Diseases in Crops – SS2 Agricultural Science Lesson Note (Second Term)

A plant disease is a departure of a plant from its normal state of health, shown by visible signs.

Behavioural objectives

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

1. define a plant disease and a pathogen;
2. state the pathogens and physiological factors that cause or encourage plant diseases;
3. name selected crop diseases and their causal organisms;
4. describe the method of transmission and the symptoms of each disease;
5. state the prevention and control measures for each disease.

What is a plant disease?

A plant disease is a departure of a plant from its normal state of health, shown by visible signs.

• Crop: a plant cultivated by man for a specific purpose, such as maize, cassava or cocoa.
• Pathogen: a disease-causing organism that passes through a regular cycle of development and reproduction.
• Symptom: a visible sign of disease on the plant, such as spots, wilting or galls.
• Vector: an agent, often an insect, that carries a pathogen from one plant to another.
• Cause: diseases are caused by pathogens and made worse by some physiological factors.

What causes plant diseases?

Plant diseases are caused by pathogens and encouraged by physiological factors.

What are the types of pathogen?

There are five types of pathogen that cause plant disease: viruses, bacteria, fungi, nematodes and, rarely, protozoa.

• Viruses: very small agents that live inside plant cells, as in cassava mosaic and groundnut rosette.
• Bacteria: single-celled organisms, as in cassava leaf blight and black arm of cotton.
• Fungi: organisms that spread by spores, as in maize smut, maize rust and cocoa black pod.
• Nematodes (parasitic worms): tiny worms that attack roots, as in root-knot of tomatoes and okra.
• Protozoa: single-celled animals that only rarely cause plant disease.

Some pathogens are carried by vectors and other agents such as wind, rain splash, tools and infected cuttings.

What are the physiological factors that encourage plant disease?

Physiological factors are conditions in the plant or its surroundings that affect how easily a plant is infected.

• Nutrient deficiency: lack of nutrients in the soil influences how susceptible the plant is to disease.
• Heat: temperature affects the plant’s susceptibility to disease.
• Inorganic salts: the presence of these salts in the soil affects susceptibility.
• Soil moisture: the moisture content of the soil affects susceptibility.

What are the fungal diseases of crops?

Fungal diseases are crop diseases caused by fungi, which usually spread by spores carried by wind, rain splash, soil or infected seed.

What are the viral diseases of crops?

Viral diseases are crop diseases caused by viruses, which are mostly spread by piercing and sucking insects.

What are the bacterial diseases of crops?

Bacterial diseases are crop diseases caused by bacteria, which spread through infected planting material, rain splash, insects and tools.

What is a nematode disease of crops?

A nematode disease is a crop disease caused by nematodes, which are tiny parasitic worms that live in the soil and attack roots.

How are crop diseases prevented and controlled?

Crop diseases are prevented and controlled by farm practices, resistant varieties and chemicals.

• Resistant varieties: plant varieties that do not easily get the disease, used against smut, rust, mosaic and others.
• Crop rotation: changing the crop on a plot each season, used against leaf spot, rust and root-knot.
• Early planting: planting at the start of the season, used against rust, rosette and onion twister.
• Uprooting and burning: destroying infected plants so the pathogen cannot spread, used against rosette, mosaic and black arm.
• Clean planting material: using healthy seeds and disease-free cuttings, used against cassava mosaic and leaf blight.
• Farm sanitation: removing diseased pods, plants and weeds, as in cocoa black pod control.
• Chemical control: fungicides against fungi, insecticides against insect vectors, and seed dressing.
• Soil sterilisation: treating soil to kill pathogens, used against root-knot and damping off.

Evaluation

1. Define a plant disease and a pathogen.
2. List four types of pathogen that cause plant diseases.
3. State four physiological factors that influence plant susceptibility to disease.
4. For rosette disease of groundnut, state the causal organism, the vector, two symptoms and two control measures.
5. State two symptoms of cocoa black pod disease and two ways of controlling it.

Assignment

Choose any three fungal diseases from the tables and state the method of transmission and two control measures for each. Then name two crop diseases spread by insect vectors and give the vector for each.