In an infectious disease there is a series of more or less district events which occur in sequence and lead to development and perpetuation of the disease and the pathogen. This chain of events is called a disease cycle. The disease cycle involve the changes in the plant and the plant symptoms as well as those in the pathogens, and spans period within a growing season and from one growing season to the next. The main events of a disease cycle include:
(i) Inoculation, (ii) pre-penetration, (iii) penetration, (iv) infection (also includes invasion), (v) growth and reproduction of the pathogen,(iv) dissemination of the pathogen, and (vii) seasonal carry-over of the pathogen.
1. Inoculation: Inoculation is the contact of a pathogen with a plant. This is the inoculum that lands on or otherwise brought into contact with plant inoculum may be adults, larvae or eggs, where as in parasitic higher plants it is plant fragment or seed.
Inoculum that survives the off season periods (winter or summer) and causes the original infection in the growing season is called primary inoculums and the infection as primary infection. Inoculum produced from these primary infection that actually spreads the disease in the field under favorable conditions, is called secondary inoculums that brings about secondary infections.
Inoculums in the absence of its host from the field services in plant debris field, soil, seed, tubers,
Transplants or other plant parts, perennial weeds, alternate hosts
The inoculums is carried to host plants and this landing or arrival of inoculums is passive by wind, water, insects etc or in some cases also by active growth as in some root-infecting fungi like Armillaria mella.
2. Prepenetration: This phase includes all the events prior to actual entry of the pathogen. Such events include (i) germination of spores and seeds, (ii) hatching of eggs (nematodes), (iii) attachment of pathogen to host, and (iv) recognition between host and pathogen (early event-not still understood clearly). Lack of specific recognition factors in plant surface may not allow the attachment of pathogen to it. Such factors in plant include lectins (proteins of glycoprotein's) and some oligo-and polysaccharides. In viral pathogen lack of recognition of its nucleic and by host ribosome's may lead to failure in disease.
3. Penetration: This is the actual entry of the pathogen into theirs host plants pathogens penetrate plant surfaces in different ways:
(i) Direct penetration through intact plant surfaces (ii) Through natural openings, and (iii) Through wounds.
4. Infection: This is the process by which a pathogen establishes contact with host cells or tissues and procures nutrients from them. This stage also includes invasion and to some extent growth and reproduction of the pathogen. During invasion, the pathogens colonise the host tissue in different ways and to different extent.
This time elapsing between penetration or more accurately spore germination and established infection is called period. We shall describe this stage in detail later.
5. Growth and reproduction of pathogen: Pathogens invade and infect tissue by growing and multiplying into them. In this way they colonise and infect more areas or paths of attacked plant. The period between infection, or more accurately spore germination and the appearance of visible symtoms is called incubation period. Thus incubation period includes the full life cycle of the pathogen. It may thus be seed that between spore germination and complete expression of the disease (symptoms), a series of events happens in the host. This chain of events between the time of infection, or more accurately spore germination and the complete expression of disease is called cycle or disease development. For spread of secondary inocula to perpetuate the disease in the field during growing season of plant dissemination is also sometimes included as a stage in disease development. Seasonal carry-over of the inoculums to next season is also similarly included by some under disease development.
6. Dissemination of pathogen: After pathogen has grown and multiplied in or on the infected host, it spread to new, healthy plants. Dissemination is the transfer of inoculums from the site of its production to the susceptible host surface. Some pathogens disperse in active manner, whereas most passively with the help of an agent of dispersal. The chief agents of dissemination are:
(i)Air, (ii) water,(iii), vectors i.e. .. Insects, mites, nematodes etc.(v)Man .
7. Seasonal carry-over of pathogen: In the absence of their hosts, the great variety of means of this seasonal carry-over.
At the on-set suitable conditions in the next growing season, these resting structure become active and produce inoculums. These inoculums then are taken to host surface.
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