Lesson Notes By Weeks and Term - Senior Secondary School 2

Diseases of crops

Date: 27th of February- 17th of March 2023

Class: SSS 2

Term: 2nd Term

Week: 8-10

Subject: Agricultural Science

Topic: Diseases of crops

Duration:45 minutes

Teaching Aid: A chart

Reference Book: 1. Essential Agricultural Science for Senior Secondary Schools by O.A Iwena (page 237-240)

  1. Comprehensive Certificate Agricultural Science For Senior Secondary Schools by L.A Are et al (page330-340)

Behavioural Objectives: By the end of this topic, students should be able to

  1. Define Disease and state its causes
  2. Mention some crop disease ,their causal organism, method of transmission, symptoms and prevention
  3. Discuss control of diseases


Step 1: Teacher revises last topics

Step 2: Teacher introduces and explains new topic

Step 3: Teacher dictates note

Step 4: Teacher evaluates students

Step 5: Teacher marks students' notes and give assignment

Previous knowledge: Students have been taught Pasture and forage crops



A plant disease may be defined as a departure or deviation of the plant from the normal state of health, presenting marked symptoms or outward visible signs.

Causes of Diseases

Diseases are caused by the following agents:

(i) Viruses (ii) Bacteria (iii) Fungi (iv) Nematode (v) Nutrient deficiency



Name of Disease

Causal Organism

Method of Transmission

Symptoms and Economic Importance

Prevention and Control Measures

Maize Snut

Fungus (ustilago maydis)

Air borne Fungus spores deposited on fruits

   i.     Reduced yield

ii.     Galls on ears, leaves and tarsels which later turn black

    i.     Destroy diseased plant.

  ii.     Use resistant varieties

iii.     Seed treatment.

Rice Blight

Fungus (piricularia oryzae)

Airbone spores on leaves

   i.     Small longitudinal red spots on leaves which turn

    i.     Use clean seeds

  ii.     Avoids heavy use of nitrogen fertilizers

iii.     Use resistant varieties

Maize Rust

Fungus (puccinia polysora)

Airborne spores deposited on leaves

   i.     Red spots on leaves

ii.     Reduced yield

iii.     Death of the crop

    i.     Early planting

  ii.     Crop rotation

iii.     Use resistant varieties

Cercopora a leaf spot of cowpea


Thr20ough wild

   i.   Reddish brown spots on leaves

ii.   Lesions on leaves

iii.   Chlorosis

iv.   Dropping or falling of leaves

      i.         Spray with fungicides

     ii.         Crop rotation

   iii.         Plant resistant varieties

Rosette disease of groundnut


By piercing and sucking insect (Aphid).

   i.   Yellow leaves with mosaic mottling.

ii.   Stunted plant with curled leaves.

iii.   Wilting and death of plant

iv.   Shortening of the internodes

      i.         Early planting

     ii.         Crop rotation

   iii.         Use insecticides

   iv.         Uproot and burn infected plants

     v.         Use resistant variety.


Cassava Mosaic


i.      Through piercing and sucking insect (whitefly) (Bemisia nigerensis)

ii.      Infected plant cutting

i.     Mottling of leaves

ii.     Mosaic pattern on leaves

iii.     Stem/leaf distortion

iv.     Stunted plant

v.     Reduction in yield

  i.     Use resistant varieties

ii.     Uproot and burn infected plant

iii.     Spray with insecticides to kill vector

iv.     Use disease free stem cutting

v.     Farm sanitation.

Leaf blight of cassava

Bacterium xanthomonas manihotis

i.     Infected cutting

ii.     Rain splashing

iii.     Insects

iv.     Tools

i.     Blighting   of leaves

ii.     Wilting of plant

iii.     Falling off lf leaves

iv.     Reduced yield

v.     Canker of stem

vi.     Die-back of stem

i.     Use resistant varieties

ii.     Use disease free cutting

iii.     Early planting

iv.     Practise crop rotation

Cocoa black pod disease

Fungus phytophthora palmivora

i.     Rain splash

ii.     Insects

i.     Brown spots on pod

ii.     Rottening of pods.

iii.     Entire pod turns black

iv.     Low yield

i.     Remove and destroy infected pods.

ii.     Regular weeding

iii.     Spray with fungicides e.g. Bordeaux mixture.

iv.     Avoid overcrowding of cocoa plant.

Coffee Leaf rust


i.     By wind

ii.     By rain splash

i.   Yellow or brown spot on leaves

ii.   Orange powdery mass on the leaf

iii.   Reduction in yield

iv.   Dropping of leaves

    i.  Plant seeds from healthy plants.

  ii.  Use resistant varieties

iii.  Spray with copper fungicides


Black arm (bacterial blight of cotton)


i.     Through leaves

ii.     Stems near the ground

i.     Angular spots on leaves

ii.     Boll rot

iii.     Exudates from affected leaves

iv.     Retarted growth and death of plant

i.     Seed dressing

ii.     Uproot and burn infected plants

iii.     Use resistant varieties

iv.     Crop rotation

Root knot of tomato/okro Damping off disease of okra





Nematodes in soil



Infected soil

  i.  Knotting or galling or roots

ii.  Retarded growth

iii.  Early death of plant

iv.  Reduction in yield

v.  Retarted growth

vi.  Cells becomes water logged

vii.  Gradual wilting of plants

viii.  Death of plant

   i.     Soil sterilization

  ii.     Crop rotation

iii.     Use resistant varieties

iv.     Uproot and burn infected plants

  v.     Spray with copper fungicide

vi.     Use resistant varieties

vii.     Sterilization of soil


Onion twister disease


      i.         Infected soil

     ii.         Water splash

   iii.         Infected bulb.

i.   Twisting of leaves

ii.   Grey patches on leaves

iii.   Reduction in yield

iv.   Death of plant

i.   Crop rotation

ii.   Use resistant varieties

iii.   Spray with fungicides

iv.   Early planting

Stored produce mould


      i.         Infected seed or fruits

     ii.         High humidity

   iii.         By soil

i.   Black mould on seeds and fruits

ii.   Pungent smell

iii.   Sour taste

iv.   Decay of seeds and fruits in store

i.   Proper drying of seed before storage

ii.   Spray with fungicides

iii.   Maintain low humidity in store

iv.   Remove contaminated seeds before storage.



Disease cause lots of damage of crop and their effects are as follows:

  1. Disease generally reduce the yield or productivity of crops
  2. They also reduce the quality of crops
  3. They cause the malformation of plants or the whole plants
  4. They can kill or cause the death of a whole palnt
  5. They cause reduction in the income of the farmer
  6. They increase the cost of production through the expenses incurred in the course of controlling them
  7. The render vegetables and fruits unattractive and unmarketable
  8. Their activities cause retarded growth in crop plants

Ways by which diseases spread on a crop farm

Diseases can spread on a crop farm through any of the following ways:

  1. By rain splash
  2. Through the use of contaminated tools and equipment
  3. The use of infected planting materials
  4. Wind blowing pathogens to other crops
  5. Through animals, especially predator during feeding
  6. Through insect vector
  7. Through visitors to the farm
  8. Through weeds which may harbor pathogen
  9. Through irrigation water.


Diseases of crop plans can be controlled by the following methods:

(1) Cultural control; (2) biological control: (3) chemical control.

  1. Cultural Control: This involves the use of crop rotation, resistant varieties, tillage practices, regular weeding, fallowing, timelines of planting, pruning, uprooting and burning of infected plant, soil treatment or sterilization, rouging or uprooting of infected crops, maintenance of farm hygiene, timeliness of harvesting, etc. to control or prevent diseases.
  2. Biological Control: This involves the use of natural enemies of the disease to reduce or totally eliminate the disease
  3. Chemical Control: This involves the use of chemical such as fungicides, nematicides, insecticides to dust or spray plants and plant materials in order to prevent or control plant disease.


Summary of General Control Measure of Diseases of Crops

  1. Weed the farm regularly to prevent the disease pathogens from being haboured by weeds.
  2. Practise crop rotation
  3. Remove and burn infected plants
  4. Plant disease-resistant varieties of crops
  5. Use healthy seeds or stems for propagation
  6. Practise seed dressing with plant protection chemicals such as Fermassan D before planting
  7. Spray plant protection chemicals such as fungicides and nematicides
  8. Early planting helps the crops to escape the period of disease occurrence
  9. Spray insecticides to control the insect vectors
  10. Destroy crops’ residues after harvesting to prevent the build-up of disease pathogen or practice good farm sanitation or hygiene
  11. Sterilize soil to control soil-borne diseases
  12. Avoid close planting to reduce the rate of spread of diseases
  13. Imported seeds and plants should be quarantined before their introduction into the country
  14. Timely harvesting reduces the period of exposure

Evaluation: How can crop rotation reduce the spread of diseases

Assignment: mention five ways diseases spread on crop farm

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