Crop Diseases SS1 Horticulture and Crop Production Lesson Note

What is a Plant Disease?

Just like humans get malaria or the flu, plants can also fall sick. A plant disease is any abnormal condition that stops a plant from growing properly or producing good fruit.

When a plant is diseased, it doesn’t just look “untydy”—its internal system is struggling.

How to tell a plant is sick (Symptoms):

• Wilting: The plant looks tired and droopy even when there is plenty of water in the soil.
• Leaf Spots: Strange brown, black, or yellow circles appearing on the leaves.
• Chlorosis: The green leaves turn yellow because the plant can’t make its food.
• Rotting: Parts of the plant become soft, smelly, and slimy.
• Stunting: The plant stays very small while others of the same age are big.

The Three “Villains” (Causes of Disease)

Most plant diseases in Nigeria are caused by three tiny living things that we usually need a microscope to see clearly.

1. Fungi (The Most Common)

Fungi love damp and warm weather. They spread through tiny “seeds” called spores that fly in the wind.

• Example: Maize Rust or Leaf Spot on tomatoes.

2. Bacteria

These are tiny organisms that enter the plant through wounds (like where a goat bit a leaf) or through the plant’s “pores.”

• Example: Citrus Canker (sores on oranges) or Bacterial Wilt.

3. Viruses

Viruses are the trickiest. They are usually carried from a sick plant to a healthy one by insects like aphids or whiteflies.

• Example: Cassava Mosaic Disease (where leaves look wrinkled and multicolored).

Common Diseases in Nigerian Horticulture

Here are a few diseases you will likely see in your school garden or local market:

1. Damping-Off

• Target: Young seedlings in the nursery.
• What happens: The stem of the baby plant rots at the soil level, and the plant just falls over and dies. It’s caused by too much water in the nursery.

2. Tomato Leaf Spot

• Target: Tomato plants.
• What happens: Dark spots with yellow edges appear on the leaves. If not treated, the leaves fall off, and the sun “burns” the tomatoes because there is no shade.

3. Cassava Mosaic

• Target: Cassava (a major food crop).
• What happens: The leaves become twisted, small, and have yellow/light green patches. The plant cannot grow big tubers.

Prevention (Keeping the “Doctor” Away)

In horticulture, “Prevention is better than cure.” It is much cheaper to stop a disease from starting than to treat it.

• Use Resistant Varieties: Some seeds are “stronger” and were born to fight certain diseases. Always ask for these at the seed store.
• Crop Rotation: Don’t plant tomatoes in the same spot every year. The “germs” that like tomatoes will stay in the soil waiting for them. If you plant corn there next, the tomato germs will starve and die.
• Good Spacing: Don’t crowd your plants. Air needs to blow between them to keep the leaves dry. Fungi love wet, crowded leaves!
• Clean Tools: If you use a cutlass to prune a sick tree, wash it before touching a healthy one. You are the “nurse”—don’t spread the germs!

Controlling a Disease (The Cure)

If the disease has already arrived, we have to fight it. We use two main methods:

1. Cultural Control (Manual)

• Roguing: This means pulling out the sick plant and burning it far away from the farm so the disease doesn’t spread to others.
• Pruning: Cutting off only the sick branches.
• Weeding: Some weeds carry diseases that jump onto our crops. Keep the farm clean!

2. Chemical Control

We use “plant medicines” called Pesticides:

• Fungicides: To kill fungal diseases (usually sprayed on leaves).
• Bactericides: To fight bacteria.
• Nematicides: To kill tiny worms in the soil that attack roots.

Safety Note: Never taste or smell these chemicals. They are poisonous to humans too! Always wash your hands after working with sick plants or chemicals.

Summary Table: Quick Diagnosis