2 Detection of Plant Pathogens

Transcript

Hello welcome to the second talk of this course that is detection of plant pathogens. In the previous talk we have seen that what is the distinguishing characteristics between detection and diagnosis. there we have talked about the various ways of detecting a plant plant pathogen and today we will be talking about the different types of symptoms and signs and they overlap and how to distinguish them in a single plant species so that we can accurately detect a proper disease. Some of the commonly observed Symptoms and Signs for example fungal leaf spots on rice there are two major diseases of rice that causes leaf spots – one is blast and another one is brown spot of rice. In both the cases one can see that dark margins with straw-colored Center spots are seen. In brown spot sometimes these spots are very dark in color and the central portion is also dark brown in color, but the distinguishing characteristic between the dark and light brown spots is that the in brown spot, the spots are relatively smaller whereas, in case of blast the spots are relatively bigger and spindle shaped. Whereas bacterial leaf spots we had see that similar brown spots are present on leaf surfaces but these are angular in nature. Basically, one can very clearly observe that the spots are limited by the veins and that is why the angular nature of the spots is very clearly evident. Tenderous symptoms which causes by viruses that is known as vein clearing and when banding. in case of vein clearing one can see that the veins are colorless basically the chlorophyll content of the veins is removed or destroyed and that is why the veins look yellow in color whereas, in case of wind banding one can see that along the vein there is a deep green color band is present in comparison to the lamina portion of the leaves where it is lighter in color or light green or yellow in color. This is what we call vain banding and that is how we can differentiate between when clearing and when banding. Similarly, in case of mosaic it is also produced by certain viruses. We can see that there are irregular shapes of green islands in a yellow background and different viruses produce different patterns of mosaic on leaves. At the same time some viruses produce certain typical symptoms which we call it as ring spot whether it may be on fruit or on leaves we can notice that the ring spots are almost circular in nature where the ring region is darker or lighter in nature whereas, the central portion is either darker or lighter but it gives a ring like appearance on both leaves and surface and that is why this symptom is known as ring spot symptom. Then leaf distortion is another symptom that is produced by different types of fungal or bacterial or viral pathogens. In case of peach the leaf distortions can be seen due to a fungal infection where you can see that this is a peach leaf which is distorted. Then, this is a chili plant that is having leaf distortion which we call it as leaf curl then it is caused by a virus whereas in case of banana, leaf swivel leaves are observed in banana plants due to Panama disease caused by a fungal pathogen. So leaf distortion can be another criteria for symptom and it is very distinct from 1 to 1 in different fungal pathogens. Then presence of spores or spores structures on Leafs or other parts of the plant such as presence of white powdery masses in case of powdery mildews, then orange color smut balls that is present on rice in case of all smut, then rust pustules on stem and leaf in case of width, then some leaf spots having fungal fruiting body pycnidia. This black spots on leaf surface is again a fungal infection, then black charcoal like powdery masses on leaves is again an characteristic feature of sooty mold diseases. So these are fungal spores or structures that are present on leaf or other parts of the plant and they are typical signs of the presence of the pathogen. Then there are bacterial cankers they can have different manifestations for example in case of citrus we can see small cankerous 05:04 legions on fruits and leaves and these are caused by bacterial pathogens, then in certain fruit trees we can see big cankerous growth on the stem portions which is dry in nature whereas, in case of certain other plants where bacterial infections that causes cankers alongwith oozing out of gummy substances which can be another typical signs of presence of bacterial pathogens on different plant parts. Then food decays and rots can also tell us some stories. The patterns of fruit decays and rots for example in case of strawberry, in case of tomato, in case of chilli or in case of mango we can have different types of fruit rots or decays and these patterns tells us about the specific nature of certain diseases and pathogens. Then we have wilts. The wilts symptoms also varies from plant to plant. The wilt symptom initially starts with drooping of leaves, then the leaves turn yellow and finally leaves turn brown in color and the entire plant is wilted. So different plants like cucurbitaceae pigeon pea then 06:19 eggplants chickpea they all have wilt causing pathogens and they affect this plant severely one can have a very clear-cut idea how they wilt symptoms look like in different plants. Then we have another symptom that is we call it as Shoot dieback or blights where the basic nature of the symptom is that the drying of the young in fluorescence or parts of the plants which dries up from the tip and it progresses towards the base of the stem. So this is typically known as Shoot dieback or blights caused by various pathogens. Then overall stunting or Decline can be another symptom of plant disease where we can see that diseased plants have very significantly low growth in comparison to a healthy plant and that is why we call it stunting and then in certain cases the entire plant they die and we call it as decline. Damping- off is another disease symptom caused by certain oomycetes pathogens and where the seedlings can be died within few hours as the pathogen infects at the base of the seedlings and because of the decay of the base of the plant the entire plant wilts and causes damping -off symptom. So certain symptoms and signs can be observed with unaided eyes and the presence of fungal structures can very well seen without any assistance for example the Sclerotium rolfsi pathogen that causes Collar rot. Here we can see that presence of mustard seed likes Sclerotia that are present at the base of the plant normally with white mycelial traits which is very distinctive in characteristics. A similar Sclerotia producing fungi fungus is known as Sclerotinia sclerotiorum we call it white mold and we hear the symptom is typically manifested by presence of white cottony growths and presence of black colored bigger-sized Sclerotia in comparison to the Sclerotia a of Sclerotinia sclerotiorum rot size which are mustard seed sized but this Sclerotia of Sclerotinia sclerotiorum can go up to one centimeter in size. Then False smut can also very well manifest it in the field condition where initially we see orange colored smut balls which then later turn into a olive green or black colored smut balls which is very distinct from the healthy plants. Then wheat rust pathogens which is manifested in stems or leaves by rust pustules and this is a very clear-cut science of a rust pathogen. Common scab pathogen consists of scabby areas on potatoes and bacterial leaf blight of rice which where the field is very distinctly visible from a distance it looks like blighted or burnt off where the symptom produces along the margin of the rice leaves from the margins and normally on rainy days or in humid conditions we see this type of yellow droplets which later dry and becomes yellow colored balls that are present on the leaf surface and very clearly indicate the presence of the Bacterial leaf blight pathogen on rice plants. Then Fusarium wilt is very clearly identified with symptoms like wilting and the vascular browning of the stems. It’s a clear cut indication of Fusarium wilt. this is a pattern we see in tomato, this is a pattern we see in pigeon pea, in both the cases and this is a pattern in chickpea where in all the cases we could very well see the browning of the vascular tissues because of the presence of the pathogen and its effect. Then bacterial leaf spot whether it is in mango or chilli in both the cases we see that angular nature of the leaf spots which is basically surrounded or limited by the veins of the leaves. In case of chili also the spots are basically angular in nature so that is the typical characteristics of bacterial leaf spot diseases. One can distinguish the bacterial leaf spot and Cercospora leaf spot because as I already mentioned that Bacterial leaf spots the spots are angular in nature whereas, in case of fungal pathogen producing leaf spots such as Cercospora leaf spots the spots are not angular in nature, it is more or less circular and the central portion of the spots are greyish or light brown in color but this definitely they are not angular in nature which is a typical distinguishing characteristic between these two leaf spots. The powdery mildew very well manifested with presence of white powdery masses on leaves on different plant species. Karnal bunt, it’s another fungal pathogen. It basically affect the embryo of the wheat kernels and this is a initiation of the disease this is advance stage and this is the most severe form of the disease where w e can see the extent of damage due to Karnal bunt and this is very well or typically it can be identified because of presence of these damaged embryos. Viral diseases can also be very well distinguished because of their symptoms that are produced like Mosaic and Leaf curl. Mosaic it is a green and yellow island which is non uniform in nature, it is scattered whereas, Leaf curl is a very typical symptom where the leaf margins are rolled upside and curled. Nematode pathogens can also be very well distinguished based on the symptoms produced on hosts like Root knots, Soybean cyst and Root nodules can be distinguished from this root knot or cysts by looking at the differences between the symptoms. In case of root knot this is basically the cell division and enlargement that causes by the nematode infection which produces galls on the roots itself whereas, cysts are produced which are extended growth of the plant parts and this ceased basically consists of either off white color or golden in color and they are typically distinguished from Root nodules which are basically produced by root inhabiting beneficial bacteria like Rhizobium which is simply an outgrowth of the bacterial root system and it is initially pink in nature, later on, it turns into brown but definitely this is having very distinct oozing characteristics from cyst as well as galls. We can use Hand lens to observe certain fungal parts. For example downy mildew where, the leaf spots are basically from the top it looks yellow and only on the undersurface of the leaves one can see the fungal growth by using a hand lens. So particularly this is evident in the morning hours where the moisture level is high in the field conditions one can use a hand lens to see the fungal structures on the below surface of the leaf just below the yellow symptoms on the upper leaved surface. Similarly, we can use hand lens to observe the growth of fungal mycelium on the lower surface of the leaves in case of Late blight of potato and we can very well distinguish the sporangios that are produced on the lower surface of the plants in morning hours when the moisture is present. We can then differentiate Early and Late blight because early blight in potato it has brown dark spots which have concentric rings in the center giving a target board like appearance whereas in case of late blight the brown patches or the dead tissues extends very rapidly and in the morning hours if we see with a hand lens we can see white fungal sporangial growth on the margins of the spot. For bacteria the typical test is Ooze test to distinguish it from a fungal wilt or fungal disease producing agent. So once we suspect a bacterial infection one can take the plants and we can break the 14:56 stem and if we see some mucilage substance coming out between the cut ends then it can very well be established that it may be a bacterial causative agents and if we have access to a glass tube or a beaker then we can put the cotton in a distilled water and we can if we see oozing out of bacteria then we can be well highly confirm that it is a bacterial disease. So we have seen varieties of symptoms and signs that are produced by different types of fungal, bacterial and nematode pathogens of plants and until and unless we distinguish these signs and symptoms which are sometimes very difficult to distinguish from each other. We need a very experienced eye to distinguish them and only by distinguishing these signs and symptoms we can initially confirm a particular disease or disease causing agent which may be further confirmed through laboratory tests but initial recognition is very very important for distinguishing between a disease causing agent and other abnormalities is due to abiotic factors. So in the next talk we’ll be talking about procedures in plant disease diagnosis and I hope you will be able to learn the steps that are involved in the process of disease diagnosis.

Thank you very much.