1. Parasite: An organism living upon or in another living organism (the host) and obtaining the food from the invading host.
2. Pathogen: An entity, usually a micro-organism that can cause the disease.
3. Biotroph: A plant pathogenic fungus that requires living host cells i.e. an obligate parasite.
4. Hemibiotroph: A plant pathogenic fungus that initially requires living host cells but after killing the host cell grows on the dead and dying cells.
5. Necrotroph: A pathogenic fungus that kills the host and survives on the dying and dead cells.
6. Pathogenicity: The relative capability of a pathogen to cause disease.
7. Pathogenesis: It is a process caused by an infectious agent (pathogen) when it comes in contact with a susceptible host.
8. Virulence: The degree of infectivity of a given pathogen.
9. Infection: The initiation and establishment of a parasite within a host plant.
10. Primary infection: The first infection of a plant by the over wintering or over summering of the pathogen.
11. Inoculum: That portion of pathogen which is transferred to plant and cause disease.
12. Invasion: The penetration and spread of a pathogen in the host.
13. Colonization: The growth of a pathogen, particularly a fungus, in the host after infection is called colonization.
14. Inoculum potential: The growth or threshold of fungus available for colonization at substratum (host).
15. Symptoms: The external and internal reaction or alterations of a plant as a result of disease.
16. Incubation period: The period of time between penetration of a pathogen to the host and the first appearance of symptoms on the plant.
17. Disease cycle: The chain of events involved in disease development.
18. Disease syndrome: The set of varying symptoms characterizing a disease are collectively called a syndrome.
19. Single cycle disease (Monocyclic): This type of disease is referred to those caused by the pathogen (fungi) that can complete only one life cycle in one crop season of the host plant. e.g. downy mildew of rapeseed, club root of crucifers, sclerotinia blight of brinjal etc.
20. Multiple cycle disease (Polycyclic): Some pathogens specially a fungus, can complete a number of life cycles within one crop season of the host plant and the disease caused by such pathogens is called multiple cycle disease e.g. wheat rust, rice blast, late blight of potato etc.
22. Collateral host: The wild host of same families of a pathogen is called as collateral host.
23. Predisposition: The effect of one or more environmental factors which makes a plant vulnerable to attack by a pathogen.
24. Physiologic race: One or a group of microorganisms similar in morphology but dissimilar in certain cultural, physiological or pathological characters.
25. Biotype: The smallest morphological unit within a species, the members of which are usually genetically identical.
26. Symbiosis: A mutually beneficial association of two or more different kinds of organisms.
27. Mutualism: Symbiosis of two organisms that are mutually helpful or that mutually support one another.
28. Antagonism: The counteraction between organisms or groups of organisms.
29. Mutation: An abrupt appearance of a new characteristic in an individual as a result of an accidental change in genes present in chromosomes.
30. Disease: Any deviation in the general health, or physiology or function of plant or plant parts, is recognized as a disease.
31. Cop Damage: It is defined as any reduction in the quality or quantity of yield or loss of revenue resulting from crop injury.
32. Deficiency: Abnormality or disease caused by the lack or subnormal level of availability of one or more essential nutrient elements.