Agriculture Form 3 – Crop Pests and Diseases

Introduction

• Crop pests and diseases cause significant losses in crop production; therefore, effective control measures are essential to minimize these losses.
• To implement proper control measures, farmers need to:
• Identify the pests and diseases affecting their crops.
• Understand the life cycles and feeding habits of these organisms.
• Recognize the damage they cause to crops.

Crop Pests

Definition of a Pest

• A pest is a living organism that destroys crops or trees either directly or indirectly by introducing pathogens (disease-causing germs).

Classification of Pests

Pests are classified according to the following criteria:

Mode of Feeding

• Pests with biting and chewing mouthparts cause physical damage and reduce the photosynthetic area of the plant.
• Pests with piercing and sucking mouthparts extract nutritious plant sap and may introduce disease-causing organisms.

Crops Attacked

• Some pests attack specific crops; for example, stem borers prefer cereal crops.

Stage of Growth of Crops Attacked

• Seedling pests attack crops when they are young, such as cutworms.
• Fruit pests attack crops at the fruiting stage.
• Grain pests attack crops when the grains have formed.

Field and Storage Pests

• Some pests attack crops while they are still in the field.
• Other pests attack produce after it has been harvested and stored.

Identification of Common Pests

Other Crop Pests Include:

• Mealy bugs – coffee
• Thrips – coffee
• Beetles – field and storage pests
• Birds – field pests on cereals and fruits
• Rodents – field and storage pests on cereals and tubers
• Nematodes – soil-borne pests on 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 reduce both the quality and quantity of farm produce.
• They increase production costs since farmers incur expenses purchasing chemicals to control them.
• They transmit diseases to crops; for example, aphids transmit streak virus disease in maize.
• Chemicals used to control pests can cause environmental pollution.
• They can exterminate crops by feeding on them, such as eating the embryo of the seed.

Control of Pests

• If pest populations cause damage beyond tolerance, they have reached the economic injury level (EIL); control measures should be implemented before this level is reached.

Before any control measure is implemented, the following should be considered:

• Knowledge of the pest’s life cycle.
• Correct identification of the pest.
• Accurate assessment of the damage caused.
• Weather conditions at the time.
• The value of the crop in question.
• The cost of the control method.

Methods of Controlling Pests

• Cultural methods
• Physical/mechanical measures
• Biological methods
• Chemical methods
• Integrated pest management

Cultural Methods

• These are farming practices aimed at reducing pest populations by disrupting their life cycles, 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: a period when a susceptible crop is not grown to control a certain pest.
• Trap cropping: planting crops that attract pests, diverting them from the main crop; the trap crop is grown alongside the main crop.
• Crop rotation: breaks the life cycle of the pest.
• Planting resistant varieties: plants with natural protective mechanisms against pests, e.g., hairy cotton against jassid bugs, goose-necked sorghum against birds, high tillering sorghum against shoot fly.
• Field hygiene: includes rogueing and removal of crop residues that harbor pests.
• Alteration of environmental conditions, such as creating a microclimate unfavorable to pests, e.g., open pruning in coffee.
• Crop nutrition: application of fertilizers and manures to strengthen crops and help them resist pest attacks.
• Destruction of alternate hosts, e.g., weeds like mallow that harbor cotton stainers.
• Use of clean planting materials to prevent introduction of pests.
• Proper spacing: well-spaced plants make it difficult for pests to move from one plant to another.
• Use of organic manure, e.g., farmyard manure discourages eelworms (nematodes).

Chemical Control

• Chemicals used to control pests are called pesticides.
• Pesticides are applied through dusting, spraying, or fumigation.

Classification of Pesticides

Pesticides are classified based on:

Mode of Entry

• Stomach poisons – ingested by the pest along with crop materials.
• Contact poisons – absorbed through the pest’s body tissues.
• Fumigants – enter through the pest’s respiratory system.
• Systemic pesticides – translocated to all parts of the plant.

Mode of Action

• Respiratory poisons – interfere with breathing mechanisms.
• Coagulants – cause the pest’s blood to coagulate.
• Neurotoxins – affect the nervous system.
• Protoplasmic poisons – cause cell disintegration.

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 Affecting the Efficiency of Pesticides

• Concentration of the pesticide.
• Weather conditions during application.
• Timing of application – highest efficiency when applied when pests are most susceptible.
• Persistence of the pesticide, including residual effects in the soil.
• Resistance developed by pests.

Advantages of Chemical Pest Control

• Fast-acting.
• Immediate results.
• Requires low labor.

Disadvantages of Chemical Pest Control

• Expensive to purchase.
• Can cause environmental pollution.
• Requires skilled labor for application.
• May kill beneficial organisms and predators.
• Target pests may develop resistance.

Mechanical Pest Control / Physical Methods

• Involves killing pests using physical methods.
• Or creating physical barriers to prevent pests from reaching crops.

Examples:

• 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 making them airtight.
• Trapping and killing, e.g., rats.
• Creation of physical barriers, such as rat bafflers and sticky materials on tree trunks.
• Proper drying of produce to make it too hard for pests to destroy.
• Scaring devices, especially in rice plantations, to control birds.
• Use of explosives thrown at bird breeding places to kill or scare them away.

Biological Pest Control

• Involves using living organisms to reduce pest populations.

Predators and Target Pests

• Parasitic wasps – target whitefly in citrus, boll worms, stalk borers.
• Birds – feed on crickets, locusts, caterpillars.
• Ladybird – feeds on aphids.
• Tachinid flies – target cotton stainers.
• Praying mantis – feeds on giant loopers.
• Majimoto ants – target scales.
• Cats – control moles, rats, mice.
• Brachonid wasps – target mealy bugs.
• Chicken – feed on cotton stainers, larvae of beetles, grasshoppers, crickets.

Advantages

• Cost-effective.
• No environmental pollution.
• Reduces labor requirements.

Disadvantages

• Time-consuming to identify the correct biological agent.
• Difficult to control pests effectively.

Integrated Pest Management

• A modern approach combining various pest control methods.
• Aims to minimize hazards to users and the environment.
• Example: attractant pheromones are used to lure pests to one location where they are sprayed and eradicated.

Legislative Method / Quarantine

• Implemented by the Kenya Plant Health Inspectorate Service (KEPHIS) through seed inspection to control pests.

Crop Diseases and Their Control

• A disease is any deviation from normal performance or function.
• A plant disease is a harmful physiological disorder caused by pathogenic agents such as viruses, bacteria, and fungi.
• The study of plant diseases is called plant pathology.

Economic Importance of Crop Diseases

• They reduce crop yield.
• They lower the quality of produce, reducing market value.
• They can cause food poisoning, e.g., ergot in wheat, aflatoxin in grain crops caused by fungi.
• They reduce the photosynthetic area of plants.

Classification and Identification of Plant Diseases

• Plant diseases are classified according to their causal agents.

Fungal Diseases

• Fungi are non-green, plant-like organisms.
• Some fungi are parasitic, while others are saprophytic.

Parasitic Fungi are Divided Into:

• Obligate parasites – depend on living organisms for food.
• Facultative parasites – can live on both living and dead tissues.

Examples of Fungal Diseases

• Panama disease (Fusarium oxysporum) – affects bananas.
• Cigar-end rot (Verticillium theobromae) – affects bananas.
• Die back – attacks tips of shoots and roots.
• Mildews – foliar disease affecting several crops.
• Armillaria root rot (Armillaria mellea) – affects coffee and tea.
• Damping off disease – affects seedlings in nurseries.
• Anthracnose (Colletotrichum spp) – affects coffee, beans, tomatoes.

• Fungi also damage stored grains that are not properly dried or if storage is damp.
• Fungi cause food poisoning and reduce seed viability; for example, Aspergillus flavus produces a highly toxic compound called aflatoxin.

Examples of Fungal Diseases

Bacterial Diseases

• Bacteria are microscopic single-celled organisms that 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

Viral Diseases

• Viruses are tiny living organisms visible only under powerful electron microscopes.
• Viruses interfere with photosynthesis, respiration, transpiration, and nitrogen utilization in plants.

Symptoms of Viral Infection

• Leaf chlorosis.
• Leaf curling.
• Mosaic patterns (light green or yellow patches).
• Malformation (distortion) of plant parts.
• Rosettes; development of abnormally short internodes.

Transmission

• Through infected vegetative materials and insect vectors such as aphids, mealybugs, and leafhoppers.

Viral Diseases

Other Causes of Crop Diseases

• Flooding produces ammonia, which is poisonous to crops and causes leaf burning.
• Chemicals that may be toxic to plants.
• Poor weather conditions, such as extreme day and night temperatures.
• Stress factors, such as irregular watering, e.g., tomato blossom end rot.

Control of Crop Diseases

• Cultural methods: use of healthy planting materials.
• Practicing field hygiene.
• Proper seedbed preparation.
• Proper spacing of plants.
• Heat treatment of planting materials, e.g., sugarcane.
• Proper drying of cereals and pulses to 13% moisture content.
• Growing disease-resistant varieties.

Legislative Method

• Involves imposing regulations and laws, especially during disease outbreaks, to prevent 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 with fungicides.