3 Procedures in Plant Disease Diagnosis

Transcript

Hello!! I welcome you to third talk of the week that is procedures in plant disease diagnosis. so we have to follow certain steps and procedures for plant disease diagnosis. It is important to know whether the pattern of the disease that is occurring in a particular field, whether the entire plant is affected, whether certain plant parts is affected, so in considering all these things only we can go ahead for the proper diagnosis of a plant disease.

So first of all it is very important to know whether all parts of the plant or a specific part of the plant is affected by the disease. So it’s important to note if the symptoms observed are associated with specific plant parts and is it a wilt observed that is correlated with a disruption of the vascular system or indicated by browning of vascular system or are the roots of the plants abnormal including knots or necrotic lesions observed during strictly on the younger leaves. So whether the symptoms are produced on the roots, or the stems, or the fruits so this is first thing to ascertain, and only because of ascertaining this particular fact which part is affected we can go ahead with diagnosis of the particular disease. Then we have to check the distribution of the symptom where we have to observe the diseased plants which are distributed on the affected area. We have to see whether distributed uniformly across an area or it is a localized disease and a definite pattern to the distribution is it only along the edges of a greenhouse near open windows or in a low spot of a field or in a planted row only or affecting the plants at random. These are certain criteria that needs to be observed when we try to distinguish a symptom and try to establish the region behind the symptom. Non-infectious problems can also occur in the field because of improper herbicide use or soil factors and they too cause certain types of symptoms on plant parts. So that needs to be distinguished or segregated from the pathogenic ones. A uniform pattern of an individual plant over a large area are usually due to a biotic agents this is a typical pattern where suppose the soil is affected by a certain mineral toxicity or say for example iron toxicity or for deficiency of certain micronutrients like boron or sulphur, it produces a symptom and which is uniform in the entire field. so this is a pattern that needs definitely to be ascertain whether it is pattern may be due to a biotic agent or an abiotic agent and that is why we need to go for checking the distribution of symptom. Then the next thing that we need to consider is that how prevalent is the problem and whether all parts of the plants are affected. Infectious problems occur over time and symptoms progress over time.

So infections normally when it is caused by a biotech agent it progresses over time and it also symptoms are produced slowly from a minimal symptom to a higher severe symptom with time and rarely all plants are affected. It is normally the few plants that initiate with and slowly if the infection is severe then it progresses to other plants or other parts of the plants. Normally, a uniform disease can be caused only if the seed locks are affected by a particular disease then too very rarely we can have hundred-person plants affected by their particular infected seeds. When a problem appears in 100% of plants it’s more commonly results from factors like soil conditions, like deficiencies and toxicities as I have already mentioned, adverse climatic factors like, high temperature, low temperature, or draught or toxic chemicals like improper pesticide used air pollutants and so on. So these things needs to be addressed before we go for final diagnosis of a particular problem. What has been the progression of the symptoms of plant is another questions that can be asked. If the symptoms all appeared at the same time and there has been no further development of symptom this could indicate a possible episodic events such as change in temperature or improper chemical uses. If it is a biotic agent then definitely the disease will progress with time but if the disease progress till a particular time period and after that the disease progression is zero or nil then definitely the cause of the symptoms producing agent is not a biotic one it may be a other measure by biotic or a abiotic factor. If symptoms started in one area and slowly spread to other areas and the severity of symptoms change over time this would most likely due to a biotech agent.

So biotech agent that there will be a pattern of symptom development it will start in one area and that slowly it will progress to other areas and the severity will be changed with time and this is the typical characteristics of a biotech agent. Now check for host specificity, sometimes in some cases, the problem occurs in only one plant species and it’s our different plant species are affected. This is a question to be asked. If different plant species are affected this may be possibility of a non-infectious problem which could be related to cultural or environmental problems, but because we know that most of the plant pathogens they are causes disease on a single host and only very limited plant pathogens are there which causes disease on multiple host or related species. For example Phytophthora and Pythium roots rots. More than they can cause root rots in more than one plants species and that we have to look for this type of pathogens also so background knowledge regarding such pathogens is also important when we go for host specificity. If there is more than one species in or plant involved and these plants closely related and they can be infected by a common pathogen, this information is highly essential to establish the biotic or biotic nature of the plant disease. Then it is also important to review the cultural practices and the growing environment of the affected plant or plant areas is that the growers may not be involved in associated or associate with the problem that is observed in his or her field but maybe it is due to the activities of his or her neighbor which lead to certain problems. So information pertaining to the growing environment is very very important for example change in the environment such as extreme temperature that is freezing or heat, rainfall, prolonged drought etc then site factors such as soil type, drainage problems as well as soil pH should also be evaluated when we go for diagnosis and these are certain aspects that needs to be kept in mind. Cultural and maintenance activities such as application, rate of pesticides, or other chemicals equipment used and unusual occurrence or weather patterns are also important to be considered when we are going for disease diagnosis because they can also have some impact on symptom development on plant species. Careful investigation by the diagnostician is required as someone may have done something improperly and may be unwilling to admit their error. This is also a phenomenon that come across by several growers that sometimes some untoward happening there takes place in crop fields and it is the role of the diagnostician to establish the reason behind it because the person who has committed the mistake is not willing to admit it for the various reasons. Then before going for final diagnosis we need to go for testing of a hypothesis.

So diagnosis can be related to testing of a hypothesis and the hypotheses are generated through observations of the plant, environment and information from the grower. When all information is sources could be consulted to determine already known causes of the disease and disease causing agents. So diagnosis of plant problems is like a detective investigating an assault on the plant species. All clues should be investigated and compilation of the clues ultimately lead to the most accurate diagnosis. Some of the common symptoms may be produced by different types of problems and detailed symptoms and science need to be studied during the diagnostic process for proper diagnosis. So it is simply just like an investigation taking on a crime that has happened in a society the tips and clues or information that is collected from the crime site or from the crime location is very crucial for establishing the disease causing agent.So initial steps for is to look for signs or biotic causal agents like for example in case of Amillaria infection we can look for white mycelia present in the bark or trees like peach trees, then in case of rust pathogens we can see the stem rust on pustules on wheat plants, then on powdery mildews we can see white powdery masks on the plant structures, then in case of bacterial pathogens along with the symptoms we can go for testing of ooze test that can help us to establish the pathogen associated with the bacterial diseases. So these are the some of the initial steps that we need to look for along with cultural and environmental conditions or information that needs to be gathered before going for final diagnosis. Then initial observations needs to be finally validated or confirmed under laboratory conditions where, the plant material is first incubated in the laboratory, then isolation and anti fixation of biotic plant disease causal agents are done. Then Diagnostics tests for identification of biotic causal agents are done. First we can go for traditional methods and then we also go for modern methods because they are time-saving and they are more accurate and we can go for both traditional and modern methods for establishing the causal agents and diagnostic takes for identification of abiotic disease causal agents is also required to be negated or established in relation to the cases it may be. Modern disease diagnostic methods can be categorized into two methods – the direct method, the indirect method.

In case of direct method we can go for Serological methods like ELIZA, Immunofluorescence (IF) Flow Cytometry Whereas in case of molecular methods we can go for Fluorescent In Situ Hybridization (FISH) method. Polymerase Chain Reaction method (PCR) DNA Arrays. In case of indirect methods we can go for Bio-marker based methods such as Gaseous metabolite profiling, Plant metabolite profiling. Then we can go for plant properties or stress based analysis that is Imaging techniques or Spectroscopic techniques. So depending on the availability of tools and techniques or in necessity of the tools and techniques we can go for either direct or indirect method of disease diagnosis utilizing various molecular or biomarker for establishing the disease-causing agent. So in this particular talk we have seen that what are the initial steps we need to do. That is first to go for observation of science and symptoms, then we also need to understand the information about the cultural practices it has been adopted, the environmental conditions that has been a face exposed by into the plant system and that’s how we can correlate the initial investigations and finally we can go for laboratory methods for confirmation of the initial observation through adopting different methods. So with this we conclude our today’s talk and in the next talk we’ll be talking about the Quarantine pathogens detection and it’s necessity and till then we will see you again in the next talk.

Thank you very much.