Introduction
- Crop pests and diseases lead to high losses in crop production hence efficient control measures are needed.
- Proper control measures require the farmers to be able to;
- Identify these organisms,
- Know their life cycles, feeding habits
- The damage they cause to crops.
Crop Pests
Definition of a Pest:
- It is a living organism that destroys crops/ trees either directly or indirectly by introducing pathogens (disease causing germs).
Classification of Pests
- Pests are classified according to the following:
- Mode of Feeding
- Pests with biting and chewing mouth parts - they cause physical damage and reduce the photosynthetic area of the plant.
- Pests with piercing and sucking mouth parts - they suck out the nutritious plant sap and in the process may introduce disease causing organisms.
- Crops Attacked
- Some crop pests attack specific crops for example, stem borers prefer cereal crops.
- Stage of Growth of Crops Attacked
- There are pests of seedlings attack when the crop is young, for example cutworms.
- Pests of fruits - attack the crops at fruiting stage.
- Pests of grains attack the crops when the grains are formed.
- Field and Storage Pests
- Some pests attack the crops while in the f
- Other pests attack the produce after it has been harvested and stored.
Identification of Common Pests
Name of Pest | Crop Attacked | Damage Done | Control Measures |
Armyworms (Spodoptera exempta) | - Cereal crops - Sugarcane - Grasses |
Defoliate the whole plant | - Early planting - Use of effective insecticides |
Cutworms (Agrotis spp.) | Young seedlngs | Cut the seedlings at the stem base | - Early planting - Use of soil applied insecticides(fumigants) - Flood/irrigation |
Boll worms (Heliathis migera) | Cotton, tomatoes, citrus, maize, beans, millet, other legumes | Eat and destroy the fruits and seeds | - Crop rotation - Field hygiene - Spraying with insecticides - Early planting |
Maize stalk borer (Busseola fusca and Chilo partellus) | Maize, sorghum | Destruction of the stem and young growing tissues | - Early planting - Field hygiene - Crop rotation - Use of stalk borer dust |
Loopers (Ascotis selena ria) | Coffee | Make windows in crop leaves | - Use of effective insecticides - Use of parasitics wasps, birds and chameleons |
Leaf miner (Leucoptera meyricki and L. caffeina) | Coffee | Makes mines in leaves reducing photosynthentic area | - Use of parasitic wasps(natural enemies) - use of effective insecticides |
Stainers (Dysdercus spp.) | Cotton | Stain the cotton lint reducing quality | - Use of parasitic tachinid flies - Spraying with insecticides - Control alternate hosts - Crop rotation |
Aphids (Aphis spp.) | Several crops such as citrus, maize, cotton, beans, cabbages and others | - Transmit viral diseases - Suck out sap leading to stunted growth |
- Natural enemies for example, lady birds - overhead irrigation - Use of insecticides |
Other Crop Pests Include:
- Mealy bugs - coffee
- Thrips - coffee
- Beetles - field and storage pests.
- Birds - field pests - cereals and fruits.
- Rodents - field and storage pests ,cereals and tubers.
- Nematodes - soil borne pests - tomatoes, potatoes, sunflower, beans.
Harmful Effects of Crop Pests
- Pests such as squirrels and rodents, unearth planted seeds, resulting in poor germination.
- Some pests like nematodes, termites and moles damage crop roots causing wilting and death of the crops.
- They lower the quality and quantity of farm produce.
- They increase the cost of production since farmers will incur expenses in purchasing chemicals to control them.
- They transmit diseases to crops for example, aphids transmit streak virus disease in maize.
- Chemicals used to control the pests cause pollution to the environment.
- They exterminate the crop by feeding on them for example eating embryo of the seed.
Control of Pests
- If pest population causes damage beyond tolerance then it is said to have reached economic injury level (EIL) hence control measures should be effected before this level.
- Before any control measure is effected ,the following should be considered:
- Know the life cycle of the pest.
- Correct identification of the pest.
- Correct assessment of the damage.
- The weather conditions.
- The value of the crop in question.
- The cost factor of the control method.
Methods of Controlling the Pests
- Cultural methods.
- Physical/mechanical measures
- Biological methods.
- Chemical methods.
- Integrated pest management.
Cultural Methods:
- These are farming practices which aim at reducing the pest population by destroying the life cycle of the pests either by exposing them to adverse conditions or denying them food.
- These include:
- Timely planting to escape pest attack.
- Timely harvesting.
- Proper tillage.
- Close season: this is the period when a susceptible crop is not grown in order to control a certain pest.
- Trap cropping: These are crops which attract pests diverting them from the main crop. The trap crop is grown together with the main crop.
- Crop rotation: It breaks the life cycle of the pest.
- Planting resistant varieties: These are plants with natural protective mechanisms against pest for example hairy cotton against jassid bugs, goose necked sorghum against birds, high tillering in sorghum against shoot fly.
- Field hygiene: This includes rogueing and removal of crop residues which harbour pests from field.
- Alterations of environmental conditions, such as, creating a microclimate which is not conducive to pests for example open pruning in coffee.
- Crop nutrition: application of fertilizers and manures to make the crop strong and able to escape pest attacks.
- Destruction of alternate hosts, for example, weeds like mallow which harbour cotton stainers.
- Use of clean planting materials. This helps to prevent introduction of crop pests.
- Proper spacing: if well spaced some pests find it difficult to move from one plant to another.
- Use of organic manure, for example, farmyard manure discourages eel worms (nematodes).
Chemical Control
- Chemicals used to control pests are known as pesticides.
- Pesticides are administered through dusting, spraying or fumigating.
Classification of Pesticides:
- Pesticides are classified on the basis of:
Mode of Entry
- Stomach - ingested by the pest together with the crop materials.
- Contact - absorbed through the body tissues.
- Fumigants - through the breathing mechanism.
- Systemic - translocated to all parts.
Mode of Action
- Respiratory poisons - interfere with breathing mechanisms.
- Coagulants - cause the blood of the pest to coagulate.
- Neurotoxins - act on the nervous system.
- Protoplasmic poisons - cause the cells to disintegrate.
Target Pests
- Insecticides - kill insect pests
- Molluscicides - kill snails and slugs.
- Rodenticides - kill rodents.
- Nematocides - kill nematodes.
Formulation .
- Dusts, granules and powders
- Emulsifiable concentrates.
- Miscible liquids.
- Wettable powders.
- Fumigants.
Factors That Affect the Efficiency of Pesticides
- Concentration of the pesticides.
- Weather conditions at the time of application.
- Timing of application - efficiency is high if applied when the pest is most susceptible.
- Persistence of the pesticide having long residual effect in the soil.
- Resistance of the pests.
Advantages of Chemical Pest Control
- Faster
- Immediate results are achieved.
- Low labour requirements.
Disadvantages of Chemical Pest Control
- Expensive to buy.
- Cause pollution to the environment.
- Require skilled labour to apply.
- Some pesticides may kill beneficial organisms and predators.
- Some target pests may build up resistance.
Mechanical Pest Control/Physical
- This involves the killing of the pests using physical methods.
- Or creating physical barriers to prevent pests from getting into contact with the crops .
Example:
- Flooding/irrigation; for example, moles are suffocated through flooding.
- Use of lethal temperatures: either too cold or too hot.
- Suffocation; commonly used in grain storage bins by being made air tight.
- Trapping and killing, for example, rats.
- Creation of physical barriers, such as, rat bafflers, sticky materials on tree trunks.
- Proper drying: this makes them too hard to be destroyed by pests.
- Scaring devices especially in rice plantations to control birds.
- Use of explosives thrown at breeding places of birds to kill or scare them away.
Biological Pest Control
- It involves the use of living organisms to reduce the pest population.
Predator Target Pest
- Parasitic wasp-------------------White fly in citrus, boll worms, stalk borers
- Birds--------------------------------- Crickets, locusts, caterpillar llars.
- Lady Bird --------------------------Aphids
- Trachnid flies -------------------cotton stainer
- Praying mantis-----------------giant loppers
- Majimoto ants -----------------scales
- Cats ----------------------------------moles,rats,mice
- Brachonid wasps-------------mealy bugs
- Chicken ---------------cotton stainer, larvae of beetles, grasshoppers, crickets
Advantages
- Cheap
- No environmental pollution
- Saves on labour.
Disadvantages
- Takes too long to get the correct agent
- Difficult to control the pest effectively.
Integrated Pest Management
- It is a new method which involves the combination of the methods mentioned above.
- The aim is to have least hazards to the user and to the environment.
- Example, attractant-pheromones are used to attract pests to one place where they are sprayed and eradicated.
Legislative Method/Quarantine
- Legislative mearsures of pest control are effected by the Kenya Plant Health Inspectorate Service(KEPHIS) through seed inspection.
Crop Diseases and Their Control
- A disease is any deviation from the normal performance or functions.
- A plant disease is any harmful physiological disorder in a plant caused by pathogenic agents such as virus, bacteria, fungi.
- The study of plant disease is called plant pathology.
Effects of Crop Diseases
- They lower crop yield
- They reduce the quality of the produce thus reducing their market value
- They cause food poisoning. E.g ergot in wheat, afflatoxin in grain crops by fungus.
- They reduce photosynthetic area of the plant.
Classification and Identification of Plant Disease
- Plant disease are classified according to their causal agents;
Fungal diseases;
- Fungi are non-green plant-like.
- Some are parasitic and others are saprophytic.
- Parasitic fungi divided into;
- Obligate parasites- those that depend on other living organisms for food.
- Falcultative parasites-those that are able to live on both the living and dead tissues.
- Examples of fungal disease
- Panama disease(Fusarium oxysperium—bananas)
- Cigar-end rot(Verticilium theobromae)-bananas
- Die back –attack the tips of shoots and roots
- Mildews-foliar disease of several crops
- Armillaria root rot(Armillaria mellea)-coffee and tea
- Damping off-disease of seedlings in the nursery
- Anthracnose (Colletotrichum spp)-coffee,beans,tomatoes.
- Fungus also cause damage to stored grains which are not properly dried or if the store is damp.
- Fungus cause food poisoning and lower seed viability for example Aspergillus flavus which produces a highly toxic compound called afflatoxin.
Examples of fungal disease
Disease/cause |
Crops attacked |
Symptoms of attack |
Control measures |
Late blight (Phytopthora infestans) |
Members of Solanaceae family (potatoes, tomatos) |
Dry patches on the leaves and fruits (necrotic lesions) |
- Crop rotation |
Rusts (Pucinia spp) |
Rice, wheat , sorghum, maize |
Red and brown pistules on the leaves, shriveled grains |
- resistant varieties |
Smuts(Ustilago spp) |
Wheat, maize , sugarcane |
Black powder mass on the spikes and the ear |
- Field hygiene, |
Blasts (Piricularia oryzae) |
Rice |
-Small blue sports on leaves with grey centre. |
- Seed dressing |
Coffee Berry Disease(CBD) (Colletotrichum coffeanum) |
Coffee |
- Dark blotches spots on the flowers |
- Resistant varieties eg Ruiru 11 |
Bacterial Diseases
- Bacteria are microscopic single-celled organisms which reproduce by binary fission
- Transmission; Through irrigation water, seeds, fertilizers, manures, wind , raindrop splash, insects, soil and mechanical means.
Symptoms of Bacterial Diseases
- Wilting
- Cankers(necrotic tissues)localized necrosis
- Gall formation in infected tissues.
Examples of bacterial diseases
Disease/Cause | Crops Attacked | Symptoms of Attack | Control Measures |
Halo blight (Pseudomonas phaseolicola) | Beans | - Irregular dark lesions on leaves and pods. - Yellow band round the lesions called "halo". - Water soaked lesions |
- Use of resistant varieties for example Wairimu. - Effective fungicide. - Crop rotation |
Fusarium wilt (Fusarium oxysporum) | Tomatoes | - Stunted growth. - Yellowing and shedding of leaves. - Wilting of the plant. |
- Use of resistant varieties. |
Black arm (Anthomonas malvacearum) | Cotton | - Small round spots on the cotyledons of young seedlings. - The spots elongate to form black lesions on the stem. |
- Field hygiene. - Use of certified seeds. |
Bacterial wilt (Pseudomonas solanacearum) | Tomatoes and potatoes | - Uniform wilting of the whole plant even with enough water. | - Use of certified seeds. - Crop rotation. |
Viral Diseases
- Viruses are small living organisms which can only be seen under a very powerful electronic microscope.
- Viruses interfere with photosynthesis, respiration, transpiration and nitrogen utilization
Symptoms of Viral Infection
- Leaf chlorosis.
- Leaf curling.
- Mosaic(light green or yellow patches).
- Malformation(distortion)of plant parts.
- Rosettes; Development of abnormally short internode.
Transmission
- Through the use of infected vegetative materials and insect vectors like aphids, mealybugs and leafhoppers.
Viral diseases
Disease/Cause | Crops Attacked | Symptoms of Attack | Control Measures |
Ratton stunting | Sugar cane | Red discoloration on the vascular bundles. | - Use of clean materials. - Treatment of seed canes. |
Maize streak | Maize | Yellow stripes alternating with green, parallel to the midrib. | - Control leaf hopper. - Use of certified seeds - Field sanitation. |
Greening disease | Citrus | - Yellow mottling of the leaves. - Die back. - Premature leaf fall |
- Use of clean tools when budding. - Control of insect vectors. |
Leaf mosaic | Sugar cane, cassava, sweet potatoes | - Yellow mottling. - Necrosis of stem. |
- Control of aphids. - Use of clean materials - Seed treatment. |
Tristeza | Citrus | - Dwarfing of plants. - Die back. |
- Stripping affected fruits - Use of clean equipment of budding. |
Other Causes of Crop Diseases
- Flooding forming ammonia which is poisonous to the crops causing a burning effect on leaves.
- Chemicals: some may be toxic.
- Poor weather: Extremes of day and night temperatures.
- Stress: such as irregular watering as in tomato blossom end rot.
Control of Crop Diseases
- Cultural method: This involves use of healthy planting materials.
- Practicing field hygiene.
- Proper seedbed preparation.
- Proper spacing.
- Heat treatment of the planting materials for example sugar cane.
- Proper drying of cereals and pulses to 13%M.C.
- Growing disease resistant varieties.
- Legislative Method
- Involves the imposing of regulations and laws especially in case of disease outbreaks to prevent the introduction and spread of diseases.
- Chemical Control
- Used as a last resort.
Chemical control measures include:
- Seed dressing before planting.
- Soil fumigation to control soil borne diseases.
- Spraying: application of fungicides.
Download Crop Pests and Diseases - Agriculture Form 3 Notes.
Tap Here to Download for 50/-
Get on WhatsApp for 50/-
Why download?
- ✔ To read offline at any time.
- ✔ To Print at your convenience
- ✔ Share Easily with Friends / Students