DISEASE ORGANISMS OF CROPS. Types of disease organisms affecting crops. A disease could be defined as an abnormal condition in the crop plant. This may shown in some part & or on the whole plant causing damage to the plant. Some organisms are capable of causing diseases to crops. These organisms are called pathogens and they include disease organisms of crops

crop disease organisms
crop disease organisms

(a) Bacteria (b) Fungi (c) Viruses (d) Nematodes.

State the main groups of disease organisms.
important crop diseases based on their symptoms.
Explain the stages of disease development and mode of transmission.
describe damages to crops. preventive and control measures of disease organisms of crops

Types of disease organisms affecting crops

A disease could be defined as an abnormal condition in the crop plant. This may shown in some part & or on the whole plant causing damage to the plant. Some organisms are capable of causing diseases to crops. These disease organisms of crops  organisms are called pathogens and they include:

(a) Bacteria (b) Fungi (c) Viruses (d) Nematodes

 Bacteria diseases of crops

These are very small unicellular organisms. They are said to be ubiquitous because they can be found everywhere even in the remotest places where other plants and animals cannot live=== disease organisms of crops

They are found in soils, dirty water, air. plant and animal bodies, can only be seen under the microscope.

The attack of bacteria on plants is manifested as rottening of plant parts accompanied with very bad smell.

Though most bacteria are destructive as they cause disease*, some of them are useful man since they help in the following ways (a) Decomposition of compost (b) Production of drinks, e.g. beer (c) Making of butter and cheese (d) Fixation of atmospheric nitrogen in root nodules of leguminous plants, thereby helping in increasing soil fertility.

There are three types of bacteria based on their shapes of disease organisms of crops

They are (spherical), bacilli (rod shaped) and spirilla (Spiral shaped).

  1. Fungi
    These are plant species but they do not possess chlorophyll stems roots and leaves. The body is made up of myceliium which of tiny delicate branches called hyphae.

The fruiting bodies originate from some of these hyphae. The spores many tiny spores through which the fungus is dispersed-.

fungi have no chlorophyll,

they cannot manufacture their own so they depend on their host for their food, thereby causing uses the host plant.

  1. Viruses
    These are minute organisms that can only be seen under the tron microscope. Plants suffering from this disease show: (

a) Coloured spots and bands on the leaves (b) Twisted and wrinkled leaves (c) Stunted growth of the entire plant. Sucking insects e.g. mealy bugs, aphids and white flies can limit virus disease from one infected plant to a healthy one.

Almost all virus diseases have no effective cure.

It is advisable to prevent and control the disease as the case may be.

  1. Nematodes These are soil-borne organisms which usually attack the roots of plants. They are capable of causing considerable damage to the plant.

Affected plants show decline in productivity and the roots become distorted with enlarged growths. Soil sterilization may help to control the spread of nematodes.

Important diseases of major crops


1 Smut Fungus (Ustilago spp) Maize, Rice, grain sorghum and other cereals By wind Grains are covered with large mass of spores The whole cob may be covered with mass of black spores thereby rendering them valueless Use clean seed s to plant. Treat seeds with fungicide

2 Streak Virus Maize By piercing and sucking insects Discoloured leaf surface forming streaks Reduces crop yield Early planting spray with insecticide. Rogue the affected plants

3 Blast of Rice Fungus (Pyricularia oryzae) Rice By wind Dead areas sho on lead surface Crop yield is reduced Grow resistant varieties. Treat seeds with fungicides

4 Leaf Rust Fungus (Punccinia spp) All cereals Wind and water Rust-coloured pustules develop on the leaves of the plant The leaves die resulting in poor production Early planting. Dress seed with fungicides.

5 Bacterial Blight Bacteria Sorghum cowpea, etc Wind and water Yellow leaves followed by defoliation and death of stem Yield is affected Grow resistant varieties. Use clean seeds.

6 Cercorspor a leaf spot Fungus (cercospora spp) Legumes By wind Yellow leaves followed by defoliation Yield is reduced
1. Plant early 2. Uproot affected plants and burn.

7 Rosette of groundnut Virus Groundnut By aphids, piercing and sucking insects Leaves are closely packed, wrinkle and mottled The leaves curl, turn yellow and this reduces yield. 1. Plant early
2. Use resistant varieties 3. Rogue affected plants disease organisms of crops

8 Mosaic Virus Cassava Yam
By Bemisia fly Yellow pale areas on leaves Leaf distortion and stunting of plant leading to reduced yield 1. Use resistant varieties 2. Use insecticide to kill the insects
3. Destroy the affected plants as they are caused by disease organisms of crops

9 Tuber Rot Nematode, fungus Yam Through soil Yam tuber becomes brown in the tissues and later becomes soft. Renders the tuber valueless 1. Destroy affected tubers 2. Use Aldrin dust 3. Use Nematicide e.g. Mucar.

Root knot Nematode
(Root knot eel worm) Tomato and okro Through soil borne Nematode Knots in the roots of plants Wilting and retardation of growth and low yield. 1. Crop rotation 2. Use nematicides

11 Downy mildew Fungus Onion Airborne-disease fungus White coating on leaves Underdeveloped or small bulbs 1. Spray with sulphide of potassium

12 Sigatoka Fungus Banana and plantain Airborne fungus Chlorosis of leaves Small bunches and few fingers in bunches 1. Clean weeding and good spacing 2. Use appropriate fungicide

13 Damping off Fungus Okra Airborne and soil borne fungus Rotting of roots and wilting of leaves Yield is reduced 1. Treat soil with fungicides

14 Leaf blotch Fungus Pineapple By air-borne fungus Streak on the entire leaf surface Yields are reduced Use copper fungicide to spray affected plants

Black arm Bacteria (Xanthomonas malyaccarum rain fungus (Phytophthora palmivora) Cotton Through the stomata Angular sports on leaves Cotton bolls rot. Use resistant varieties

16 Black pod Cocoa Rain splash, insects and animals Brown spots on pod leading to rotten and black pods The pods become useless leading to reduction in yield 1. Regular weeding of farm 2. Remove and bury all affected pods 3. Spray with fungicide like perenox and Bordeaux mixture

17 Swollen shoot Virus Cocoa By Nymphs or meaty bug and white flies Swelling on the growing parts of the stem and roots Distorted leaves and reduced growth in plants. There is underdevelopment of pods and low yield

  1. Destroy all affected plants 2. Use resistant varieties

18 Freckle Fungus Oil palm By wind insects Brown spots on leaves which later dry and die Growth of palm is affected and maturity is delayed 1. Spray with fungicides such as captan, or diethane M45

19 Collar Rot Fungus (Ganoderma Lucidum) Oil palm Soil and wind borne fungus Rotting of old palms near the base of truck The disease can kill the palm tree
1. Disinfect with fungicides 2. Burn all infected trees

20 Antrannose Fungus Oil palm By wind and insects Leaves show clear spots. Spots turn dark-brown or black later and with yellow halo.
Growth is affected in the nursery 1. Space germinating seedlings properly
2. Cut off affected parts of leaved and burn
3. Spray with fungicides, Cumin, Dithane M45, cAptan or Ziran in pre-nursery

21 White root Fungus (Fumes lignosus) Rubber Through the soil Leaf defoliation Taproots are attacked by fungi leading to the breakdown of their tissues. Plants die eventually

  1. Burning farmland especially old trunks after clearing before planting of seedlings.

22 Mould Fungus Grains and seed By wind Formation of whitish spores on stored produce Grains or seeds are rendered useless

  1. Proper drying of produce 2. Avoid dampy stores 3. Protect store from rain or flood.

23 Gummosis Fungus (Phytophhthrora spp) Citrus Through the soil and aided by moist condition

The cortex of the citrus plant is killed Gum pockets occur in the cambium. There is exudation of gum from the stem The stem tissue collapse and plant may fall and die

  1. Plant resistant varieties e.g. stocks or four orange and rough lemon
  2. Bud citrus plants 45cm from the ground and allow no branch below 1m 3. Proper farm drainage and sanitation reduce incidence

24 Tristeza Virus Citrus Through aphid (Toxoptera citricida) Yellowing of terminal shoots in plants; there is stem pitting Growth and yield are affected.

The disease can kill the citrus plants. Control by budding plants on Cleopatra tangerine, lake and Sampson. Tangelo stocks and rough lemon

Nature and Effect of Disease Damage to Crop

The nature of damage caused by a disease on crops depends on the type of disease causing organism. It also depends on the crops being affected. Generally, there is a reduction in, the yield of crops affected by the disease.

The market value of the product is also reduced. It is estimated that crop diseases account for about 20% loss of yield in crops all over the world. Disease attack also leads to i of planting materials.

Stages of Diseases Development and Transmission,

There are four major stages of disease development and transmission in crops. These are:

  1. Invasion: This is the stage where ‘the disease causing agent comes in contact with the host plant.
  2. Establishment At this stage the disease causing agent enters and establishes it in the crop tissues.
  3. Injury This is the stage where the disease pathogen multiplies itself leading to the actual attack of the host.
  4. Mission/Spread This is the stage where the disease organism spreads from one plant to a healthy one. The process is aided by wind, water, insects contact and others.

7.5 Prevention and Control of Crop Diseases It is better and more economical to prevent a disease from, crops than to control it when it has occurred. The sure way cutting diseases, in farms is to prevent the disease causing organism from reaching the crops.

This is possible through regular removal of rotten materials from the farm, routine spraying of crops planting resistant varieties and so on. However, crop diseases can be controlled in the following ways: 1. Cultural Control The techniques adopted in this control measure include:

(i) Proper care for plants such as regular weeding. This practice reduces the amount of disease attack on the crops. (ii) Bunting, of farmland: This involves setting farmland on fire before cultivation. The fire helps to kill disease causing agent such as bacteria, fungi, virus and others in the soil.

(iii) Karly Planting: Early planting enables the crops to escape the outbreak of some crop diseases.
(iv) Practicing Crop Rotation: This practice, ensures that crop which suffer from similar diseases do not follow each other in the same rotation.

This is to make sure that the cycle of the disease development is disturbed.
(v) Removal of dead plants and other disease harboring agents from the farm. The source of the disease is removed when this is done.

  1. Biological Control

This involves the use of good resistant crops varieties to plant. It is a very successful method of controlling most serious^ plant diseases.

Over the years, scientists have developed crop types which have great resistance to some deadly crop diseases. The use of which helps to control most diseases of crops.

  1. Chemical Control This involves the use of chemicals to kill the disease organism affecting the crops. For example: (i) Fungicides such as perenox. Bordeaux mixture, and copper sulphate are used in the control of fungus diseases.

(ii) Virus diseases can be controlled by using appropriate chemicals such as vetox 85 to kill the vectors e.g. aphids. mealy-bugs white flies, etc. that transmit the virus diseases.

(iii) Most bacterial diseases can now be treated using appropriatle chemicals (bacteriacides) such as Agrosan 5W mercurial duss and acid.
(iv) Nematodes can be controlled by using nematicides such a nemagoiu and DDT.

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