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Distribution Maps of Plant Diseases

A leading resource for worldwide distribution data of plant diseases of economic or quarantine importance

Producing the Maps

Behind the simple appearance of a map, and its associated references, lies a complex and detailed piece of research. The aim is simple, to ensure that the map that is published, contains the best and most accurate assessment of the known distribution of a particular organism. Our approach is described in more detail below.

The general criteria for species selection are: existence or currency of maps of pests of economic or phytosanitary importance, recent reports of spread, new proposals for phytosanitary categorization, feedback from customers, revised taxonomy which may render old map editions invalid, current interest requiring separate maps for subspecies, coverage in other CABI/EPPO programmes, and various topical reasons. Validators (see below) are sent provisional species lists for comment and to assist final selection.

Collecting the data, checking sources and ensuring geographical accuracy

For each species selected for mapping, specially designed distribution map editing software is used to extract from CAB Abstracts a set of records containing geographic terms for the given species name and its synonyms. The abstracts are sorted by country and the most appropriate selected for inclusion on the map. Full papers are consulted as required. Notes and sources can be added and subnational distribution decided for large countries. A comprehensive range of additional sources is also searched, including material from EPPO, and records are added as appropriate.

In order to produce an accurate distribution map, the precise taxonomic identity and synonymy of each pest must be understood in order to target literature searches as accurately as possible. Expert taxonomic input is also important at the verification stage, to ensure that only literature based on accurate identifications is included in the distribution data. Only when the specific limits of an organism have been ascertained can there be certainty as to its presence in a defined area.

The area where a pest is present was originally indicated on the Distribution Maps by one or more continuous solid lines demarcating areas of known presence, with broken lines used occasionally to indicate uncertain limits to the distribution. These lines would divide a country if subnational information was available. However, for phytosanitary purposes relating to international trade, it is national level political units that are important. It was thus decided in 1996 to change the map style to note presence as discrete points and shading, by country, with subnational distribution points as appropriate.

Assuring the accuracy of individual records is a key role of the mapping process. Preferably, records should be from primary sources, published in peer-reviewed publications, and from persons or authorities free from political or commercial bias. These preferences cannot always be reconciled. For example: The results of official surveys and diagnoses are mostly not published in peer-reviewed journals; Articles in peer-reviewed journals are not necessarily examined closely for the validity of geographical distribution information; Official reports from countries (of presence or absence) often take the form simply of a signed letter, which is not ideal, but they are a valuable source which must in principle be taken into account.

Quite often, records have to be taken from reviews, which are only secondary sources and may be based on unsubstantiated or unreliable primary sources. In a few cases, where other sources are lacking, unpublished information from a scientist of known and respected international reputation may be used. Every effort is made to ensure that the maps are based on the best and most reliable data available.

Expert validation of the data

All draft maps are sent to expert validators for checking. The validators receive: the distribution list with references, the printed map, a validation sheet with queries that may have arisen during compilation, the previous map if the map is a revision, and abstracts, web pages or other information if deemed necessary. Coordination and validation of most Pest Maps (arthropods) is presently undertaken at the Natural History Museum (NHM), London, of most Disease Maps (fungi, bacteria and nematodes) by CABI Bioservices and of viruses by independent consultants. Specimen records from CABI, NHM or national collections are included where possible. The coordinator incorporates all validators’ comments and additional country records, and conducts any further searches or enquiries (to national official contacts) as required. Second map drafts are sent to EPPO for final validation. EPPO’s role includes: (1) providing and checking records from the PQR database which are often derived from complementary sources to the published literature records compiled at CABI; (2) confirming the status of quarantine pests for, or in, Europe; (3) addressing the phytosanitary implications of new records; and (4) using the EPPO network of contacts to resolve queries.

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