Crop Pests and Diseases - Agriculture Form 3 Notes

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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
- effective fungicides
- treated seeds
-resistant varieties

Rusts (Pucinia spp)

Rice, wheat , sorghum, maize

Red and brown pistules on the leaves, shriveled grains

- resistant varieties
- Recommended fungicides
- Early planting

Smuts(Ustilago spp)

Wheat, maize , sugarcane

Black powder mass on the spikes and the ear

- Field hygiene,
- certified seeds,
- resistant varieties,
-crop rotation

Blasts (Piricularia oryzae)

Rice

-Small blue sports on leaves with grey centre.
-Attack inflorescence to cause ‘’empty heads’’

- Seed dressing
- Resistant varieties eg        sindano
- Destruction of affected plants
-fungicides

Coffee Berry Disease(CBD) (Colletotrichum coffeanum)

Coffee

- Dark blotches spots on   the flowers
- Brown concentric rings on the leaves
- Dark sunken wounds on the berries.

- Resistant varieties eg Ruiru 11
- Proper pruning
- Effective fungicides
- strippung

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. 
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