Crop Diseases – SS1 Horticulture and Crop Production Lesson Note (Second Term)

A plant disease is any abnormal condition that stops a plant from growing properly or producing good fruit.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define a plant disease and list five symptoms of a sick plant;
2. name the three main causes of plant diseases, with one example of each;
3. describe damping-off, tomato leaf spot and cassava mosaic;
4. state four ways of preventing crop diseases;
5. explain cultural control and chemical control of crop diseases.

What is a plant disease?

A plant disease is any abnormal condition that stops a plant from growing properly or producing good fruit.

• Comparison: plants fall sick just as humans get malaria or flu.
• Appearance: a diseased plant looks untidy and weak.
• Pathogen: the living thing that causes a disease, such as a fungus, a bacterium or a virus.
• Symptom: a sign on the plant that shows it is sick.

What are the symptoms of a sick plant?

The main symptoms of a sick plant are wilting, leaf spots, chlorosis, rotting and stunting.

• Wilting: the plant droops even when the soil has plenty of water.
• Leaf spots: brown, black or yellow patches appear on the leaves.
• Chlorosis: green leaves turn yellow because the plant cannot make enough food.
• Rotting: parts of the plant become soft, smelly and slimy.
• Stunting: the plant stays small while others of the same age grow big.

What causes plant diseases?

There are three main causes of plant diseases in Nigeria: fungi, bacteria and viruses.

1. Fungi

Fungi are the most common cause of crop diseases, and they thrive in damp, warm weather.

• Spread: by tiny spores carried by wind and splashing rain.
• Examples: maize rust, leaf spot of tomato, powdery mildew and damping-off.

2. Bacteria

Bacteria are very small single-celled organisms that enter a plant through wounds or natural openings.

• Entry points: wounds (for example, where a goat bit a leaf) and the leaf pores.
• Examples: citrus canker (sores on citrus fruits and leaves) and bacterial wilt.

3. Viruses

Viruses are extremely small disease agents that live inside plant cells and are hard to cure.

• Spread: by insects such as aphids and whiteflies, which carry them from sick plants to healthy ones.
• Example: cassava mosaic disease, where leaves are wrinkled and patchy.

How do the three causes compare?

How do crop diseases spread?

Crop diseases spread from sick plants to healthy ones in several ways.

• Wind: carries fungal spores from plant to plant.
• Rain and irrigation water: splash germs from the soil or sick leaves onto healthy ones.
• Insects: aphids and whiteflies carry viruses.
• Dirty tools: cutlasses and knives carry germs from sick plants to healthy ones.
• Infected planting material: sick seeds, seedlings or cassava stem cuttings start the disease in a new farm.

Which diseases are common in Nigerian horticulture?

Damping-off, tomato leaf spot and cassava mosaic are common diseases in Nigerian gardens and farms.

1. Damping-off

• Target: young seedlings in the nursery.
• Cause: soil fungi that thrive when the nursery is too wet.
• Symptoms: the stem rots at soil level, and the seedling falls over and dies.
• Control: avoid overwatering, sow thinly and use well-drained soil.

2. Tomato leaf spot

• Target: tomato plants.
• Cause: fungi.
• Symptoms: dark spots with yellow edges appear on the leaves.
• Effect: infected leaves fall off, and the fruits are exposed to strong sun and may be scorched.
• Control: remove infected leaves, space plants well and spray with a fungicide.

3. Cassava mosaic

• Target: cassava, a major food crop.
• Cause: a virus spread by whiteflies and by infected stem cuttings.
• Symptoms: leaves become twisted and small, with yellow or light green patches.
• Effect: the plant grows poorly and gives small tubers.
• Control: plant resistant varieties and healthy cuttings, and rogue sick plants.

How can crop diseases be prevented?

Crop diseases are prevented by good farm practices that stop the disease from starting.

• Resistant varieties: some seeds are bred to resist certain diseases, so ask for them at the seed store.
• Crop rotation: do not plant the same crop on the same plot every season.
• Good spacing: air blowing between plants keeps leaves dry, and fungi do not like dry leaves.
• Clean tools: wash a cutlass after pruning a sick plant and before touching a healthy one.
• Healthy planting material: use clean seeds, seedlings and cuttings from healthy plants.
• Good drainage: avoid waterlogged nurseries and beds.

Why does crop rotation prevent disease?

• Germs that attack tomato stay in the soil waiting for another tomato crop.
• If a different type of crop such as maize is planted next, the germs have no host.
• The number of germs in the soil falls over time.

How is a crop disease controlled?

A crop disease that has already appeared is controlled by cultural methods or chemical methods.

1. What is cultural control?

Cultural control is the use of farm practices and manual work to reduce a disease.

• Roguing: pulling out sick plants and burning them far from the farm so the disease does not spread.
• Pruning: cutting off only the sick branches or leaves.
• Weeding: removing weeds, because some weeds carry diseases that pass to crops.
• Burning crop remains: destroying diseased residues after harvest.

2. What is chemical control?

Chemical control is the use of pesticides to kill the organisms that cause crop diseases.

What are the safety rules for using pesticides?

• Do not taste or smell: pesticides are poisonous to humans too.
• Wash your hands: do this after handling chemicals or sick plants.
• Follow the label: use the amount and method written on the container.
• Keep away from children: store chemicals in a safe place, away from food.

How can you diagnose a disease quickly?

The table below links what you see on the plant to the likely disease and the first action to take.

Evaluation

1. Define a plant disease.
2. List five symptoms that show a plant is sick.
3. Name the three main causes of plant diseases and give one example of each.
4. What is roguing, and why is it done?
5. State the pesticide used to kill fungal diseases, and give one safety rule for handling pesticides.

Assignment

1. Write a short note on cassava mosaic: its cause, its symptoms and its effect on the plant.
2. Explain how crop rotation and good spacing help to prevent crop diseases.