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HomeNotesAGRICULTURE NOTESCROP PESTS AND DISEASES.

CROP PESTS AND DISEASES.

CROP PESTS AND DISEASES.

CROP PESTS AND THEIR CONTROL.                                                   

Crop pest.

Living organism that destroys crops either directly by causing physical damage or indirectly by introducing disease-causing organisms into crops.

HARMFUL EFFECTS OF CROP PESTS.

  • Pests like squirrels and mice unearth planted seeds resulting in low plant population.
  • Some e.g. nematode, termites and moles damage crop roots causing wilting and death of plants.
  • Pests destroy crop leaves lowering the photosynthetic area resulting in reduced yields.
  • Sucking pest deprive the plants of its food by sucking plant sap resulting in retarded growth.
  • Some pests attack fruits, berries and flowers, thus lowering their quality and quantity. 6) Some pests destroy the embryo of seeds, thus lowering their germination potential.
  • Some transmit crop diseases e.g. aphids and mealy bugs transmit viral diseases while wounds caused by pests’ acts as route of secondary infection.
  • Some pests e.g. stalk borers, eat the growing points causing retarded growth.
  • In crops where the leaf is the major product, pest damage lowers the quality and quantity through defoliation.
  • Pests reduce the marketability of crop produce by lowering quality e.g. weevils bore into maize grains lowering their economic value.

CLASSIFICATION OF PESTS.

  1. Their mode of feeding.

This is determined by the nature of their mouth parts:

  • Those with biting and chewing mouth parts.
  • Those with piercing and sucking mouth parts.
  1. Crops attacked.

Most pests are crop specific. That is prefer certain crops or a family of crops. E.g. coffee pest.

  1. Stage of development of the pest.

Some pests are harmful to crops when they are in the larval or nymphal stage while others affect crops during their adult stage.

Most attack at larval stage such as moths and butterflies.

  1. Stage of growth of the crop attacked.

Some pest attacks the crops when it is young and tender while others attack the crop at flowering or when mature. E.g. cutworm after transplanting and American bollworm at flowering stage or the fruits.

  1. Scientific classification.

May be classified according to their scientific grouping. E.g. insects, birds, nematodes, mites etc.

  1. The level of damage.

Some cause less damage to crops thus called minor pests while others causes great damage thus called major pest.

  1. The place where they are found/habitat.

May be classified according to their ecological niche or where they are found.

Some pest cause damage to the crops while in the field (field pests) others cause damage to stored produce. (Storage pest)

CROP PESTS AND DISEASES.

IDENTIFICATION OF COMMON PESTS.

  1. a) Insect pest.

Most destructive class of tropical pests. More serious in tropics as tropical conditions are ideal for their breeding.

Some are beneficial e.g. pollinators and pest predators.

Damage caused by insect pest depend on feeding habits of the insects.

  1. Insects with biting and chewing mouth parts.

Include: locusts, grasshoppers, crickets, and maize stalk borers, army worms, cut worms, bollworms and termites.

Have specialised biting and chewing mouth parts with developed cutting and grinding teeth.

Damages caused.

  1. Leaves

Damage on leaves reduces photosynthetic area of plant lowering yields.

Reduces quality in leaf vegetable.

  1. Stems

Damage interferes with transport system leading to death of plant. May cut and lead to falling of stems.

  1. Roots and tubers.

Affect nutrients and water uptake. Lowers quality of tubers e.g. sweet potato attacked by weevils.

  1. Flowers and fruits.

Leads to their fall also reducing quality of those that do not fall.

  1. Insects with piercing and sucking mouth parts.

Includes: aphids, scales, adult stages of butterflies and mouth, cotton leaf hoppers, mealy bugs and thrips.

Have mouth parts modified into needle-like stylet that the use to suck sap.

Effects.

Introduction of disease causing organisms e.g. viruses and fungi present in their saliva.

Some inject toxic saliva which may cause distorted growth in plants e.g. antestia bug in coffee.

Name of pest.

 

Crop attacked. Disease transmitted.
Aphids (Aphis  crassivora) Groundnut.

 

Groundnut rosette virus.
Aphids (Aphis  persicae) Groundnut.

 

Groundnut mosaic virus.
Aphids (Myzus persicae) Tobacco.

 

Tobacco mosaic virus.
Tobacco whitefly.  (Bemisia  tabaci) Tobacco.

 

Tobacco leaf curl virus.
Tobacco whitefly.  (Bemisia  tabaci)  

Cassava.

Cassava mosaic virus.
Pineapple mealy bug.

(Dysmicoccus  brevipes)

Pineapple. Pineapple virus.

CROP PESTS AND DISEASES.

  1. b) Mites

Order: Acarina.

Class: arachnida.

Lack wings and antennae, have four pairs of legs and two body parts (Cephalothorax and abdomen)

Adapted to piercing and sucking. Some are microscopic.

They are primarily parasitic. Are moved from one place to another by wind due to their small size. They usually attack underside of leaves.

Examples.

  • Red spider mite.
  • Yellow tea mite.

Their population is lowered by natural enemies.

  1. c) Namatodes

Legless worms with elongated and unsegmented bodies. Belong to class Nematoda  Some are free-living while others are parasitic in plants (eelworms) and animals (Ascaris).

Have separate sexes. Have mouth parts modified into stylet adapted for piercing and sucking.

Attack leaves, stems, roots and bulbs.

Examples.

Meloidogyne    spp      attack solanacea family crops.

Practylenchus   spp         attack pyrethrum leaves.

 Damages by nematodes.

  1. Injecting toxic substances into the plant tissues stimulating structures like galls (swellings).

E.g.  Meloidogyne spp.

  1. Galls do not rub off easily unlike root nodules. Galls block xylem vessels restricting water movement leading to wilting.
  2. Some feed on plant roots causing root stunting limiting water and nutrients uptake. This causes wilting which is severe in dry spell and when crops are young.
  3. Causes wounds in plant tissues through which secondary infection may take place.

Rodents

Gnawing mammals. Include squirrels, moles, cats, porcupines, field mice and hares.

Have specialised teeth for gnawing hard substances such as grains.

Porcupines and hedgehogs are serious pests to green maize, root and tuber crops e.g. potatoes, cassava and cocoyam.

Moles eat roots and pull plants underground. Spoils pastures by covering them with soil from burrowed tunnels.

Squirrels unearth and eat sown seeds, eat roots and tubers.

  1. Birds

Most are grain eaters while others eat fruit.

Examples.

  1. Sudan Dioch (Quelea Quelea aethiopica)

Causes severe losses on sorghum, millet and other grain crop.

Move in large number thus it is considered an epidemic.

Control.

  • Use of explosives.
  • Resistant varieties e.g. gooseneck sorghum varieties.
  1. The common weaver bird.

Causes serious damage on grains. Causes serious damage to the maize crops at milky stage.

  • Mouse bird.

Damages fruit crops (most serious fruit eating bird) Damage bean seedlings in early stages.

  1. Domestic fowl.

Become pests when not properly reared. Cause damage to flowers, pods, grains, leaves and tubers.

  1. f) Large animals.

Wild animals such as elephants, buffaloes, monkey, hippos etc.

STORAGE PESTS.

Pests which cause damage to crops while in store. Include: rodents, insects and fungi.

Rodents.

Common rat (Rattus rattus) and giant rat. (Cricetomy gambianus)

Cause damage to grains in store by eating and contaminating the grains with excreta.

Harbour fleas that transmit bubonic plague to human.

Insects.

Include: maize weevils, beans bruchids, grain borers and floor beetles.

True maize weevil.

Species.

 

Scientific Name. Stored crop affected.
Weevil. Stiphilus      spp Maize, sorghum, wheat, rice.
Lesser grain borer. Rhizoperita    dominza Rice, maize, wheat.
Khapra beetle. Trogederma      grananium Maize, wheat, pulses, sorghum.
Saw-toothed grain beetle. Orzaephilus     spp Maize, wheat, groundnut.
Pulse beetles. Callosobruchus     obtectus Beans.
Flat beetles. Laemophiloeus    pusillus Grain flour.
Aqoumois grain moth. Sitotraga   cerealella Maize, wheat.
Tropical/warehouse moth. Epestia    coutelle Rice, maize, wheat.
Potato tuber moth. Pthorimaea    operculella Stored Irish potatoes.

Termites are also storage pests.

Fungi.

Causes damage to stored grains that are not properly dried or in a damp store.

Include: penicillum and Aspergillus.

Causes:

S Food poisoning e.g. (Aspergillus flavus) S Lowers seed viability.

CONTROL OF CROP PESTS.

Integrated pest management. (IPM)

Use of a combination of both chemical and cultural control methods.

Economic injury level. 

Where pest population causes damage beyond tolerance and control measures have to be undertaken. Measured in terms of number of pests per plant.

CROP PESTS AND DISEASES.

PEST CONTROL METHODS.

  • LEGISLATIVE METHOD/QUARANTINE.

Government regulations that prevent introduction of new pests into the country. Any plant material entering the country must be inspected to ascertain it does not contain new pests.

Done by Kenya Plant Health Inspectorate Service (KPHIS) at Muguga and Mbita in south Nyanza.

  • PHYSICAL MEASURES.

 

  1. Use of lethal temperature.

Involves use of extreme temperatures to control pests.

E.g. control of pink bollworms in cotton seeds by hot water treatment.

  1. Proper drying of produce.

Drying of grains makes them too hard for pest to penetrate. It also discourage growth of moulds. Should be dried to 11-13% MC.

E.g. cut worms and army worms will drown if the field is flooded. Kills underground pests such as moles.

Use of hermetic Cyprus bins and hermetic bags where CO2 build up to suffocate pests.

  1. Physical destruction of pests.

Hand picking or trapping and killing them. E.g. picking and killing giant loppers and trapping moles.

  1. Use of scarecrows.

Scaring large animals and birds out of the farms.

  1. Use of physical barriers.

Materials that prevent pests from getting into the crops. E.g. rat deflectors, construction of fences around the fields and trenches to control large animals.

  1. Use of Electromagnetic Radiation.

Radiators such as radioactive radiations. Deactivates enzymes in some insects. Moths are attracted by U-V and aphids by Yellow light. Thus can then be destroyed.

CULTURAL METHODS.

Farming practices employed to alter the environment making it unfavourable for survival of pests.

  1. Timely planting.

Helps crops escape pest attack e.g. maize stalk borers in maize.

  1. Timely harvesting.

Some storage pests e.g. grain weevil attack crop while in the field thus early harvesting will enable it to escape attack.

  1. Proper Tillage.

Field cultivation exposes pests that are soil borne e.g. white grubs to sun or predators. d) Close season.

Period when a susceptible crop is not grown in order to control a certain pest or group of pests.

Crop residues should also be destroyed. E.g. in control of pink boll worm in cotton.

  1. Trap cropping.

Trap crop. Planted before or together with the main crop purposely for attracting pests away from the main crop.

Trap crop is planted before if not preferred by the pest or planted along with the main crop if more preferred by the pest than the main crop,

  1. Crop rotation.

This will starve the pest to death by rotating crops that are not preferred by the pests.

E.g. rotating maize and beans which are preferred by nematodes with groundnuts that are not preferred.

  1. Planting resistant crop varieties.

Breeding has resulted into plants with natural protective mechanisms against pest attack. E.g. goose necked sorghum is resistant to birds attack and highly tillering sorghum resistant to shoot fly.

  1. Field hygiene.

Keeping filed free from plant materials harbouring pests. Include: rogueing and removal of crop residues from the field.

  1. Alteration of Environmental conditions.

Includes creating micro-climates not conducive to some pests. E.g. open pruning in coffee discourages antestia bugs and mulching reduces thrips.

  1. Crop nutrition.

Application of fertilizers and manure makes crops strong thus able to resist and escape attack. However excessive Nitrogen makes crops succulent hence susceptible to pest attack.

  1. Destruction of Alternative host.

Some weeds are alternate hosts to crop pests thus should be controlled. E.g. mallow harbours cotton stainers.

  1. Use of clean planting materials.

Helps to prevent introduction and spreading of crop pests.

E.g. suckers in bananas should be free from pests such as banana weevils.

  1. Proper spacing.

Proper spacing makes it difficult for pests to move from one plant to another. Closer spacing in groundnuts discourages Aphids.

  1. Use of organic manure.

Effective in control of nematodes.

Overhead irrigation controls aphids in cabbages.

CROP PESTS AND DISEASES.

3) CHEMICAL CONTROL.

Use of pesticides to control pests.

Classification of pesticides.

Form in which a pesticide is available. Includes: Wettable powders, fumigants, aerosols dusts, liquids and emulsions.

  • Target pest.

Classified according to specific pests killed by a pesticide.

Include:

  • Kills insects.
  • Kills nematodes. S Rodenticides. Kills rodents S Fungicides. Kills fungi.

3) Mode of action.

Include:

  1. Stomach poisons.

Kills when the pest eat plant part sprayed/dressed with chemical. Kills biting insects. It is selective as it kills only the pests that consume the crop sprayed.

  1. Systemic poisons.

Circulated to all parts of the pest once it has eaten the sprayed part. More selective as only the pest with systems that can circulate them are killed.

  1. Contact poisons.

Kill pests when they are absorbed in the body through skin or cuticle.

Effective when applied directly on the target pest. Are not selective and may kill beneficial organisms such as pest predators, pollinators and decomposers.

Kills by interfering with the breathing system of a pest after being inhaled. Pests with a tough cuticle need Suffocants.

Anti-feedants.

Inhibit (prevent) feeding in insects and other pests starving them to death.

Keep the pests away from the plant.

FACTORS AFFECTING THE EFFICIENCY OF PESTICIDES.

The correct concentration will be more effective in killing the target pest.

  1. Timing of application.

Should be applied at the stage of development when the pest is most vulnerable. E.g. leaf miners at larval stage before cocoon formation.

  1. Weather conditions at the time of application.

Rain falling immediately after application washes off/dilute the pesticide reducing its effectiveness. Apply when there is no likelihood of rain.

Pesticides should remain effective long enough to achieve all desired effects.

CROP PESTS AND DISEASES.

Advantages of chemical control.

  • It faster compared to other methods of control.
  • Results are more predictable than most other methods.

 Disadvantages.

  • They are expensive.
  • Most are non-selective thus kill useful insects such pollinators and pest predators.
  • Some develop resistance to some pesticide and thus becoming a bigger problem than before.
  • Most are toxic to man, livestock and other animals. Indiscriminate use interferes with the ecosystem.
  • Use of pesticides requires skill in handling and application.

4) BIOLOGICAL PEST CONTROL.

Use of living organisms to control pests. Based on predators-prey relationships. Predator is a living organism that kills another for food. Every living organism has its own natural enemies.

Predator. Target pest.
Lady bird. Aphids.
Wasps. Coffee mealy bugs.
Preying mantis. Giant lopper.
Majimato ants. White scales.
Chicken. Cotton stainers.
Cats. Moles, rats, mice.
Chameleons. Most insects.

 CROP DISEASES AND THEIR CONTROL. 

Disease.

Alteration in the state of an organism or its parts which interrupts or disrupts its proper performance or function.

Harmful effects of crop diseases.

  • Lower crop yields.
  • Production of quality product reducing their market value.
  • Cause food poisoning e.g. ergot in wheat, barley and rye causes nerve poisoning. Aflatoxin in moist stored grains is poisonous.

CROP PESTS AND DISEASES.

CLASSIFICATION AND IDENTIFICATION OF CROP DISEASES.

Classified according to their causal agents.

Include: fungi, virus, bacteria, poor weather conditions and deficiency of essential elements.

a) Fungal diseases.

Diseases caused by fungi.

Fungi lack chlorophyll. Exist as parasites, saprophytes or both.

  • Obligate parasite.

Completely dependent on other living organisms for food.

  • Facultative parasite.

Are able to live on both the living and dead tissues.

Saprophytes live as decomposers of dead and decaying plant and animal remains.

Groups of parasitic fungi.

  • Those with all the mycelia and the fruiting bodies on the surface of the host. E.g. Erysiphe Causes Powderly mildew.
  • Those with the mycelia inside the plant tissues and the fruiting bodies on the surface of the host. E.g. Phytophthora Puccina spp. And Ustilago spp.
  • Those having the mycelia and fruiting bodies all inside the host. E.g. Fusarium spp.

Examples of common fungal diseases.

Late blight.

Caused by Phytophthora infestans. Attacks solanacea family especially Irish potatoes and tomatoes. 

Develops a network of many small branching hyphae in leaves and other parts.

Feed by sending hyphae (haustoria) into the host cell.(Absorbs plant sap leading to death of cell)

Reproduce by spores and spread quickly in warm moist conditions. Spores are transmitted by wind or raindrop splashes.

Symptoms.

  • Dry patches (necrotic lesions) on leaves and fruits. S Affected fruits appear rotten and falls prematurely.

Control.

Spray copper based fungicides.

Rusts.

Caused by Puccina spp. E.g. Puccina graminis, Puccina sorghi.  Puccina polysora.

Symptoms.

  • Infected leaves have red or brown pustules. S Field appear rusty.

Pustules on leaves reduce photosynthetic area resulting in low yields.

Control.

Spraying copper fungicides and Bordeaux mixture.

Smuts.

Caused by Ustilago spp.

  • Ustilago zeas. In maize.
  • Ustilago scitiminea in sugarcane. S Ustilago noda in wheat.

Produces large number of black spores that forms black masses on maize tassels and cobs.

Control.

  • Hot water treatment of barley and wheat seeds.
  • Use of certified seeds.
  • Crop rotation.
  • Field hygiene.

Coffee berry disease. (CBD)

Colletrotrichum coffeanum. Attacks Arabica coffee.

Causes crop losses up to 80%.

Favoured by high rainfall, high relative humidity and cold temperatures.

Symptoms.

  • Flowers have a dark brown blotch or streak on the white petal.
  • Green berries have small dark sunken patches or lesions that spread rapidly and cover the whole berry.
  • Infected berries drop to the ground or remain on the tree in a black mummified condition.
  • Berries are empty.
  • Ripe berries have dark sunken patches with minute black dots.
  • Serious infection is on berries that are spreading.

Control.

  • Use of appropriate copper fungicides. Start before onset of rains and should be repeated every four weeks. S Open pruning.

CROP PESTS AND DISEASES.

Other fungal diseases.

  • Powdery mildew.
  • Fusarium wilt.
  • Root rot.
  • Downy mildew. S Early blight.
  1. b) Viral diseases.

Caused by viruses.

Viruses are extremely small living organisms. All are parasitic. They are obligate parasites. When outside living tissues, viruses form spores (cysts) and live in inactive form for many years.

 Symptoms of viral infection in plants.

  1. Leaf chlorosis.

Due to impairment of the plants ability to synthesise chlorophyll.

  1. Leaf curling.

Production of light green or yellow patches of various sizes and shape. Irregularly distributed among normal tissues.

Distortion of plant parts e.g. small leaves, galls and overgrowth on leaf lamina. e) Rosetting. 

Development of abnormally short internodes resulting in stunted plants.

Transmission of viral diseases.

  • Use of infected vegetative materials.
  • Insect vectors (sucking mouth parts)

 Examples of viral diseases. 

  • Cassava mosaic.
  • Brown streak of cassava.
  • Potato leaf roll.
  • Tobacco mosaics.
  • Groundnut rosette.
  • Greening disease of citrus fruits.
  1. c) BACTERIAL DISEASES.

Bacteria are microscopic single celled organisms that reproduce by binary fission.

Transmission.

  • Irrigation water.
  • Seeds, fertilizers, manure and wind.
  • Rain splashes.
  • Cultivation implements and pruning knifes.

Symptoms.

  • Wilting due to blockage of xylem vessels by masses of bacteria.
  • Localised infections resulting in death of plant tissues (necrotic tissues) S Gall formation in infected tissues.

Examples of bacterial diseases.

Bacterial Blight of Coffee. (BBC)

Caused by: Pseudomonas syringe. Found on plant surfaces (leaves and green shoots.

Enters through the stomata and wound. It is severe at high altitude.

Symptoms.

  • Necrotic lesions which when active, have water-soaked margins.
  • On shoots it causes die back originating from flowering nodes and shoots.
  • Infection on mature bark and wood resulting in cankers that girdle the main stem killing the whole plant.
  • Fine scorch appearance in epidermis.

Control.

  • Use of appropriate chemicals e.g. supanil dust before, during and after flowering.

Other bacterial diseases. 

  • Black rot of cabbage.
  • Black arm of cotton.
  • Bacterial wilt of potatoes, tomatoes etc. S Halo blight of beans.
  1. d) Nutritional disorders.

Deficiency symptoms. 

  • Yellowing of leaves.
  • Drying of leaves.
  • Falling of leaves, flowers and fruits.
  • Stunted growth.
  • Death in extreme conditions.
  1. e) Other causes of crop diseases.

 

  • Poor weather.

Extremes of day and night temperatures cause Hot and Cold disease in coffee. (Elgon Die Back).

Stressful conditions e.g., irregular watering causes blossom-end rot in tomatoes.

CROP PESTS AND DISEASES.

CONTROL OF CROP DISEASES.

1) CULTURAL METHODS.

  • Using healthy planting materials.
  • Filed hygiene such as burning of diseased crop residues.
  • Proper seed bed preparation to control armillaria root rot in tea and coffee.
  • Proper spacing to control damping off in cabbage seedlings in a nursery bed and rosette disease in groundnut closer spacing is used.
  • Heat treatment e.g. in control of ratoon stunting disease in sugarcane.
  • Proper drying of cereals and pulses before storage.
  • Use of disease resistance varieties e.g. Ruiru 11 and batian resistant to CBD and coffee leaf rust.
  • LEGISLATIVE METHOD.

Imposing regulations and laws to prevent introduction and spreading of diseases.

  • CHEMICAL CONTROL.
  1. Seed dressing.

Application of fungicides before planting seeds.

  1. Soil fumigation.

Application of fumigants in the soil to control soil borne pests e.g. bacterial wilt.

Application of fungicides to control diseases

CROP PESTS AND DISEASES.

ALL AGRICULTURENOTES FORM 1-4 WITH TOPICAL QUESTIONS & ANSWERS

PRIMARY NOTES, SCHEMES OF WORK AND EXAMINATIONS