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3 Components of plant disease development


Hello! Welcome back to this particular course today it’s the third talk and we will be discussing about components of plant disease development. So plant disease although it looks very simple that a pathogen comes and attacked the plant and it causes plant diseases. But it is not that simple because there are different events that takes place during infection process to the symptom development part and this is very important to understand before we proceed to understand the plant diseases so there is a concept of disease triangle where three particular factors are essential for a disease to occur and this three important components of plant disease or disease triangle is there should be a susceptible host, a virulent pathogens and a favorable environment. Until and unless all these three things are combined together there won’t be any disease. If there is pathogen and host but the environment is not favorable then there won’t be any disease. Likewise, if the pathogen is there and environment is conducive but the susceptible host is not there, there won’t be any disease. So we can see that this disease triangle is very very important from that particular point. Of course, these are not three only issues that deal with the disease triangle there are other components like time. So for a disease to develop there’s along with this susceptible host favourable environment and a favorable pathogen or a virulent pathogen the time of interaction between the host and the pathogen under a favorable environmental condition is very very essential. So if the time of the favorable environmental condition is very very short then there may not be a successful interaction between plant and pathogen and that may not lead to disease development. So here we can see that only a proper combination of these three components can lead to disease. Here you can see this is combination of two components like susceptible host and virulent pathogen but there won’t be disease there.

Again at the same time if there is environmental condition is favorable and susceptible host there won’t be no disease along with there won’t be no disease if there is only pathogen and conducive environment interact. so there are three components should interact together so for that development of plant diseases. So properties of plant pathogens to cause successful disease are also important. Main or the most significant one is the Level of Virulence the pathogens should be highly virulent otherwise it won’t be able to cause disease. Similarly, the pathogens should have better Adaptability to different environmental conditions. Then it should have a good Dispersal efficiency then the pathogen should also have survival efficiency and it should have a good Reproductive fitness. So all these characteristics of the pathogens are important for developing or establishing a successful infection in the host. Then what should be the properties of the host that may lead to disease development. The host should be Susceptible that is the first criteria. Then the Growth stages all pathogens may not attack the plant at all growth stage so the susceptible host should encounter the virulent pathogen at a particular growth stage which is susceptible to the pathogen. Say for example if the pathogen is able to cause disease in the seedling stage and the pathogen is absent in the seedling stage then, there won’t be any disease. Even if the pathogen comes back at the later stage say for example at the maturity stage again there won’t be any disease because the pathogen is able to cause disease to the plant only in the susceptible condition i.e the early growth stage of the plant. Then Population density and structure of the agriculture field that is very important. If the population density of the plants are high then their micro canopy within the plant system will have a high humid condition and the temperature will also be very low.

These conditions may be very very favorable for the pathogen to develop or to establish infection on the host. So that is also very very important and general health of the plant. If the plant health is poor then it will naturally become susceptible to the pathogen rather than if the plant is very robust and having a very good vigor or very good growth then the plant may be escaping the pathogen infection. Similarly, the properties of the environment includes temperature, rainfall then leaf wetness period, soil properties and wind. So all these things favour a development of a particular disease and all these things may also discourage development of a particular disease. Say for example if a pathogen is able to cause disease at a particular temperature if the environmental temperature is higher than that particular temperature then there won’t be any successful establishment of the disease. Similarly, if the pathogen requires some moisture on the plant surface to cause disease or infection and there is no humidity or rainfall during the crop season then again it will help or it will help the plants to escape the infection process by the pathogen. So, that is why environmental condition is very very important in this context. Let us take the example of potato late blight development. So for the Phytophthora infestans that cause potato late blight to have a successful disease development on potato it requires a cool and wet condition. So if the nights are generally cool and the days are normally warm and the extended wet conditions due to rain and fog if they combine together then there is a successful development of late blight pathogen, if the pathogen is present in the field where potato is being cultivated. Next if we just look into the chain of events that lead to disease development then we can see that there are different stages like Inoculation, Penetration, Infection, Incubation, Reproduction and Dissemination. So these all steps need to be followed for establishment of a successful disease all leading to an epidemic condition. So let us see what are these conditions one by one. Inoculation – inoculation is arrival of the pathogen to the host. Then after arrival the pathogen needs to penetrate the host. So it is very very important that the pathogen penetrate the host after its arrival and for that it needs again favorable environmental condition. The next step is Infection. So penetration may alone may not lead to development of disease until and unless they start establishing a parasitic relationship with the host plant. That is what we call it as infection. So successful establishment of pathogen with host cells or tissues and then deriving nutrition from them is what we call it as infection. Then after infection the next step is Incubation. So this is the time interval between arrival of inoculum on the host and symptom development. So incubation is the period after penetration or arrival of the pathogen on the host surface till development of a symptom. So it takes little bit of time and that time period is what we call it as incubation period. Then after successful establishment of the pathogen it needs to reproduce on the host tissues until unless the pathogen is able to reproduce on the infected host it will not help or it will not help the cause of developing of an epidemic condition. So it is very very important that the pathogen reproduce on the host surface. Then after reproduction the next important step is dissemination. So dissemination is equally important because the pathogen need to move from the infected host plant and arrive a say for example a non-infected or fresh plant to cause a new disease or infection. So that is why, why I call it dissemination is also very very important. So depending on the type of lifestyle we classify disease cycles into two major groups. One we call it as monocyclic and the other we call it as polycyclic. That means in case of monocyclic the most suitable examples are Smuts and rust fungi. Normally once they start producing primary inoculum they come and infect on the primary host and the inoculum that is built on the primary host they then over season for the next season of the crop to develop a successful infection. In case of polycyclic the examples are late blight of potato, powdery mildews and so on. So here what happens after arrival of the primary inoculum it goes and infect the primary host causing the initial infection and then it developed secondary inoculum in the same host. And this secondary inoculum then may disperse from this plant to other plants in the field or nearby field to cause secondary infection.

So this cycle may continue several times in a single crop season. So after the end of the crop season this secondary inoculum then again overwinter further or they remain in the field or in the dormant form till the next season where the new crops are grown. So looking into this we can again say that monocyclic disease have the lesser potentiality to cause epidemic within a season whereas polycyclic diseases has more chances of causing epidemic in a single growing season. So ultimately we can now say that it is not only the arrival of the pathogen that leads to development of a successful infection rather it’s successful infection depends on at least three components i.e a virulent pathogen, a susceptible host and a favorable environmental condition so these three things when coincides then it leads to development of a successful infection and successful infection may lead to development of an epidemic situation and this may finally lead to development of plant diseases. So understanding the components of plant disease is very very important because we can develop management strategies based on this disease development components.

Thank you




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