Welcome to Crop Protection: Disease Control!
Hi future agriculturalists! Protecting crops from disease is one of the most important jobs on the farm. A small infection can wipe out an entire harvest, which affects both the farmer's income and our food supply.
In this chapter, we will learn how tiny enemies—bacteria, fungi, and viruses—attack plants, and more importantly, how we can stop them. Don't worry if this seems tricky at first; we will break down each type of disease step-by-step!
Why is Disease Control Essential?
Plant diseases cause massive economic losses globally by reducing crop yield and quality. If you can keep your crops healthy, you maximize your yield and ensure a better market price.
Quick Review: What is a Plant Disease?
A plant disease is any abnormal condition that impairs the plant's ability to function normally (e.g., photosynthesize, absorb water, or reproduce). It is usually caused by a living organism, known as a pathogen.
1. Control of Bacterial Diseases
Bacteria are single-celled organisms that can cause serious plant infections, often leading to rapid decay or wilting.
Named Example: Tomato Bacterial Wilt (Ralstonia solanacearum)
This is a particularly destructive disease that affects tomatoes, potatoes, tobacco, and other important crops grown worldwide.
1. Mode of Infection (How it Spreads):
• Bacterial wilt is primarily soil-borne. The bacteria live and survive in the soil for long periods.
• They enter the plant through wounds (caused by insects, tools, or natural damage) or through the root tips.
• Spread is easily accomplished by irrigation water, contaminated tools (like hoes or spades), and movement of infected soil or planting material.
Did you know? Once inside, the bacteria multiply rapidly and clog the plant's vascular system (the xylem), stopping the flow of water—like rust clogging a pipe!
2. Harmful Effects:
• The most noticeable effect is sudden wilting of the plant, often during the hottest part of the day, even if the soil is moist.
• The stem base and roots may show a slimy, brown discoloration when cut.
• Eventually, the entire plant collapses and dies, leading to 100% crop loss in severe cases.
3. Prevention and Control:
Bacterial diseases are difficult to treat once established, so prevention is key!
• Prevention (Cultural Methods): Use crop rotation, avoiding planting susceptible crops (like tomatoes) in the same soil for several years.
• Use certified, disease-free seed or seedlings.
• Improve soil drainage, as waterlogged conditions favour bacterial growth.
• Control (Sanitation): There is no effective chemical cure (bactericide) available to farmers for this disease.
• Immediately remove and destroy (burn or deep bury) all infected plants and surrounding soil.
• Sterilize tools using bleach or heat after working in an infected area to stop mechanical spread.
Key Takeaway: Bacterial wilt is soil-borne and kills by blocking water flow. The best defense is sanitation and crop rotation.
2. Control of Fungal Diseases
Fungi are perhaps the most common cause of plant disease. They reproduce using tiny spores, which often spread through the air or water.
Named Example: Late Blight of Potato (Phytophthora infestans)
Late Blight is famous for causing the Irish Potato Famine in the 1840s. It remains a serious threat to potato and tomato crops globally.
1. Mode of Infection (How it Spreads):
• Spread primarily by airborne spores, especially when humidity is high and temperatures are cool (around 10–20 °C).
• Rain or irrigation splashes spores onto healthy leaves.
• The fungus can survive in infected seed tubers left over from the previous season (called 'volunteer' plants).
2. Harmful Effects:
• On leaves, it appears as dark, water-soaked spots that quickly turn brown or black.
• In humid conditions, a characteristic white, fuzzy mold (the fungal growth) appears on the underside of the leaves.
• The disease spreads rapidly, killing the leaves and stems (haulms), preventing the plant from producing carbohydrates.
• If the spores wash into the soil, they infect the tubers, causing a dry, reddish-brown rot beneath the skin, making them inedible and unmarketable.
3. Prevention and Control:
• Prevention (Cultural): Plant resistant varieties (cultivars).
• Earthing up (mounding soil around the base of the plant) helps prevent spores from washing down and reaching the tubers.
• Ensure proper spacing to improve air circulation and reduce humidity around the leaves.
• Control (Chemical): Apply specific fungicides (chemical sprays) on a regular schedule, especially when weather conditions favour the disease (cool and wet).
• Destroying the haulms (by chemical or mechanical means) just before harvest prevents infection from spreading to the tubers.
Key Takeaway: Fungal blight is airborne and thrives in cool, moist conditions. It is mainly controlled using resistant varieties and fungicides.
3. Control of Viral Diseases
Plant viruses are extremely tiny particles that must infect a living cell to reproduce. They cannot be killed with traditional chemicals once inside the plant.
Named Example: Tobacco Mosaic Virus (TMV)
TMV is one of the oldest known plant viruses. It infects hundreds of species, including tobacco, tomatoes, peppers, and beans.
1. Mode of Infection (How it Spreads):
• Unlike fungi, viruses cannot move on their own. They require help.
• Mechanical Spread: TMV is easily spread by contact—when farm workers handle infected plants and then touch healthy ones, or via contaminated tools.
• Vectors: Spread by sucking insects, known as vectors (e.g., aphids, leafhoppers), which bite an infected plant and then feed on a healthy one, transferring the virus.
2. Harmful Effects:
• The virus disrupts the production of chlorophyll, leading to characteristic discoloration known as a mosaic pattern (light green and dark green mottling) on the leaves.
• Leaves may become distorted, crinkled, or dwarfed.
• Overall, viral infection severely reduces photosynthesis, resulting in stunted growth and significantly lower crop yields and poor quality fruit.
3. Prevention and Control:
• Prevention: Use only certified, virus-free seeds and planting material.
• Practice excellent field hygiene: constantly wash hands and sterilize equipment (tools, knives) between working on different plants.
• Vector Control: Use insecticides to control the aphids or other insects that spread the virus.
• Control: Just like a computer virus, there is no chemical spray that cures an infected plant.
• Rogueing: Immediately remove and destroy (ideally by burning) any infected plants to prevent the spread to neighbours.
• Use resistant cultivars developed through selective breeding.
Common Mistake to Avoid: You cannot cure a plant virus with a fungicide or a bactericide!
4. Summary of Disease Control Strategies
Regardless of whether the pathogen is a bacterium, fungus, or virus, farmers use a combination of methods for crop protection. These fall into three main categories (similar to pest control methods):
A. Cultural Control (Farm Management Practices)
These methods aim to make the environment less hospitable to the pathogen.
• Crop Rotation: Breaks the disease cycle, especially for soil-borne diseases (like Bacterial Wilt). If a pathogen only feeds on maize, rotating to beans starves the pathogen.
• Field Sanitation: Removing and destroying plant debris or infected plants (like 'rogueing' for TMV). This reduces the source of infection.
• Good Drainage: Reduces the moist conditions that favour fungal spores (like Late Blight).
• Use of Resistant Varieties: Planting cultivars that have been bred to naturally fight off specific diseases.
B. Chemical Control
This involves using chemicals to kill the pathogen or the vector.
• Fungicides: Chemicals specifically designed to kill fungal pathogens (e.g., used to control Late Blight). They are usually applied as preventative sprays.
• Insecticides: Used to kill insect vectors (like aphids) that spread viral diseases (like TMV).
Note: Chemicals are generally ineffective against bacterial or viral diseases already inside the plant tissue.
C. Biological and Biotechnical Control
These methods use living organisms or modern breeding techniques.
• Breeding: Developing plants with genetic resistance is the most sustainable way to control many diseases.
• Biocontrol Agents: Although less common for general crop disease in IGCSE scope, sometimes beneficial microorganisms are used to suppress pathogens in the soil.
Quick Review Box: Disease Summary
| Disease Type | Named Example | Primary Mode of Spread | Key Control Method |
| Bacterial | Tomato Bacterial Wilt | Soil/Water, Mechanical (Tools) | Crop Rotation, Sanitation, Destruction |
| Fungal | Late Blight of Potato | Airborne Spores, High Humidity | Fungicides, Resistant Varieties, Earthing Up |
| Viral | Tobacco Mosaic Virus (TMV) | Vectors (Insects), Mechanical (Contact) | Vector Control, Sanitation, Rogueing |
You have now mastered the different enemies of your crops and the strategies needed to defeat them! Always remember that in farming, an ounce of prevention is worth a pound of cure—especially when dealing with diseases.