The Fungal diseases Courtesy: The American Phytopathological Society.

The Disease Cycle: Stages of Disease Development

Who haven’t seen plants with some kinds of abnormalities? We all have, right? Sometimes in the form of some black spots or white powders in the leaves; or sometimes those rotten roots or stems; or may be those holes in the fruits that are making them inedible …. and many more. Perhaps we all are to some extent, observers of nature and in being so, we always have and we always will come to see these illness in plant bodies.

We always care about our body, but for plants? Who are there to care for them? Who are going to treat them; or cure them forever? Or should it be really necessary to care for plants? And most importantly what are those tiny aliens that are making those plants ill? And how do they develop? Or is it already too late to cure plants when it provides us with symptoms? If you are intrigued by these questions, you have come to the right place. This article has  answers of some of these questions.

Statistics showed that, the human beings are dependent on only about 20 economic plants for their staple foods. These economic plants along with the other non-staple ones are a great concern for botanists like us as their well-beings and well-development each year will feed the population round the globe.

Many times in their life cycles, crops show the mentioned features and many other symptoms. This attack by various pathogens to develop symptoms and continue their spread is very alarming for us as the plants of our concern get partially or fully destructed and do not bear up to our expected production. Sometimes the whole crop field has to be destructed as the disease can spread very easily.

In order to fight these problems, the botanical science has developed a distinct discipline only devoted to this problem and its solution known as Plant Pathology. This beneficial science discusses many things such as the problems by pathogens to plants, the solution, the environmental concern, the genetic interaction between plants and pathogens, the parasitism and pathogenicity etc..

Being an old discipline, this science has made a huge progress and made some formulas of its own to face the challenge of both the problem and the formulation of solution. One of such key concept of plant pathology is The Disease Cycle.


The Disease Cycle

The disease cycle is a chain of interconnected successive events of a pathogen’s infection in a host plant. It usually coincides with the life cycle of the pathogen with a correlation to its host and the environment.

  • Each cycle includes two alternating phases; the parasitic phase and the survival or over-summering or over-wintering phase.

The distinct events in a disease cycle are very much important as they provide us information about how and when we should stop the spread of the disease easily. Besides, the prediction about other disease cycles by other unknown or less known pathogens can be formulated using the info of one of such cycle.

Good to know:

  • By the time we notice symptoms on plants, the pathogen has already gone through three or four stages unnoticed. Which are: inoculation, incubation, penetration, infection and symptoms.

Steps in a disease cycle

  • Inoculation
  • Penetration
  • Infection
  • Invasion
  • Colonization
  • Dissemination
  • Over-wintering and over-summering


(The stages are discussed briefly here. For further detailed study, follow each link after each description)


Inoculation is the first contact of a pathogen with its host in a place where infection is possible. The pathogen may be in any of its stages of its life cycle when being inoculated.

  • Most pathogens rely on rain, wind, insects or human to carry them to their host plants.


Pathogens penetrate plant surfaces by direct penetration of cell walls, through natural openings, or thorough wounds. It is to remember that, penetration doesn’t necessarily cause infection. There are many cells that can be penetrated but yet not susceptible to infection. In this case, the organism can not proceed beyond penetration and die without producing disease.

Link: Inoculation and penetration


When the inoculum starts procuring nutrients from the susceptible tissues of the host, the infection starts. This is the first stage from where the disease starts to develop.

As the pathogen devours soluble products from the cell, various symptoms start appearing.

  • The time interval between inoculation and the appearance of disease symptoms is called the incubation period.


Some pathogens which have motile stages such as nematodes, oomycetes, zoosporic fungi, and bacteria can easily disseminate very short distances on their own power. Fungal hyphae can grow between tissues in contact and sometimes through the soil toward nearby roots for a few to may centimeters. Both of these means of dissemination are quite limited. However, there are some fungi which can expel their spores up to a few centimeter in the wind above their sporophores.

Link: Infection and dissemination


Over-seasoning is an evolutionary developed technique followed by many pathogens to survive the adverse period of their life cycle. It has been developed in the pathogens so that they can complete their life cycle and can spread their infections in the next year. The pathogens may survive this harsh period either as spores, sclerotia like hard over-seasoning structures or as a whole.

Link: Over-seasoning of pathogens

So, these are the successive steps of disease development by a pathogen. The science of plant pathology deals with these steps and always try to find a way to stop the cycle so that the disease is not spread further.

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About Abulais Shomrat

Currently in 4th year (Hons) in the Department of Botany, University of Dhaka. Planned to have multiple careers one by one but promised to be with 'Plantlet' as long as it's primary stage remains unfinished. Email: abulaisshomrat@gmail.com. Minimum monthly resolution - Publish (1), Revise (3), Share (5)

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