Diseases of Crops


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In today’s Agricultural Science class, We will be learning about Diseases of Crops. We hope you enjoy the class!



  • Meaning and causes of diseases
  • Details of selected diseases


crop diseases agric classnotesng



A crop is a plant cultivated by man for a specific purpose. A plant disease is a deviation of the plant from the normal state of health, presenting outward visible signs. Diseases are caused by pathogens and enhanced by some physiological factors.



Plant diseases are caused by pathogens. Pathogens are disease-causing organism which passes through a regular cycle of development and reproduction. Examples of pathogens that cause plant disease are viruses, bacteria, fungi, parasitic worms and rarely protozoa. Some of these pathogens are carried by vectors and other agents.


Physiological factors such as nutrient deficiency in the soil, heat, presence of inorganic salts in the soil and soil moisture content have a major role to play in influencing plant susceptibility to diseases.



Name Casual organism Method of transmission Symptoms and economic importance Prevention and control measure
Maize Smut Fungus (Ustilago maydis) 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) Airborne spores on leaves (i)   Small longitudinal red spots on leaves which turn grey or brown

(ii)  Reduced yield

(i)   Use clean seeds

(ii)  Avoid heavy use of nitrogen fertilizers.

(iii)  Use resistant varieties

Maize Rust Fungus (Puccinis polysora) Airborne spores deposited on leaves (i)   Red spots on leaves.

(ii)  Reduced yield

(iii) Death of crop.

(i)   Early planting

(ii)  Crop rotation

(iii) Use resistant varieties.

Cercopora  a Leaf spot of Cowpea Fungus Through


(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 Virus By piercing and sucking insect (Aphid) (i)  Yellow leaves with mosaic mottling.

(ii) Stunted plant with curled leaves.

(iii) Wilting and death of the 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 Virus (i)   Through piercing and sucking insect (whitefly)


(Bemisia nigerensis)


(ii)   Infected plant cutting


(i) Mottling of leaves or leaf curl

(ii) Distortion of leaves and stems.

(iii) Vein clearing

(iv) Stunted growth

(v)  Development of yellowish pale areas alternating with green patches on the leaves or mosaic pattern on the leaves

(i)     Use resistant varieties

(ii)    Uproot and burn infected plant

(iii)   Spray with insecticide to kill the vector

(iv)  Use disease-free stem cuttings

(v)    Farm sanitation.

Leaf blight of Cassava Bacterium

Xanthomonas manihotis

(i) Infected     cuttings

(ii)  Rain splashing

(iii) Insects

(iv)    Tools

(i)           Blighting of leaves

(ii)          Wilting of plant

(iii)         Falling off of leaves

(iv)        Reduced yield

(v)         Canker of stem

(vi)        Die-back of stem

(i)         Use resistant varieties

(ii)        Use disease-free cuttings

(iii)       Early planting

(iv)      Practise crop rotation

Cocoa black pod disease Fungus Phytophthora palmivora (i) Rain splash

(ii)          Insects

(i)       Brown spots on pods

(ii)      Rotting of pods

(iii)     The entire pod turns black

(iv)    Low yield

(i)         Remove and destroy infected pods

(ii)        Regular weeding

(iii)       Spray with fungicides eg Bordeaux mixture

(iv)      Avoid overcrowding of cocoa plants.


Leaf rust

Fungus (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) Bacterium (i)   Through leaves

(ii)  Stems near the ground

(i)     Angular spot on leaves

(ii)    Boll rot

(iii)   Exudates from affected leaves

(iv)  Retarded growth and death of the plant.

(i)         Seed dressing

(ii)        Uproot and burn infected plants

Root Knot of




Nematodes Nematodes in soil (i)     Knotting or galling of roots

(ii)    Retarded growth


(iii)   Early death of the plant

(iv)  Reduction in yield

(i)         Soil sterilization

(ii)        Crop rotation

(iii)       Use resistant varieties

(iv)      Uproot and burn infected plants

Damping-off Disease of Okra Fungus Infected soil (i)     Retarded growth

(ii)    Cells become waterlogged

(iii)   Gradual wilting of the plant

(iv)  Death of plant

(i)       Spray with copper fungicide

(ii)      Use resistant varieties

(iii)     Sterilization of soil




Fungus (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 fungicides Fungus (i)   Infected seeds 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

(iii)       Maintain low humidity in the store

(iv)      Remove contaminated seeds before storage.



  1. What is plant disease?
  2. List four common disease-causing organisms.




We have come to the end of this class. We do hope you enjoyed the class?

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.

In our next class, we will continue learning about Diseases of Crops. We are very much eager to meet you there.


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