Disease cycle

Disease cycle

  • The development of disease involves a chain of events reaction and interaction for its successful manifestations in a host plant.
  • This chain of events leading to the development of the disease is called disease cycle.
  • The whole chain of events in a disease cycle can be divided into two phases:-
  • Dormant phase or survival
  • active phase for pathogenesis

Dormant phase

  • In this phase the pathogens remain in dormant condition as an example they overcome the unfavourable environmental condition so that they can survive until the coming growing session and start with active phase of pathogenesis this is called perrennation.
  • Perrennation :- mostly the pathogens in fact the perennial and where and deciduous plant.
  • In perrenial hosts the pathogen usually perrenate in /on the host in their vegetative state.
  • But the pathogen that attacks the deciduous host cannot perrennate in a vegetative state and thus developed special structures. (structure like oospores, cysts endospores.)

Active phase

  • Inactive phase the pathogen associated with the tissue shows the pathogenicity.
  • Actually what happened on the advent of the favourable condition the dormant phase comes to an end and the pathogen become active to start with the disease cycle at this stage the perrenating atom structure either act as primary inoculum or the produce reproductive bodies to earth as a primary inoculum.
  • The following is the sequential arrangement of the stages of a disease cycle which completes the active phase after the activation of the parrenating structure.

Dissemination or dispersal:-

  • The primary inoculum is disseminated or dispersed on the cultivated host through different agents such as wind, water, insects air, man, other animals.
  • Chemotaxis organism within the soil are altracleid to the plant root.dispersal is necessary because there are few pathogen that are autonomous and even if they are autonomous they are of less significance like bacteria and nematodes whereas virus and the most sports of fungi are unable to move themselves.
  • Ex-wind disseminated pathogens- Alternaria
  • Water disseminated- xanthomonas campestris
  • Dissermination by men -phytophthora infectans

(2) infection(mode /entry)

  • After coming in contact with the active host through dissemination the pathogen first prepares to enter the host and then enters into the host these are called penetration.


  • In this stage the pathogen comes to its vegetative form then it germinates on the affected host by producing a germ tube which grows on the host surface and the pathogen attaches itself by a specific structure called appressoria. The appressoria then give rise to one or more thin and finally pointed infection threads or hyphae which adopts various ways to gain entry into the interior of the host.

Attachment to the host:-

  • Fungi and bacteria produce a gelatinous substance to help them to stick to the leaf surface.

Spore germination:-

  • Mainly in fungi sports germinates forming germ tube and moves along host to find an opening or weak point.
  • Appresorium formation
  • Recognition between host and pathogen
  • Triggers plant resistance genes if present promotes or prevent infection.

(2) pennetration:-

  • The infection or Haifa enters into the host either through artificial openings and through natural openings or by penetrating directly through host surface.
  • Direct penetration of the host surface can take place through mechanical pressure foot by the infection thread or through secretion of an enzyme by the infection trade or mostly by the combination of both.

Disease cycle
Establishment of infection
Penetration by fungi:-

  • For a successful establishment, certain conditions must be satisfied.
  • The infecting organism must be at pathogenic stage.
  • The host must be in susceptible condition.
  • the environmental condition must be favourable.
  • At this point a struggle is seen between the pathogen and the host as an example the pathogen release a number of toxins and other harmful substances to affect the structural integrity of the host cell and their physiological process and other hand mechanisms to protect itself from pathogen if the pathogen overcomes the defence mechanism of the host then a successful infection is established and disease development and if the host wins the disease is discarded and the host lives a healthy life.

(4) Invasion:-

  • Here the pathogen spread through the plant after the successful establishment of the infection.
  • Fungi and bacteria spread by intracellular and intercellular by using enzyme and hormones localised.
  • Nematodes move intracellularly localised.
  • Viruses, viroids and Jhelum slash phloem limited bacteria move cell to cell intracellularly systemic.

(5) growth and reproduction of the pathogen:-

  • Pathogen continues to spread until the infection is stopped or the plant is dead.
  • Fungi forms spores either inter or intracellular surface and interior both.
  • Bacteria buy cell division either inter or intracellular surface.
  • Viruses intracellular and inside cells only.
  • Parasitic plant by seeds, the exterior of the plant only.
  • Nematodes inter or intracellular surface and interior both.
  • Rates of colonization vary on pathogen present, environment and the host.

(6)Exist of pathogen:-

  • When the host tissues fall short to support the overcrowded population of pathogens the letter finds exist and disperse and determinant with the help of various agencies to escape death due to overcrowdedness and to continue their disease cycle.
  • Come in contact with the healthy host in the same growing season causing secondary host and resulting in the secondary infection cycle.
  • The secondary infection-causing pathogens or it’s reproductive units are called secondary inoculum.
  • Secondary infection is more destructive and results in greater loss of crops.
  • After that, they enter a dominant phase at the end of the growing season to overcome the unfavourable conditions.

Disease epidemics:-

  • Monocyclic:-completes one disease cycle in a year.
  • Polycyclic:- Two or more disease cycle in a year causes explosive epidemics.