protectant (contact) fungicides such as mancozeb and copper stay on the leaf surface and must be applied before infection. Systemic fungicides such as metalaxyl and the triazoles move inside the plant and give some curative action.
Single-site systemic products select strongly for resistance. Manage it by alternating FRAC groups, tank-mixing with a multi-site protectant, and never using more than the labelled number of applications per season.
4. Safe use and pre-harvest interval
Always state protective equipment, correct dosage, calibration of the sprayer, and observance of the pre-harvest interval (PHI) — the minimum days between the last spray and harvest. Examiners award marks for safety points that most candidates forget.
Key definitions
- IPM
- Integrated Pest Management — combining cultural, biological, physical and chemical methods to keep pests below economic damage levels.
- Economic threshold
- Pest density at which control action must be taken to prevent loss exceeding the cost of control.
- Pre-harvest interval
- Minimum legal period between the last pesticide application and harvest.
Exam tips
- Never answer a control question with chemicals only — you will lose most of the marks.
- Name actual active ingredients (mancozeb, copper oxychloride, metalaxyl) rather than 'a fungicide'.
Practice exam questions & model answers
Q1.Outline four cultural methods of controlling soil-borne plant diseases.
Crop rotation with non-host crops to starve the pathogen; use of certified disease-free planting material; sanitation by removing and destroying crop residues; and improving drainage and spacing to reduce soil moisture and humidity around the plant base.
Q2.Explain why alternating fungicides from different FRAC groups reduces resistance.
Resistant individuals selected by one mode of action are still killed by a product with a different mode of action, so no single resistant strain can dominate the population.
Q3.Distinguish between a protectant and a systemic fungicide.
A protectant remains on the plant surface, forms a barrier and must be applied before infection; a systemic is absorbed and translocated within the plant tissue and can act after infection has begun.
Continue revising
Plant Cell Structure and Function
Plant BiologyPhotosynthesis: Process, Equation and Limiting Factors
BotanyLeaf Structure, Stomata and Transpiration
Agricultural SciencePlant Nutrition, NPK and Soil Fertility
Put the theory to work — diagnose a real leaf photo or browse the disease library.