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2 Introduction to plant pathogens


Hello! I welcome you again to the second topic of this particular course and today’s topic is introduction to plant pathogens. so to understand plant pathology. It is very important for us to understand plant pathogens. So what are plant diseases or what we call it as plant diseases? So if you just look into the term then anything that prevents a plant from performing its maximum potential is called a plant disease. The cause may be abiotic or a biotic factor so anything that it has life or something that doesn’t have life may cause disease to plants so abiotic or non-infectious diseases could be due to say for example nutritional deficiencies in soil or soil compaction salt injury ice or even sun scorch. All this abiotic causes may lead to a non infectious diseases we can take an example of zinc deficiency in pea where you can see that the plants if they are facing zinc deficiency in soil the pea plants develop a white colored borders in zinc deficient plants and soil and comparison to non zinc deficient soil.

Similarly, if we just look into sodium chloride stresses when Jatropha plant was subjected to sodium chloride stresses then you can see that the interveinal space in Jatropha become white in presence of excess sodium chloride. So these are the abiotic or non infectious diseases that lead to certain symptom development when the plants are under stress conditions. So biotech or infectious diseases are the second category of plant diseases where the cause of the disease is due to living organisms. They are called plant pathogens when they infect plants the pathogens can spread from plant to plant and may infect all types of plant tissues including leaf shoots, stems, grounds roots, tubers, fruits, seeds and vascular tissues. So you can say that all plant parts are affected by plant pathogens. No part is spared then what we call plant pathogens or what are the plant pathogens? So if we just look into what a plant pathogens then they may be fungi or fungi like organisms, bacteria, Phytoplasmas, viruses, viroids, nematodes and even higher parasitic plants that cause plant diseases. So all these are plant pathogens because they have the potentiality to cause plant disease and reduce crop yield. So plant pathogens are of interest for a number of reasons because they can make an ecosystem very fragile and increase the demand to protect food supply. So these are major reasons why we need to understand plant pathogens. So plant pathogens that cause plant diseases reduce growers ability to produce crops and can infect almost all types of plants. So no plant on this earth is spared of plant pathogens but the only difference is that different plants are attacked by different pathogens. So, plant pathogens as I mentioned earlier may be fungi, bacteria, virus, viroids, phytoplasma and nematode.

So what are fungi? Fungi are basically branched filamentous and spore producing mycelium and the spores vary from fungi to fungi. Bacteria are single-celled organisms that can cause disease in certain plants. Viruses are basically nuclear proteins which may be either cylindrical or isometric in morphology and they are also able to induce certain diseases in different plants. Similarly viroids are basically virus like organisms. But it divides of the protein coat so these are the basically infectious nucleic acid that cause plant diseases, fight Phytoplasmas. Again, they are basically single cellular bacteria like organisms but the major difference between Phytoplasmas and bacteria is that the phytoplasma does not contain a cell wall so their size are different in a single cell then nematodes are worm like organisms. All nematodes are not phytopathogenic in nature. Those nematodes which can cause disease in plants are basically have a stylet in their mouth part. This is an indication that nematode of this type can produce inside plant diseases. Then parasitic higher plants are of basically two types stem parasites and root parasites. Stem parasites comprises of two major groups of higher plants that is Loranthus and Cuscuta or Dodder. They infect higher plants and they parasitize on this plant lead to even death of the plant after the period of time. Similarly, Striga and Orobanche key are basically root parasites of different crop plants and they can also cause a significant loss to the growers when they are attacked by this particular parasites.

So plant pathogen produced different types of symptoms. What are in their symptoms? How we can categorize plant pathogens based on these symptoms? At times plant symptoms can tell us what may be the causal agent? It may be because the symptoms are unique to this particular pathogen. So plant symptoms is very important to understand. So that we understand the cause behind this production of this symptom. So the type and external symptoms in most cases indicate the nature of pathogen responsible for the disease. So plant pathogens can attack in a number of different ways some colonize the tissue in the plant or others settle on the surface of the plant and others may go for specific areas such as roots, stems and leaves. So depending on the pathogens they may either go into the plant system, they may remain on that surface of the plant or they may be tissue specific like they may infect only the roots or only the stems or at times only the leaves. So pathogens commonly cause problems like tissue death, browning and a decrease in fruiting and problems with setting of flowers and so forth. So these are the some of the major indications that plants are suffering from certain pathogens infection. So if we just look into the disease symptoms that are produced by some common fungal species we can see that bird eye spot on berries which we also call as anthracnose.

This is one of the major symptom produced by fungi then damping off of seedling, it is another major symptom that is produced some a group of phytopathogens including Phytophthora leaf spot or septoria. Brown spot this is another fungal symptom produced by a fungus chlorosis is also a symptom that is produced by certain fungi. So these are some of the major classes of symptoms that are produced by some fungal infection. Similarly when the plants are infected by some bacterial pathogens we may come across symptoms like leaf spot surrounded by a yellow halo, then we may see some fruit spots on the fruits, then we can see cankers or you can say rough corky cellular growth on the different parts of plants like leaf stems and fruits, then crown gall that is it’s a formation of tumor like growth either in the stem or on the root system so, viral symptoms are little different from either fungi or bacteria. So in case of viruses we can see a pattern of mosaic in the leaves, then we can also see there is crinkling type of symptoms on the leaf surface, then some leaves may become completely yellowed or even plants become stunted. So these are some of the characteristics of symptoms that are produced by different viruses. Here you can see that this is a symptom of leaf spot. Normally, it is caused by a fungal pathogen. This is the symptom of blight where you can see that the leaves are just like burnt. This is a wilt symptom where the plants are getting wilted because of infection by a pathogen on the root system, these are scabs on potato and apple and these scabs are nothing but hard corky layer of cells on the surface of potatoes and apple. This is the symptom of mosaic where you can see that there are some green islands on the background of a yellow background. Then these are the typical example of galls where you can see the crown area is tumourized by infection of certain viruses or some nematodes and this is an example of another symptom that is what we call it as grassy shoot where the stems become grass like appearance. So altogether we must understand that plant diseases are caused by some biotech agents as well as abiotic agents. Biotech agents are mostly living organisms like fungi, bacteria, virus, viroids, nematodes and micro plasmas. Whereas, abiotic agents maybe it is the extreme temperature either in the lower side or higher side. There may be nutritional deficiencies in soil or there may be nutritional toxicity higher amount of certain nutrients are present in the soil or it may be due to extreme heat. So all these are abiotic causes. But both these causes lead to these development of plant diseases and produce different types of symptoms. Many a times we see that symptoms are characteristics of a particular individual pathogen and we can immediately point it out what may be the causal agent by looking into the symptom. So that is why it is very important to understand the plant disease concept along with plant pathogens so that we are able to identify the plant diseases that and at the same time by looking into the plant system diseases we are able to at least tell major groups of pathogens that may be involved in causing those diseases.

Thank you



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