The below mentioned article provides a study note on inoculum.
During unfavourable conditions like sudden change in temperature (high or low), effect of poisonous gas, disturbance of soil moisture relation etc., pathogen may occur inside the perennating organ or organs. With the onset of favourable condition, it becomes active and causes disease. The portion of the pathogen responsible for infection is known as inoculum (pl. inocula).
In presence of susceptible host, virulent pathogen and favourable environmental condition, a disease may not develop due to variation in the amount of inoculum. If the amount of inoculum is less in the diffusate, the pathogen will get more food and will germinate properly.
But the amount of enzymes and toxins secreted by them become diluted in the comparatively bigger water or dew-drop and are not able to damage the host cell wall, so there will be no disease. On the other hand, if the amount of inoculum is more in the diffusate, the pathogen will get comparatively less amount of food in their share.
Now they will fight for the food, by secreting more enzymes and toxins. For the above facts, the concentration of the enzymes and toxins will be more in the available water or dew-drop, thereby the germination and/or growth of the pathogen will be hampered and there will be no disease.
So optimum concentration of inoculum is essential to develop a disease. The amount of inoculum which is responsible for successful infection is known as inoculum potential.
The above situation may also be noticed in your class room. Consider that all the conditions in a class room are normal, except the number of students. If the number of students present in the class is very low, then the teacher will think that he has to repeat the syllabus again and the output of the class will be less satisfactory.
On the other hand, if the students present in the class is more than normal, then they are not able to sit properly and will create disturbance in the class, thereby the output of the class will be less satisfactory. So the optimum number of students in a class is essential in addition to other conditions, for the normal output of a class.
The inoculum which develops from the perenating organ or comes from the neighbouring field is the primary inoculum. The primary inoculum then disseminates by wind, rain, insects, or by other agencies or directly comes in contact with the host tissue and establishes infection is the primary infection.
The primary infection that produces disease symptom is the primary symptom, that which produces next crop of inoculum is known as secondary inoculum. So the secondary inoculum may be defined as the inoculum which develops from the primary symptom.
Like primary inoculum, the secondary inoculum causes secondary infection and secondary symptom. The second phase i.e., the production of secondary inoculum and development of secondary symptom may repeat several times in a growing season.
The chain of events which takes place in cyclic order to complete the life process is known as life cycle; the disease cycle is the life cycle of a pathogen including the series of events starting from the time of infection to disease expression on a host.
In case of obligate parasite, disease cycle and life cycle are interdependent, but in other cases, like facultative parasite, facultative saprophytes etc., the disease cycle is dependent primarily on the parasitic part of the life cycle.
Generally the disease cycle consists of two cycles, one produced by the primary infection is the primary disease cycle and the other one which is produced by the secondary infection is the secondary disease cycle. In some case, the secondary disease cycle may be absent and the pathogen produces only one cycle in its life process (wilt disease etc.).
Diseases Cycle of Inoculum:
1. Monocyclic Plant Disease:
When pathogens are able to complete only one or part of disease cycle in one year, they are called monocyclic pathogens and the disease as monocyclic disease. In monocyclic disease, pathogen develops primary inoculum which is the only inoculum available for the entire season.
The secondary inoculum and secondary infection are totally absent in such diseases, e.g., smuts (loose smut of wheat, c.o. Ustilago segetum), root rot (root rot of turmeric, c.o. Pythium aphanidermatum) and vascular wilt (wilt of pigeon pea, c.o. Fusarium udum), etc.
2. Polycyclic Plant Disease:
When pathogens are able to complete more than one generation (2-30) in a growing season, they are called polycyclic pathogens and the disease as polycyclic disease. The amount of inoculum in each cycle is multiplied manyfold, e.g., leaf spot (brown spot disease of rice, c.o. Helminthosporium oryzae), blight (late blight of potato, c.o. Phytophthora infestans), powdery mildews (powdery mildew disease of cucurbits, c.o. Erysiphe cichoracearum), etc.
3. Polyetic Plant Disease:
When pathogens are not able to complete a disease cycle within one year and take several years before the inoculum is developed on plant and can be disseminated and may initiate new infection, they are called polyetic (multiyear) pathogen and the disease as polyetic disease.
This is common in diseases of trees. After the development of inoculum they cause further infection. The amount of inoculum per plant per year remains almost constant, but with the increase in the number of infected plants the disease may cause severe epidemic, e.g., Dutch elm disease, peach decline etc.
In brief, the terms and definitions can be described as:
1. Host:
An organism on which disease takes place is the host:
(a) Resistant host:
The host which possesses the qualities that hinder the development of a given pathogen.
(b) Susceptible host or Suscept:
The host that can be attacked severely by a given pathogen is called susceptible host.
(c) Immune host:
The host which cannot be infected by a given pathogen, even under the most optimum condition.
(d) Resistance:
The ability or efficiency of an organism to overcome completely or in some degree the effect of a pathogen is referred to as resistance.
(e) Susceptibility:
It is the condition of a plant that makes it suitable for attack by a given pathogen or other harmful factors.
(f) Immunity:
It is the ability of a host to resist against the infection of a given pathogen, even under the most optimum condition.
2. Pathogen:
An organism which generates a disease is called pathogen. The pathogen is also known as causal organism. In brief it is written as c.o.
The efficiency of a pathogen to cause disease is the pathogenecity, and pathogenesis is the process or chain of events of disease development.
3. Hypersensitivity:
It is the excessive sensitivity of plant tissue to some pathogens; thereby the affected cells are killed quickly and block the progress of the pathogen.
4. Casual complex:
In addition to pathogen, other environmental factors, such as atmospheric humidity, temperature, wind speed, light etc., are essential for the development of disease on a host. Pathogen along with other environmental factors are collectively called causal complex. Causal complex = causal organism + other environmental factors.
5. Penetration:
The initial invasion of the pathogen inside the host tissue is called penetration.
6. Infection:
The establishment of the pathogen inside the host tissue is called infection.
(a) Visible infection:
The infection which is visible externally on the host surface is called visible infection.
(b) Invisible infection:
The infection which is not visible externally on the host surface is called invisible infection.
7. Incubation period:
The interval of time between infection and symptom expression is called incubation period.
8. Symptom:
The manifestation of the disease is called symptom.
External symptom:
When the symptom becomes visible externally, it is known as external symptom, e.g., brown coloured spots on leaf and on other parts of rice plant indicate that it is brown spot of rice.
Internal symptom:
When the symptom becomes internally invisible, it is called internal symptom, e.g., browning of woody tissue in wilt of pigeon pea.
9. Sign:
When the disease is identified by seeing (with naked eye) the pathogen externally on the host surface, it is called sign, e.g., powdery mildew of cucurbits.
10. Syndrome:
When a number of symptoms are available for a particular disease, they are collectively called syndrome, e.g., wilt of pigeon pea, where symptoms like; i. lodging of the leaves (external), ii. browning of the woody tissue, and iii. presence of fungal mycelium inside the host tissue (internal) are available. These three symptoms are collectively called syndrome.
11. Inoculum:
The portion of the pathogen responsible for infection is called inoculum. The inoculum may be spore, mycelium or any other part of the fungus, but in bacteria and virus the entire body behaves as inoculum.
12. Inoculum potential:
The amount of inoculum responsible for successful infection is called inoculum potential.
13. Primary inoculum:
The inoculum which develops from the perennating organ or comes from the neighbouring field is the primary inoculum.
14. Primary infection:
The establishment of infection caused by primary inoculum is called primary infection. The primary infection leads to the development of primary symptom.
15. Secondary inoculum:
The inoculum which develops from the primary symptom is called secondary inoculum.
16. Secondary infection:
The establishment of infection caused by secondary inoculum is called secondary infection. The secondary infection leads to the development of secondary symptom.
17. Disease cycle:
It is the life cycle of a pathogen including series of events, starting from the time of infection to disease expression on a particular host.
18. Parasites:
The organisms which grow on living tissue are called parasites.
(a) Obligate parasite:
The organism which grows only on living tissue and not in axenic culture (culture free from any other organism), is called obligate parasite, e.g., Plasmopara viticola (c.o. of downy mildew of grapes).
(b) Facultative parasite:
The organism which can live as saprophytes for long period in pure culture or otherwise under certain condition it becomes pathogenic is known as facultative parasite, e.g., Phythium debaryanum (c.o. damping off of tobacco).
19. Saprophytes:
The organisms grow on dead organic tissue or on inorganic material are called saprophytes.
(a) Obligate saprophyte:
The organisms which have no relation with living host and collect their nutrition from dead organic tissue or from inorganic materials are called obligate saprophyte, e.g., Rhizopus sexualis.
(b) Facultative parasite:
The organism which is actually a parasite, in certain circumstances behaves as saprophyte, and is called facultative parasite, e.g., Ustilago (smut fungi).
20. Infectious disease:
It is a type of disease in which a transmissible causal organism or particle like virus is a part of the causal complex.
21. Symbiosis:
When two organisms live in close association with one another non- pathogenically and are essential and/or beneficial to each other’s development, this phenomenon is called symbiosis, e.g., lichens — formed by the association of algae and fungi.
22. Symbionts:
The organisms, taking part in symbiosis are known as symbionts, e.g., lichen — both algal and fungal partners are the symbionts.
23. Epiphytes:
When one plant grows on other plant without making any parasitic or symbiotic relation, the former is known as epiphyte.
24. Endophytes:
The organisms living entirely within the host plant are called endophytes. Endophytes may be parasitic or symbiotic.
25. Predisposition:
It is the effect of one or more meteorological factors which makes a plant vulnerable to attack by a pathogen.
26. Etiology:
The determination vis-a-vis study of the cause of a disease is called etiology.
27. Necrotroph:
The microorganisms which feed only on dead organic tissues, e.g., Mucor, Rhizopus etc.
28. Biotroph:
The organism that can live and multiply only on another living organism, e.g., Vasicular-arbuscular mycorrhizae (VAM), powdery (Erysiphe) and downy (Peronospora) mildews.
29. Disease triangle:
The interactions of the three components of a disease (host, pathogen and environment) have often been visualised as a triangle, commonly referred to as disease triangle.
The disease triangle can be discussed now:
The disease of plants is caused either by biotic or, less commonly, by abiotic means. In the first case, both host and pathogen must interact with each other and the result of their interaction will depend on the different environmental conditions like temperature, humidity etc. (Fig. 5.3).
Without the requisite environmental factors pathogen will not be able to develop disease even on a susceptible host. All the three components (host,- pathogen and environment) show considerable variation in nature.
The intensity of the disease in individual plant or in plant population varies with the variation of any one component:
A. Variation in respect of host:
1. Degree of resistance of the host.
2. Age of the plant.
3. Size of the plant, etc.
B. Variation in respect of pathogen:
1. Degree of virulence.
2. Amount of inoculum, etc.
C. Variation in respect of environment:
1. Range of temperature,
2. Range of humidity, etc.
The disease triangle consists of three components — host, pathogen and environment, placed at the three sides of the triangle. The length of each side is proportional to the sum total of the characteristics of each component that favours disease. The shape of the triangle varies with the variation of the factors.
Such as:
1. If the plants are resistant and/or at wrong age, the amount of disease will be very less or zero. So in this condition, the host arm will be very small and thus the central area of the triangle will be less so that the prevalence of disease will be very little or nil.
2. On the other hand, if the plants are susceptible and at proper age of development of disease, then the disease will be to a greater extent. So, in this condition the host arm will be comparatively larger and thus the central area of the triangle will be more, so that the prevalence of disease will be much more.
3. Similarly, if the pathogen is more virulent, in more amounts and active then the pathogen arm will be larger and there will be larger central area i.e., the extent of disease will be much more prevalent. The result will be reversed with less virulent pathogen if present in fewer amounts.
4. If the environmental conditions like temperature, moisture etc., become favourable, then larger will be the environment arm and simultaneously larger will be the central area i.e., the amount of disease will be much more. The result will be reversed with less favourable environmental condition.
5. Lastly, it can be concluded that if the extent of any one of the three components is zero, then the amount of disease will be zero i.e., no disease will develop.