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Crop pests and diseases - Agriculture Form 3


1. Learners to practice application of chemicals on a plot of cabbages to control cutworms.
2. Learners to visit the school farm and collect and identify crop pests. For each of the pest classify it on the basis of damage to crops as I evident I the crop attacked.
3. Collect some common insect pest and preserve them.

Background Information

In Form one, among the factors influencing Agriculture were biotic factors such as pests and pathogens. This chapter will present various pests and diseases and how they affect Agricultural production. Pests and diseases cause a lot of damage to crops both in the field and in stores.


Integrated pest management (IPM) - refers to when a combination of both chemical and cultural pest control methods is used. Economic injury level (EIL)-the lowest pest population that will cause economic damage. Legislation - the passing of laws to control activities. Trap crop- is a crop which is planted before or together with main crop purposely for attracting pests away from the latter. Rogueing- its the removal of and destruction of plants which are infested with pests. Rossetting- development of abnormally short internodes resulting in stunted bushy looking plants. Stomach poison- a poison which operates by absorption through the intestine after having been ingested by the insect. Suffocants- are pesticide that kill by interfering with the breathing system of a pest after being inhaled. Repellants- are pesticides that keep the pests away from the plant.

Topic Objectives

By the end of the topic, the learner should be able to:
Define pests and disease,
State the main causes of pests and diseases
Describe the harmful effects of crop pests and diseases
Carry out general disease and pest control measures.
Identify and classify some of the common pests and diseases.

Crop Pests and Diseases

Crop Diseases.

Definition-a crop disease is any alteration in the state of a crop or its parts which interrupts or disturbs its proper performance or function

Harmful effects of crop diseases.

Some of the harmful effects of crop diseases include;

  • Low crop yields
  • Production of poor quality products
  • Food poisoning e.g aflatoxin in grains

Classification and identification of diseases.

Crop diseases are classified according to their causal agents. These include;

  • Fungi
  • Viruses
  • Bacteria
  • Poor weather conditions
  • Lack of essential elements.

Identification of diseases

Diseases are identified by recognising the symptoms of infection by specific micro organisms or or deficiency symptoms.

Symptoms of diseases may include; leaf chlorosis, leaf curling, mosaics, malformations, dry patches, Wilting and resetting among others.

Fungal Diseases.

These are diseases caused by fungus. Fungus are non-green plants that are incapable of manufacturing their own food due to lack of chlorophyll. There are three main groups of parasitic fungi;

  • Those with all the mycelia and the fruiting bodies on the surface of the host.
  • Those with mycelia inside the plant tissue but the fruiting bodies on the surface of the host .
  • Those having the mycelia and the fruiting bodies all inside the host.

Common fungal diseases.

Late blight It is caused caused by phytophthora infestans it infects most the members of the solanacea family especially Irish potatoes and tomatoes. Symptoms of the disease include dry patches(necrotic lesions), on leaves and fruits Control measures: spraying bordeax mixture and other copper fungicides.


They are caused by

Ustillago spp

. The species is host specific for example, Maize smut is caused by ustillago ,

ustillago nuda

attacks wheat and

ustillago scitiminea

attacks sugarcane


Head smut in maize produces large number of black spores which forms black masses on the maize tassels and cobs.

Rusts .

Rusts are caused by puccinia spp. They attack the leaves and stems of most cereal crops. Infected leaves have red or brown pustules. These pustules reduce the photosynthetic area resulting in low yields
Rust in wheat

Rusts are controlled by spraying copper fungicides.

Viral diseases.

These are diseases caused by viruses.
Viruses are extremely small living organisms which can only be seen under very powerful electronic microscope. All viruses are parasitic and are only able to multiply in living tissues.

Symptoms of viral diseases include;

leaf chlorosis, leaf curling, mosaics, malformations and resetting.

Cassava mosaic

An infected leaf has white or pale-yellow or pale-green spots. It is often twisted, distorted, and stunted. The leaflets have uniformed or localized mosaic pattern. Heavily infested plant is distorted, retarded, and dwarfed.

Tomato leaf roll

Symptoms of the disease include; leaf chlorosis, mosaics and malformations .

Maize streak

Symptoms of the disease include; yellow longitudinal stripes which run parallel to the midrib leaf chlorosis, mosaics, malformations and rosetting.

Groundnut rosette

The most destructive virus disease on groundnut in Kenya. Three casual agents are invoved in GRD such as Groundnut rosette assistor virus (GRAV), Groundnut roselte virus (GRV) and a satellite RNA (satRNA) Trasmitted by an Aphid,

aphis craccivora.

Symptoms of the disease include;

Leaf chlorosis, mosaics, malformations and rosetting. Both forms of the diseases cause plants to be severely stunted, with shortened internodes and reduced leaf size , resulting in a bushy appeaerance of plants. In chlorotic rosette , leaves are usually bright yellow with a few green islands and leaf lamina is curled. In the green rosette , leaves appear dark green,with light green to dark green mosaic.

Bacterial diseases.

These are diseases caused by bacteria. Bacteria are microscopic single-celled organisms which reproduce by binary fusion. They vary in shape and size. They are able to live in both dead and living tissues. Not all bacteria are harmful.
Symptoms of bacterial diseases include:

  • Wilting
  • Cankers
  • Gall formation

Halo blight of beans

Symptoms of the disease include:

  • Cankers
  • Gall formation

Control of crop diseases.

Before any control measure is adopted in disease control, the causal agent must be identified. Some of the control measures include; cultural practices, legislative methods and chemical methods.

Chemical control.

Chemical measures used in control of diseases include:

  • Seed dressing
  • Soil fumigation
  • Spraying.

Crop pests

Classification of crop pest

Pests can be classified on the basis of: a) Mode of feeding b) Crops attacked c) Stage of growth of crops attacked d) Habitat

Field pests

Aphids feeding on cabbage forage

Locusts feeding on crop forage

Storage pests

Weevils feeding on grain

Potatoe boler feeding on tuber

Modes of feeding

A pests mode of feeding is determined by the type of mouth parts the pest has. The two modes of feeding of insect pests are

  • Biting and chewing.
  • Piercing and sucking

Crops are usually attacked at different growth stages.

Mouse bird feeds on ripe fruits such as tomatoes.

Cutworms attack plant at seedlings stage.

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