In this article we will discuss about:- 1. Aspects of Plant Diseases 2. Dissemination 3. Epidemiology.
Aspects of Plant Diseases:
The Development of a Plant Disease is dependent upon three aspects:
(i) A susceptible host,
(ii) A virulent pathogen, and
(iii) A favourable environment of sufficient duration to allow infection by pathogen and subsequent development of the disease.
As such, the study of plant diseases can be divided into four interrelated aspects:
(i) The causes of disease- etiology,
(ii) The interaction of host and pathogen (including pathogenesis, the development of disease in the plant);
(iii) The interaction of populations of plants, pathogens and environment- epidemiology; and
(iv) the prevention and control of plant diseases.
The practical grower is especially interested in the prevention and control of plant diseases only. But it is almost impossible to treat a plant successfully for a disease without a thorough knowledge of the cause. However, in some instances, methods have been devised for keeping diseases under control without a complete knowledge of the organism causing them.
A disease may be serious on one species or variety of plant, and of little importance or not present at all on one closely related; or it may be serious under certain climatic conditions, and harmless under other conditions. For these reasons, a disease- producing organism, when introduced from one part of the world to another, may become more or less virulent than in its original home.
It may find new host plants which may be more or less susceptible than its previous hosts, or it may find the climatic conditions more or less favourable to its growth. It is also true that when such an organism is introduced into another part of the world, it is liable to meet enemies which will endanger its existence.
If the disease is due to a specific organism, the organism may bring about the unhealthy condition in its host:
(a) By feeding upon its tissues or sucking its juices or otherwise for food;
(b) By covering the surface of the leaves or stem and thus interfering with the action of the light and proper transpiration;
(c) By clogging up the passage of the fibro-vascular bundles and interfering with the circulation of water and other fluids;
(d) By attacking the growing regions, especially the root tips, and interfering with or preventing their development;
(e) By attacking the cambium;
(f) By living inside the host and feeding upon it and secreting toxic substances;
(g) By attacking the fruits but doing little or no damage to the plant itself;
(h) By injuries to the leaves, interfering with their activity;
(i) By malformations of various parts of the host.
Although the symptoms and effects of certain diseases caused by agencies other than specific organisms are well-known, the factor or combination of factors producing them is still questionable. Details of etiology, interaction of host and pathogen, epidemiology, and prevention and control of plant diseases are incorporated in pages to follow.
Dissemination of Plant Diseases:
Dissemination of plant diseases is basically dependent upon the dissemination of inoculum whose origin and behaviour are again controlled by various factors.
To understand this aspect one has to consider the various aspects of inoculum:
(i) Source
(ii) Development
(iii) Dissemination
(iv) Behaviour.
It has already been indicated that these aspects follow in regular sequence in a cyclic order and are interdependent for disease dissemination followed by disease development. Here only mechanism of dissemination of plant diseases with special reference to dissemination of inoculum is discussed in detail.
The spread of parasitic diseases from host to host and from locality to locality is primarily dependent upon the dissemination of pathogens (inocula).
The dissemination may be:
(i) Direct when it takes place without the help of any external agency, e.g., explosive spores of Taphrina deformans that cause peach leaf curl;
(ii) Indirect when dissemination of pathogens (inocula) takes place through agencies or media like, seeds, propagating stocks, plant debris, manure, soil, field operations and implements, insects, air currents, water, etc.
For Effective Dissemination the inoculum should be:
(i) Sufficiency productive
(ii) Suitable for liberation
(iii) Suitable for dissemination
(iv) Able to stand influence of the agency
(v) Virulent.
Epidemiology of Plant Diseases:
Epidemiology is the study of interactions of populations of host plants, pathogens and environment. It is very much concerned with epiphytotics. Whereas pathogenesis deals with sickness of an individual host plant. Though epidemiology of plant diseases comparatively receives less attention than pathogenesis, but a planning of plant disease control needs a proper understanding of epidemiology.
Since epidemiology includes seasonal persistence and spread of pathogens, and the effects of environmental conditions on their incidence, hence it is also of great importance in forecasting of plant diseases.
Pathogens may be permanently present throughout the year in and around a population of plants which are perennial, although not necessarily active all the time. But so far as the population of annual plants is concerned, some means of perennation are necessary for the pathogens to carry over intercrop period when the crop is absent.
This is very essential for the continued survival of pathogens serving as potential source of inoculum of the cropping season.
i. Organs of Perennation of Pathogens:
Principal organs of perennation of the pathogens are:
(i) For fungi—mycelium, sclerotia, spores of different kinds with or without being associated with fruiting structures;
(ii) For bacteria—vegetative cells with or without remaining along with host plant parts, and
(iii) For viruses —symptomless carriers or tolerant plants, different infected host plant parts, insect vectors, eggs of viruliferous leaf hoppers.
ii. Media of Pathogen Perennation:
Media in which organs of perennation of pathogens survive during intercrop period vary on the nature of pathogen and climatic conditions.
But in general, they may be:
Soil, organic manure, infected crop residues of harvest, seeds and other propagating stocks, collateral hosts, subsidiary hosts, plants which are missed during harvesting–commonly known as ground-keepers, mummified fruits, etc.
iii. Perennating Organs and Climatic Conditions:
Climatic conditions can directly affect the longivity of perennating organs of the pathogens which again depend on the nature of perennating organs and the media in which they perennate. For soil borne pathogens, water-holding capacity and pH of soil are very important for the survival of pathogen and development of inoculum. Similar is the situation with the organic manure.
As to the crop residues of harvest, pathogens usually persist longer in woody residues than in leaves which disintegrate much more rapidly. The pathogens which perennate in infected crop residues are usually relatively unspecialized parasites that are able to survive in dead tissue, but obligate parasites may also survive in the form of thick-walled resting structures.
In the seeds, propagating stocks and various plant parts the pathogens remain in a dormant condition during the intercrop period and become active with the return of suitable climatic conditions causing infection to crop plants during cropping season by producing inoculum.
Subsidiary, collateral and similar other hosts in which pathogens survive during intercrop period normally do not cause any economic loss but on the contrary produce inoculum during cropping season to induce diseases.
iv. Crop Infection by Pathogens:
With the return of cropping season and favourable -climatic conditions—both macro- and microclimatic, inoculum development, spread and host infection start simultaneously. The virulence of disease development depends on the virulence of pathogens, favourable medium of inoculum dissemination, susceptibility of host plants and the surrounding climatic conditions (temperature and moisture).
When the incidence of disease is quick and widespread it will lead to epiphytotic condition.
As such, the development of an epiphytotic condition is a chain reaction of survival of pathogens during intercrop period —development of inoculum under suitable climatic conditions during cropping season—dissemination of inoculum through suitable agency—inoculum arrival on host surface—host infection leading to disease incidence and further development of inoculum which in turn will be disseminated resulting quick spread of disease and destruction of the entire crop plant population.