Stages in the development of plant disease

The term disease cycle is used to describe the relationship between pathogen and host. The events in a disease cycle involve the survival of pathogen during periods that are not favorable for disease development, inoculation, penetration, establishment of infection, colonization, growth and reproduction of the pathogen, and over seasoning of the pathogen.

Inoculation

The process whereby a pathogen comes in contact with the host is called inoculation. Inoculation include fallowing things.

i) The inoculum: It is a sample of microbes that can initiate infection.

ii) The source of inoculum: It consists of the plant debris or soil on which the inoculum has over seasoned. Some times it comes from infected parts of the crop or plant.

iii) Inoculum dispersal: Inoculum are usually disseminated by wind, water, soil, plant parts or by a vectors, some times human beings.

a) Dissemination by wind: The spores of many pathogens are disseminated by the wind. sometimes, the diseased plant parts help dissemination of diseases. 

b) Dissemination by water: The disease like wart disease of potato, caused by Synchytrium endobiotichum is disseminated from one field to another through irrigation or flood water. Many bacterial disease disseminate by splashing rain drops.

c) Dissemination by insects: Insects act as important agents in the spread of plant diseases. Anumber of viral diseases are disseminated by insects like leaf hoppers, mealy bugs and aphids. Some insects are carrier of inoculum while feeding.

d) Dissemination by animals and humans: Certain plant diseases are disseminated by nematodes, snails, birds and other wild animals. Farm animals and unskilled farmers are also disseminate the disease in the farm.

iv) Penetration: For disease to develop, the pathogen must penetrate the suscept, invade tissues and begin to grow. Pathogen may enter plants through wounds, or natural openings, or direct penetration of plant surfaces.

v) Infection and Colonization: After the pathogen has entered the host tissues, it establishes contact with the susceptible cells or tissues and procures nutrients from them. The establishment of the pathogen in the host tissue is called infection. Subsequently, pathogen grows or multiplies, or both, within the plant tissues and colonizes host tissue. During infection, pathogens release in the host a number of biologically active substances that affect the structure of host cells and their physiological processes. As the pathogen continues to grow in the plant, symptoms will begin to appear at a certain time. Many biochemical and physiological changes have usually seen in the plant before symptoms develop. The time between inoculation and symptom development is known as incubation period.

The disease cycle is completed when the pathogen reaches the survival stage. The pathogen numbers in the plant body increases and finally leads to the death of the plant.