Terms and Concepts in Plant Pathology At the end of this lesson, you will be able to explain the different terms and concepts in plant pathology. You are entering a vegetable stall to pick some vegetables for your family. You can find fresh fruits and vegetables arranged all over the shop. Do you pick them randomly without examining?
Never! You closely examine every fruit and vegetable before purchasing. won't you? Among the displayed items, some fruits may be fresh, while some may have small spots, blisters, cracks, wrinkles, etc. You reject the damaged ones and pick only those which appear fresh, rigid and bright.
Many of us assume that the damage caused due to vegetables and fruits may be due to long storage or transportation. Of course, it may be one of the reasons, but The most important reason causing huge damage to fruits and vegetables is the diseases and disorders caused by biotic or abiotic agents. Such damaged foods must never be consumed.
In this lesson, we shall cover the terminologies in plant pathology that will help us to know about various diseases or disorders of plants. The word disease comprises two terms, namely dis and ease. meaning uneasiness.
Plant disease is an abnormality in the structure and function of a plant that results due to continuous irritation caused by a pathogenic agent or an environmental factor. Let us now discuss the terms related to plant diseases. The diseases of plants may either be infectious or non-infectious.
Infectious disease refers to the disease caused by a pathogenic organism such as a fungus, bacterium, mycoplasma, virus, viroid, nematode or phanerogamic plant, a parasite, it can spread from a diseased plant to a healthy plant. Non-infectious disease refers to the disease caused by an abiotic agent that is by an environmental factor but not by a pathogen. Disorder is an abnormality in the plant caused by abiotic factors such as nutrient deficiency, adverse climatic or soil conditions, etc.
A pathogen is an organism capable of causing disease in a particular host or range of hosts. A causal agent is a general term used to describe biotic or abiotic factor which incites and governs disease and injury. A causal organism is a pathogen of biotic origin. When a pathogenic agent is virulent, that is the capacity of the pathogen to cause a disease and if the agent is avirulent, it is a variant of a pathogen that does not cause a severe disease.
Pathogenicity is the capability of a pathogen to cause disease. Pathogenesis refers to the chain or series of events involved in the development of disease. in a host, ranging from initial contact of the pathogen to the host until the appearance of symptoms. A parasite is an organism living on or inside another living organism, that is, the host for obtaining its food from that organism.
To identify the causative agent of a particular disease, Robert Koch, a German physician and microbiologist, postulated four criteria which include the microorganism or other pathogen will be present in all the cases of a disease. The pathogen can be isolated from the diseased host and grown in pure culture. The pathogen from the pure culture should have the ability to cause the disease when inoculated into a healthy susceptible organism. The pathogen re-isolated from the new host should be the same as the originally inoculated pathogen. Now, Let us discuss host and its related concepts.
Host is a plant that is invaded by a parasite to obtain its nutrients. The host may be resistant or tolerant or susceptible. Resistance is the ability of an organism to overcome completely or to some degree the effect of a pathogen.
Tolerance is the ability of a plant to sustain the effects of a disease or suffering without causing serious injury or crop loss. Susceptibility is the inability of a plant To resist the attack of a pathogen, hypersensitivity is the excessive sensitivity of plant tissues to certain pathogens. On entry of the pathogen, the cells around the point of entry die, thereby preventing the spread of further infection.
On disease incidence, the plant's experience changes in their structure. The first indications that you notice in plants are referred to as signs and symptoms. Signs are the appearance and or physical evidence of the causal factor of the plant's abnormality.
It is also an evidence of physical damage caused by biotic or abiotic agents such as the pathogen, pests, spores, fruiting bodies, chemical residue, bacterial ooze and so forth. Symptoms are the visible response of a plant to biotic and or abiotic factors that result in a change or abnormality in the plant such as galls, chloroces, ring spots, wilt, rot and so on. A syndrome is the totality of the effects demonstrated in a host by one disease, whether simultaneously or successively, and whether visible to the unaided eye or not.
After disease incidence, the pathogen grows in the host. Let us proceed with the terms related to disease infection. Inoculum is the pathogen or its parts that can cause infection, wherein the portion of individual pathogens are brought into contact with the host. Propagules are any structures, fragments or parts of an organism that can propagate the organism. The propagules such as spores, sclerotia, etc. that survive and initiate an infection are called as primary inoculum.
Secondary inoculum is produced by infections that take place during the same growing season. Infection is the establishment of a parasite on or within a host cell or tissue. Primary infection is the first infection of a plant by the pathogen. Secondary infection is any infection caused by the inoculum produced as a result of a primary or a subsequent infection.
The life cycle of an infectious disease is the sequence of distinct events, such as sexual reproduction occurring between the appearance and reappearance of the causal organism. Disease cycle is the chain of events involved in disease development, including the stages of development of the pathogen and the effect of the disease on the host. Pathogens are transmitted by many factors which include biotic, abiotic and environmental factors. Vectors are active agents such as insects, mites, nematodes and other animals that carry and transmit a pathogen that is inoculum to a plant.
Now, let us discuss the conditions suitable for disease occurrence. Three conditions are necessary for the disease to appear. First, it is necessary to have a susceptible host plant.
The plant must be in a stage of development, susceptible to infection by the disease agent. The second requirement is the presence of an active pathogen. Also, the pathogen must be in a stage of development conducive to infect or affect the host plant. The third condition is an environment suitable for the pathogen to cause diseases in plants.
The interaction of host, Pathogen and environment can sometimes be represented by a triangle. The disease triangle cannot be constructed unless all three legs are present simultaneously. Breakage in any one of the legs of the triangle will hinder the occurrence of diseases. Disease control strategies can focus on breaking the leg of the triangle. Success in controlling a plant disease can be achieved by combining the following principles.
Avoidance is the first principle of disease control in which plants are grown at times when the pathogen is inactive or not present. Exclusion is a method of disease prevention in which the pathogen or infected plant material is excluded from the crop production areas. Eradication is the control of plant disease by eliminating the pathogen after it is established or by eliminating the plants.
that carry the pathogen. Protection involves the modification of the environment or cultural practices and use of chemicals to protect the plants from diseases. or control the vectors that cause the diseases. Thus, a basic knowledge of terms and definitions related to plant pathology can help us in enhancing the research process for effective control of diseases.
Summary Let us now do a recap of what we have learned so far. Plant disease is an abnormality in the structure and function of plants caused by a pathogenic agent or an environmental factor. Plant pathogen refers to the organism that is capable of causing a disease in plants or host. The study of terminologies is important to get a clear idea on the subject and design the research activities.