FUNDAMENTALS OF CROP DISEASE MANAGEMENT.

What is a crop disease?

A crop disease is a condition when one/ more physiological functions of the plant are affected to the extent that it affects plant metabolism.

Agents of crop diseases are categorized into two Biotic agents and Abiotic agents.

Biotic Agents.

These include; fungi, bacteria, viruses, phytoplasmas, nematodes and parasitic plants. Biotic pathogens are infectious, meaning that they can spread from one plant to another.

Pathogenic micro-organisms usually cause diseases in crops by disturbing the functions of plant cells, interfering with the release of plant growth regulators, food and water absorption. Some cause multiple growth of plant vessels that transport water and food hence cause blockage of xylem and Phloem.

Abiotic agents.

Are the stresses imposed on the plant that eventually results into a disease. The causes are not living and infectious, these are normally environmental factors leading to environmental, physiological / non-biotic diseases.

These stresses include extreme soil nutrients, extreme moisture, extreme temperature, soil pollution, air pollution, soil compaction, mechanical damage, effects of toxic chemicals.

What is a disease symptom?

A disease symptom refers to observable crop structure changes, different from the normal. Categorized into localized symptoms and systemic symptoms.

Localized symptoms are confined to a specific area of the plant while systemic symptoms spread through out the plant.

Common crop disease symptoms.

Lesions; localized area of discolored/ malformed tissue.

Necrotic; dead crop tissue sometimes described as blight or spot.

Change of colour; an abnormal change of crop tissue colour mainly from green to different colour.

Stripes; these are tinny, long lesions commonly observed with crops with a parallel leaf venation.

Abnormal tissue growth; formation of galls/tumors a condition called "hypertrophy".

Abnormal decrease in size; a condition referred to as "hypoplasia", when a crop fails to grow and attain a normal size.

Gummosis; excretion of excessive sticky liquids from infected plant tissues commonly in tree crops.

Cankers; Are sucken, dead areas on the surface of stems. They are often rough.

Leaf curling; margins of infected crops bend upwards/ downwards, commonly caused by viruses.

Wilting; a condition where all/ parts of crop tissues loose turgidity and collapse.

Rots; a condition where plant tissues are disintegrated. In most cases rots turn into brown/black tissues. Accompanied with the flow of liquids from tissues in this cause, this known as "wet rot".

Rusts; these are powdery sporing pustules on leaves/ stems. Usually in yellow, orange or brown colour.

Mosaic; patterns of light and dark green especially on leaves. The light green patches cannot photosynthesize effectively hence lowering crop yields.

Mottling; irregular pattern of indistinct light and dark areas, especially on leaves. It is mostly a characteristic of viral infection.

Replacement of usual organs into other crop structures; infected organs/ crop parts change into other structures that are not useful like smuts in cereals.

What is a crop disease sign?

Refers to visible pathogens, pathogen parts, pathogen reproductive structures or pathogen products on an infected plant tissues. These include ooze, mycelium and reproductive structures of pathogens.

Crop disease triangle concept.

This concept explains the occurrence of a disease on the crop and how much damage results from the disease. 

This concept allows farmers to select possible disease management measures. It allows farmers to use the most effective disease management approaches which are time and cost effective.

The crop disease triangle explains that plant diseases are as a result of an interaction of three components.

1. Pathogen; refers to a disease causal agent/ organism usually biotic.

2. Host; refers to a crop on which a disease develops.

3. Environment; refers to the surrounding of the host and pathogen.

In the crop disease triangle concept for a disease to develop on a crop and become severe, the three components must be present and interact favourably.

The host crop must be susceptible meaning that the crop variety lacks the ability to resist attack by a given pathogen. A resistant variety implies that they cannot be attacked by a given pathogen even when it is present and environmental conditions are favourable.

A suitable pathogen has the ability to infect a given variety of a host crop

A suitable environment is a collection of conditions surrounding the crop and pathogen. These include air, soil temperature, relative humidity, soil type, soil pH, soil moisture and soil fertility. These conditions may promote disease development/ may prevent it under unfavorable conditions.

Exploiting the concept crop disease triangle to manage a crop diseases.

"A recap" for the development of a diseases on a crop depends on the susceptibility of the crop variety, presence of a suitable disease-causing agent and favourable environment, if any of the three components is missing the disease fails to develop.

So to manage / control a crop disease, we try as much as possible to make one of the three components unfavorable this is equivalent to breaking one side of the triangle. For example " planting a crop variety that is resistant to a given disease prevents a disease from occuring". Here the host crop breaks the triangle hence the disease disease cannot occur. In most cases it is difficult for the pathogen strain to be easily changed, the environment can easily be change to manage the disease.

The disease cycle.

The disease cycle describes how a pathogen progresses through critical phases of it's life cycle and how the host reacts to the pathogen.

Knowledge of the disease cycle is very important for farmers to effectively and easily manage crop diseases. This knowledge aids to find the weakest component within a given host-environmen-pathogen- relationship where interventions would easily result in disease control. The disease cycle usually has five parts.

1. Inoculation and penetration 

2. Incubation and infection

3. Reproduction 

4. Dissemination 

5. Dormancy 

1. Inoculation and penetration; the host and parasite come together and the pathogen enters the host. The pathogen is spread by inoculum, which is any part of a parasite that cause the infection.

2. Incubation and infection; when pathogen gets into a plant, they do not immediately begin to grow. They spend some time getting used to the environment within the crop and physiologically ready, they begin to multiply and invade new cells and disease symptoms are now visible.

3. Reproduction; meanwhile the pathogen continues to reproduce and multiply hence more inoculum is produced.

4. Dissemination; this is the spreading of the inoculum/ pathogen to new hosts/areas. The pathogens depends on environmental forces/ other organisms for transport to new host. These include wind, water, insect vectors, humans.

5. Dormancy; after producing inoculum, it is spread to new host. If conditions for new infections are not good, then pathogens may produce "survival structures". The survival structures may serve as inoculum when conditions for infections are good. They allow pathogens to last through unfavorable environmental conditions.

PRINCIPLES OF CROP DISEASE MANAGEMENT.

Crop disease control refers to the prevention of disease / reducing the increase in the incidence and severity of disease. It is usually directed at crop populations rather than at individual crop. Satisfactory control of most diseases requires the adoption of multiple control measures.

A well planned control programme should be based on knowledge of the characteristics of the pathogen and the host plant, the cultural and climatic conditions under which the crop is grown and knowledge of available disease control procedures including cultural, genetic, chemical and biological approaches.

Note

Plant diseases management is a costly exercise. It involves labour, purchasing some inputs and time. If cost is attached to all of these, you may realize that disease management is expensive.

A crop grower may end spend more resources than the value of the harvest hence the crop disease management approaches chosen must be cheap as much as possible.

In crop disease management choices of management are guided by the concept of "spheres of disease management".

The spheres of crop disease management.

The concept of spheres of crop disease management helps crop growers to make wise choices when deciding on how to manage crop diseases. Disease management is costly and may result into a farmer spending more than the produce eventually harvested from a crop

Disease management approaches/ choices are arranged in concentric spheres with the most desirable sphere occupying the outermost circle as described below.

1. Avoidance sphere 

2. Exclusion sphere 

3. Inoculum reduction sphere 

4. Altering effectiveness of inoculum sphere.

5. Eradication sphere.

1. Avoidance sphere.

Refers to the avoidance of the disease inoculum. There can be a disease bif the inoculum doesn't get in contact with the host crop. Therefore the cheapest means of managing crop diseases, these include 

    - Raising crops from disease free stocks/ disease free planting materials.

    - Planting at times of year when inoculum is absent.

    - Planting crops in geographical areas where disease inoculum is rare/ ineffective.

    - Planting crops in sites within local areas where disease does not occur.

2. Exclusion sphere.

Refers to exclusion of inoculum from the area where a crop is grown. Applicable in areas where a given disease is known to occur but farmer intends to grow a crop in a field where the disease is questioned to exist. Exclusion as a disease management approach involves.

  - Treating planting materials.

  - Using certified planting materials.

  - Observing restrictions imposed by quarantine.

  - Eliminating vectors.

3. Inoculum reduction sphere.

Diseases that affect most of our crops are unfortunately already established in our environments. Their inoculum is in the environment almost all the time thus means that our crops certainly get in touch with these pathogens and get diseased. Under this sphere aim is focused at reducing the inoculum in the fields so that the resultant disease does not cause much damage to the crops. Disease management methods that aim to reducing inoculum include;

    - Crop rotation with crops of different species and growth habits.

    - Fallowing to allow the pathogen inoculum reduce in a given time.

     - Host eradication by roguing of agricultural fields to reduce pathogen inoculum.

    - Field sanitation by burning / burying diseased plant residues to kill pathogen inoculum.

    - Destruction of volunteer plants often growing from seeds, vegetative materials that remain in gardens after harvest.

    - Creating conditions unfavorable for the pathogen/ vector of the pathogen for example "close spacing in ground nuts to manage ground nut rosette disease."

    - Intercropping with intercrops of different species and growth habits.

    - Application if green manure into the soil to stimulate the multiplication of non-pathogenic soil microbes.

    - Biological control to suppress disease causing organisms for example " application of fungus trichoderma spp to manage pythium root rot disease.

    - Hot water treatment of planting materials mainly used to kill bacteria in botanical planting materials.

4. Altering effectiveness of inoculum sphere.

Crop growers can manipulate the growing environment to reduce the amount of inoculum and therefore make inoculum ineffective and also grow crop varieties that make it difficult for the pathogen to grow and multiply easily. below are methods to reduce the effectiveness of pathogen inoculum.

     - Growing resistant crop varieties.

     - Modification of nutrition.

5. Eradication sphere.

This is the eradication of the pathogen from a location after it has already been introduced and established it's self on the crop. This  becomes important to suppress the growth and spread of the pathogen.

     - Complete eradication of pathogens is often difficult but well chosen, timed and applied chemicals can eradicate pathogens.