General Effects of Diseases on Crop Production – SS2 Agricultural Science Lesson Note (Second Term)

A crop disease is any abnormal condition that harms the growth, structure or yield of a plant. It is mainly caused by pathogens such as fungi, bacteria and viruses, and nematodes can also damage crops.

Behavioural objectives

1. State five general effects of diseases on crop production.
2. List ways by which disease spreads on a crop farm.
3. Name the three general methods of controlling crop diseases.
4. Give examples of cultural and chemical control methods.
5. Name common crop diseases and the type of organism that causes each.

What is a pathogen?

A pathogen is a living organism that causes disease in a plant, animal or human being.

• Fungi: cause rots, rusts, blasts and moulds, for example rice blast.
• Bacteria: cause blights, wilts and soft rots, for example cassava bacterial blight.
• Viruses: cause mosaic, streak and leaf curl, for example cassava mosaic disease. Insect vectors usually carry them.
• Nematodes: are tiny worms in the soil that attack roots and cause swellings called galls.

What are the signs of disease in crop plants?

The signs of crop disease are the changes in a plant that show it is infected.

• Leaf spots: brown, black or yellow patches on leaves.
• Wilting: the plant droops even when the soil has enough water.
• Mosaic: leaves show patches of light and dark green.
• Rot: roots, fruits, tubers or stems become soft and decay.
• Stunted growth: the plant stays small and weak.
• Galls: swellings on roots or stems.

What are the general effects of diseases on crop production?

Crop diseases lower the quantity and quality of produce and raise the cost of farming.

• Reduced yield: infected plants produce fewer or smaller fruits, grains, tubers and seeds.
• Reduced quality: produce is spotted, rotten, shrivelled or discoloured, so it sells for less.
• Malformation: parts of the plant grow out of shape, for example curled leaves, stunted growth and twisted shoots.
• Death of plants: severe infection can kill a whole plant, and a whole field may be lost.
• Reduced income: low yield and poor quality mean the farmer earns less.
• Higher cost of production: the farmer spends more on chemicals, labour and replanting.
• Food shortage: heavy losses reduce the food supply and can raise prices.
• Health risk: some moulds on stored grain and groundnut produce toxins that harm people and livestock.

What are some common crop diseases in Nigeria?

How do diseases spread on a crop farm?

Crop diseases spread when pathogens move from infected plants, soil or materials to healthy plants.

• Rain splash: raindrops carry spores from infected plants or soil to healthy plants.
• Contaminated equipment: cutlasses, hoes and sprayers carry pathogens from one plant or field to another.
• Infected planting materials: diseased seeds, stem cuttings, tubers and seedlings bring the disease into the new farm.
• Wind: it blows spores and other pathogens to nearby crops.
• Animals: livestock and wild animals carry pathogens on their bodies or feet.
• Insect vectors: insects such as whiteflies and aphids carry pathogens, especially viruses, as they feed.
• Visitors to the farm: people carry pathogens on their clothes, footwear and hands.
• Weeds: they act as alternative hosts where pathogens survive and later move to crops.

How are crop plant diseases controlled?

There are three general methods of controlling crop diseases: cultural, biological and chemical.

What is the cultural method of disease control?

The cultural method is the use of good farm practices to prevent or reduce crop disease.

• Crop rotation: growing different crops in turn on the same land breaks the life cycle of pathogens.
• Resistant varieties: planting varieties that do not easily catch the disease.
• Tillage practice: ploughing and ridging bury or expose pathogens in the soil.
• Regular weeding: removes weeds that shelter pathogens and vectors.
• Pruning: cutting off diseased or crowded branches improves air flow and removes infection.
• Healthy planting materials: using clean seeds and cuttings keeps the disease out of the farm.
• Farm sanitation: removing and burning infected plants and crop remains stops further spread.

What is the biological method of disease control?

The biological method is the use of a natural enemy of the pathogen or vector to reduce disease.

• Example: ladybird beetles feed on aphids that spread viruses.
• Advantage: it leaves no harmful chemical residue on food or in the soil.
• Limit: it is usually slower than chemical control.

What is the chemical method of disease control?

The chemical method is the use of pesticides to prevent or control crop disease.

• Fungicides: chemicals that kill fungi, for example Bordeaux mixture.
• Nematicides: chemicals that kill nematodes in the soil.
• Insecticides: chemicals that kill insect vectors that spread disease.
• Advantage: it works fast and can protect a large farm quickly.
• Disadvantage: it is costly and can harm people, useful insects and the soil if misused.
• Safety: wear protective clothing, follow the label and keep chemicals away from children.

Evaluation

1. State five general effects of diseases on crop production.
2. List five ways by which disease spreads on a crop farm.
3. Name the three general methods of controlling crop diseases.
4. Give four examples of cultural control methods.
5. Name the type of organism that causes cassava mosaic disease, rice blast and cassava bacterial blight.

Assignment

Explain the cultural, biological and chemical methods of controlling crop diseases. Name the pesticides used in chemical control.