Disease and pest of 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

SELECTED DISEASES OF CROPS

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	Fungus	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	Virus	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	Virus	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	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 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	Nematode Fungus	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	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	Fungus	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.

GENERAL EFFECTS OF DISEASES ON CROP PRODUCTION

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

Disease generally reduce the yield or productivity of crops
They also reduce the quality of crops
They cause the malformation of plants or the whole plants
They can kill or cause the death of a whole palnt
They cause reduction in the income of the farmer
They increase the cost of production through the expenses incurred in the course of controlling them
The render vegetables and fruits unattractive and unmarketable
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:

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

GENERAL CONTROL OF CROP PLANT DISEASES

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

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

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.
Biological Control: This involves the use of natural enemies of the disease to reduce or totally eliminate the disease
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

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

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SS2 Second Term Agricultural Science Senior Secondary School

Disease and pest of crops

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