4 Identification and Quantification of Airborne Inoculum

Transcript

Hello! Welcome to the fourth talk of this week and that is Diagnosis of plant pathogens of quantiryl implications. In the previous talk we have seen the procedures of plant disease diagnosis and today we will be talking about the diagnosis of plat pathogens that have quarantine significance. So, quarantine significance is that, if the pathogen is new to the particular location or to a country then we need to check its spread over a specific geographical location and this is very important for a management of the plant health because of the probable damage it can cause. So for that reassessment is very important and after assessing the risk of the causal organism that can cause certain harm to the plants we here can prevent its introduction and further spread of organisms to other geographical locations. So proper diagnosis and detections are critical for the categorization and assessment of pests that are or may qualify as quarantine pests, and for appropriate application of phytosanitary measures. So, reassessment is very essential from this point of view.The plant health legislation has both National as well as International regulation framework. The International plant health regulatory framework is set by Sanitary and Phytosanitary (SPS) Agreement of the World Trade Organization (WTO). Member countries have right to take sanitary and phytosanitary measures based on scientific principles. Phytosanitary measures shall be based on International standards. Our guidelines and recommendations developed by FAO Secretariat of the International Plant Protection Convention that is in short known as (IPPC). The scientific principles Phytosanitary measures are based on risk assessment, which includes the evaluation of the likelihood of entry, establishment or spread of a pest or disease within the territory of an importing country. The plant health legislation under the framework of International regulation is very very essential for checking and prevention of a particular disease under the basis of risk assessment.

Diagnosis of Plant Pathogens and Plant Health Regulations – so what is the need of diagnostics, because pest identification is key in the process of pest risk analysis. Pest risk analysis (PRA) consists of three stages: initiating the process for analyzing risk, assessing pest risk and managing pest risk. So these are the three basic stages which are important for pest risk analysis. Initiating the process involves identification of pest as that may qualify quarantine pest or of pathways for which risk analysis is needed, either of these two starting points can involve pests already present in pest risk analysis (PRA) area not widely distributed and under official control as well pest absent from the pest risk area, since both are covered by the quarantine pest definition. So it is important that quarantine pathogen whether it has entered to a particular location or not it is need to be established whether it has already entered to a particular location then it needs to be checked for further spreading on to a new areas. So pest risk assessment determines whether each pest identified or associated with a pathway is a quarantine pest. So, it is very important to establish whether a pest that has entered to a new location is referred to be quarantine pest or not, because certain other species or related species may be either present on that particular locality or it may not have enough risk to the crops and that may not be the criteria therefore to be qualified as a quarantine pest. So pest risk management involves developing, evaluating, comparing and selecting options for reducing the risk. So pest risk management simply involves developing of and even evaluating, comparing and selecting viable options for reducing the risk. Proper detection and identification is critical for the appropriate application of phytosanitary measures. In addition, diagnostic procedures are needed for determination of pest status in a area, pest reporting and the diagnosis of pest in imported consignments.

Problems for Risk Assessment Posed by Taxonomic Changes: Risk assessment in plant health is challenged by the changes in taxonomy and nomenclature that have taken place in the pathogen. It’s a continuous process and we have seen that certain pest or pathogens that were taxonomically placed in other position earlier now has been moved from that particular taxonomic position to a newer position and this has posed a great challenge for the analysis of pest risk because the name or species or the name of the organism is changed from what it is reported earlier. This leads to a reevaluation of pest records and doubts as to whether a regulated organism is actually the one of concern. So this double extra work as we need to reevaluate whether the pest that we are reporting is already reported in that area as a quarantine pest or not so, it is needed because of the changes in the taxonomic position of the particular biologic organism. Pathogens that were previously considered strains or varieties of a single species, have now in some cases been elevated to specific species status. So those varieties or strains that were considered of a particular species has now been transferred to or named and placed in a new species. So, this information is very important for establishing the new pest as a quarantine pest. Further, pathogens recognised as species complexes have been partitioned into new species using molecular methods. So, with the advancement in molecular method it is now possible that some of the pest that were basically or were originally considered to be species complex is now moved to a new taxonomy species and for that molecular tool has been of great help.

Let us consider some of the examples, say for example Phytophthora Diseases. A pathogen Phytophthora ramorum, it was considered exotic in North America and European nurseries which is responsible for sudden oak disease (SOD) in both USA and European countries. The species had never previously been described in either continent and the distribution was either geographically limited to California of the United States , or clearly associated with the nursery trade in Europe. The high susceptibility of hosts in natural settings and the presence of different mating types in the two continents have made it a big confusion from where this particular Phytophthora pathogen has arrived and further investigations from California forest populations which was able to distinguish from the European nursery isolates and all together three genetically distinct lineages of Phytophthora ramorum was identified and it was found that the North American isolates were distinct from the European isolates and it was also observed that the entry of the pathogen has taken place into the new location through multiple times of trading not only a single time, that is how new strains had been reached into the new location.

Then considering the bacterial diseases for example Erwinia chrysanthemi, it was first included in the genus Erwinia as a pathogen of chrysanthemum, but it was found to be in affective in a variety of plants and that is why 1984 six pathovars has been created that is the dianthicola, zeae, chrysanthemi, parthenii, paradisiaca and dieffenbachiae according to their host specificity.

Futher in 1998 Erwinia chrysanthemi was moved to the new genes Pectobacterium based on 16S analysis. And in 2005 using 16S DNA-DNA hybridisation and biochemistry Pectobacterium chrysanthemi was moved into a new genus called Dickeya, which comprises 6 species , namely dianthicola, dadantii, zeae, chrysanthemi, paradisiaca and dieffenbachiae. So, here we can see the progression of changes of a bacterial genus and species overtime based on different tools and techniques used or differentiating it from the existing ones. so in 2011, the species Dickeya dieffenbachiae was moved to within the species Dickeya dadantii. And recent analysis we will get the presence of a number of potential in new Dickeya species i.e Dickeya solani, but to date none of these has been officially recognised. So Dickeya species are soft rotting pathogens that causes disease primarily through production of various plant cell wall-degrading enzymes. And description of Dickeya dianthicola follows Pectobacterium parthenii-dianthicola, described on Dianthus species and strains belong to Pectobacterium chrysanthemi biovars 1,7 and 9.

According to several phylogenetic studies including 16S, recA and dnaX sequence analysis, DNA-DNA hybridisation and REP-PCR, Dickeya dianthicola is now considered to be the most closely related to Dickeya dadantii and exhibits little diversity between strains, with no obvious delineation between isolates from different host plants. So we have seen very clear here that how a particular bacterial species has been changed or moved from one particular species to multiple species then t the new genus and then how the new genus is now again been correlated to other existing Dickeya species and probably with time will be able to establish a proper specific identification of this particular pathogens particularly in bacterial pathogen this is causing a big problem.

Similarly, incase of Nematodes the Potato cyst nematodes (PCN), like Globodera rostochiensis and Globodera pallida they are considered to be threats to world potato production and they are able to cause approximately 75% of the total potato ill globally under severe form. So diagnostic of Globodera species based on morphological characteristics of cyst and juveniles is time-consuming and requires specialized taxonomic expertise. Further, visual identification is not always unambiguous and it leads to errors. So biochemical techniques were developed to separate Globodera rostochiensis and Globodera pallida. It distinguish these two species properly as it was not able to do with visual observations. Investigations on differences between European and South America populations of PCN were based on biological and molecular studies and PCN present in Europe represent only a minor subset of the full biological diversity present in South America and that the range of virulence present in South America is far greater than that present in European PCN populations. So, although it’s a global pathogen the biological diversity shows that the South American strains are very highly build in comparison to the European population and the European population represent only a subset of the total biological diversity of the nematode pest that is present in the South America. So with this we have seen that in today’s talk that how quarantine pest is important and how risk assessment is need to be done to establish a pest or pathogen to be a quarantine pest and why it is important to check in a particular location and what are the challenges that is associated with categorizing a particular pathogen to be a quarantine pathogen and one of the major hurdle is due to the taxonomic variations that is taking place with time and that is why it is causing more problem to establishing the newer species to a quarantine pathogen with this challenges.

So, in the next talk will be talking about the detection and diagnostic services that are available for helping the farmers and associated growers and we will be talking in detail of the services available at National global and in local level and till then will be having a good time. Thank You very much we will meet again on the next talk.

Thank You.