According to Encyclopedia Britannica-
Plant disease is an impairment of the normal state of a plant that interrupts or modifies its vital functions.
The effects of a disease that can be observed is called the symptoms. A disease can be identified depending on the symptoms as different diseases show different symptoms.
Plant Disease Concepts
Ancient people considered plant disease as a punishment to people by GOD because of their sins. The Greek philosopher Theophrastus (300 B.C) was the first to study plant disease. In his book “Enquiry Into Plants” he discussed about plant disease. But he could not find out the actual reason behind plant disease. He considered it as a curse.
After inventing Compound Microscope scientists found out that many microorganisms are responsible for diseases of plant.
Based on the research on plant diseases, two basic concepts have been evetually developed. One is The Disease Triangle and another is The Disease Cycle.
Both of the concepts are described below briefly:
The Disease Triangle
Over the years, pathologists have come to learn that disease development in a plant population is determined primarily by the interactions among three major factors. These factors are:
- The Susceptible Host
- A Virulent Pathogen
- A Favourable Condition
The interactions among these three factors have been evetually conceptualized into the Disease Triangle.
When these three components are present at the same time, the pathogen will be able to cause disease.
As shown in the above diagram, the pathogen will not be able to affect a plant under unfavorable condition even if the host is susceptible. Likewise, a pathogen can not affect a plant under favorable condition if the host is not susceptible.
So conceptually, this triangle dictates that if either the host is less susceptible, the pathogen is less virulent or the environment is less favorable, the disease will either occur at a reduced level or it will not occur at all.
This should be kept in mind that there are numerous variables within each of the three factors which may affect the incident and severity of a disease. These variables include environmental condition, genetic diversity, biology and life cycle of the host plant and pathogen etc. These variables are described below in brief:
- Environmental Condition
There are many variables in the environment that affect the incident and severity of disease such as temperature, moisture, rainfall, sunlight, seasons etc. Generally a pathogen remains restricted in a certain area based on the micro-climate. The Micro-climate can be defined as “The prevailing climatic conditions in a certain geographical area”. Each geographical region can have several micro-climates depending on wind, amount of sunlight, moisture etc.
- Genetic Diversity
Genetic Diversity is defined as The number of different alleles and their frequency in a population.
Genetic diversity within host plants can greatly affect the susceptibility of hosts. If the host plant is genetically resistant to any particular disease, the pathogen will not be able to affect the plant body even under favorable condition. The severity of a disease get also influenced by genetic diversity.
- Biology And Life Cycle Of The Host Plant & Pathogen
The life cycle of both pathogen and host can influence the occurrence of a disease. A pathogen may have to be in a critical stage of it’s life cycle to cause the disease. Likewise, a host may only be affected by a disease in a particular stage of life cycle.
The Disease Cycle
Another important concept related to plant disease is Disease Cycle. A generalized figure of disease cycle is given below:
For a disease to be developed the pathogen has to go through a disease cycle to successfully invade the host plant. The Disease Cycle is the chain of events that occur during the development of a disease.
There are several stages of a disease cycle through which a disease eventually develops in host plant. These stages are:
As shown in the figure above, the first stage of the disease cycle is inoculation when a pathogen is introduced to a susceptible host plant. Different pathogens employ different inoculation methods. Generally pathogens are equipped with various mechanisms that help in inoculation. For example, some pathogens spread by air current while some other spread by vectors (insects, birds etc).
After inoculation comes the penetration stage where the pathogen enters the host tissue. Some pathogens enters through wounds, stomata etc and some have special mechanisms that can directly penetrate the host tissue. Fungi and nematodes are able to actively penetrate the host tissue under favorable condition.
After invading the host plant a pathogen may undergo an incubation period. During incubation the pathogen do not affect the host tissue and remains latent for a specific period of time.
After invading the host tissue the pathogens form parasitic relationship between the host plant and pathogen by absorbing nutrients from the host tissue and cause infection in the affected area of the plant body. This is the stage where the symptoms of a disease are visible. Depending on the pathogen the infections can be of various types based on which a disease can be identified
A pathogen can reproduce both sexually and asexually. Reproduction helps the pathogen to spread the disease to other susceptible hosts. For example fungi produce spores through asexual reproduction and spread it through the air or water current which germinate on another susceptible host and invade it.
Pathogens have developed different mechanisms to survive in host body under unfavorable condition. These mechanisms help them to survive for prolonged period of time. For example, soyabean cyst nematodes lay their eggs within a cuticle casing. The casing is very hard which prevents other microbes and chemicals to penetrate killing the eggs prior to hatching.
Any disturbance in any stage of the disease cycle influences the severity of the disease.
The knowledge of the ‘Basic Concepts Of Plant Disease’ is very important as it helps in disease management.
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