DISEASES OF CROPS

Subject:

Agricultural Science

Class:

SS 2

Term:

Second Term / 2nd Term

Week:

Week 6

WEEK SIX

Date:

TOPIC: DISEASES OF CROPS

CONTENT

Meaning and causes of diseases
Details of selected diseases

MEANING AND CAUSES OF DISEASE

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.

CAUSES OF DISEASE

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 has a major role to play in influencing plant susceptibility to diseases.

SELECTED DISEASE OF CROPS

Name	Casual organism	Method of transmission	Symptoms and economic importance	Prevention and control measure
Maize Smut	Fungus (Ustilago maydis)	Fungus spores deposited on fruits	Reduced yield Galls on ears, leaves and tarsels which later turn black	Destroy diseased plant. Use resistant varieties. Seed treatment.
Rice Blight	Fungus (Piricularia oryzae)	Airborne spores on leaves	Small longitudinal red spots on leaves which turn grey or brown Reduced yield	Use clean seeds Avoid heavy use of nitrogen fertilizers. Use resistant varieties
Maize Rust	Fungus (Puccinis polysora)	Airborne spores deposited on leaves	Red spots on leave. Reduced yield Death of crop.	Early planting Crop rotation Use resistant varieties.
Cercopora a Leaf spot of Cowpea	Fungus	Through Wind	Reddish brown spots on leaves Lesions on leaves Chlorosis Dropping or falling of leaves.	Spray with fungicides Crop rotation Plant resistant varieties.
Rosette disease of Groundnut	Virus	By piercing and sucking insect (Aphid)	Yellow leaves with mosaic mottling. Stunted plant with curled leaves. Wilting and death of plant. Shortening of the internodes.	Early planting Crop rotation Use insecticides Uproot and burn infected plants. Use resistant variety.
Cassava Mosaic	Virus	Through piercing and sucking insect (whitefly) (Bemisia nigerensis) Infected plant cutting	Mottling of leaves or leaf curl Distortion of leaves and stems. Vein clearing Stunted growth Development of yellowish pale areas alternating with green patches on the leaves or mosaic pattern on the leaves	Use resistant varieties Uproot and burn infected plant Spray with insecticide to kill vector Use disease-free stem cuttings Farm sanitation.
Leaf blight of Cassava	Bacterium Xanthomonas manihotis	Infected cuttings Rain splashing Insects Tools	Blighting of leaves Wilting of plant Falling off of leaves Reduced yield Cabker of stem Die-back of stem	Use resistant varieties Use disease free cuttings Early planting Practise crop rotation
Cocoa black pod disease	Fungus Phytophthora palmivora	Rain splash Insects	Brown spots on pod Rottening of pods Entire pod turns black Low yield	Remove and destroy infected pods Regular weeding Spray with fungicides eg Bordeaux mixture Avoid over crowding of cocoa plants.
Coffee Leaf rust	Fungus	By wind By rain splash	Yellow or brown spot on leaves Orange powdery mass on the leaf Reduction in yield Dropping of leaves	Plant seeds from healthy plants Use resistant varieties. Spray with copper fungicides.
Black arm (bacterial Blight of cotton)	Bacterium	Through leaves Stems near the ground	Angular spot on leaves Boll rot Exudates from affected leaves Retarded growth and death of plant.	Seed dressing Uproot and burn infected plants
Root Knot of Tomatoes/ Okra	Nematodes	Nematodes in soil	Knotting or galling of roots Retarded growth Early death of plant Reduction in yield	Soil sterilization Crop rotation Use resistant varieties Uproot and burn infected plants
Damping off Disease of Okra	Fungus	Infected soil	Retarded growth Cells become water logged Gradual wilting of plant Death of plant	Spray with copper fungicide Use resistant varieties Sterilization of soil
Onion Twister Disease	Fungus	Infected soil Water splash Infected bulb	Twisting of leaves Grey patches on leaves Reduction in yield Death of plant	Crop rotation Use resistant varieties Spray with fungicides Early planting
Stored produce mould fungicides	Fungus	Infected seeds or fruits. High humidity By Soil	Black mould on seeds and fruits Pungent smell. Sour taste Decay of seeds and fruits in store.	Proper drying of seed before storage Spray with Maintain low humidity in store Remove contaminated seeds before storage.

Presentation :

The topic is presented step by step

Step 1: The class teacher revises the old topic

Step 2: The class teacher introduces the new topic

Step 3: The class teacher allows the pupils to give their own contributions and gives room for pupils” participation

Class Teacher and Pupils Activities. Interaction or Participation

This involves class teacher and pupils’ interaction, conversation, imitation or modeling through discussion, play method or just by recitation or asking and answering questions that are related to the topic that has just been taught.

EVALUATION

What is plant disease?
List four common disease causing organisms.

Conclusion :

The class teacher concludes the lesson by giving the pupils some notes on the topic that has just been taught. He goes round to mark and he does the necessary corrections.

Tags:SECOND TERM

Subject:

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