[intro]<\/p>\n
Bremia lactucae<\/em>, the causal organism of downy mildew in lettuce, is a major threat to lettuce production. Farmers often need to use both fungicides and resistance genes to prevent heavy losses. Reliable information about resistances in relation to the local strains of Bremia<\/em> is essential for a successful and durable disease control strategy. [content2cols index=”1″] IBEB decides on the composition of a set of host differential lines with distinct diagnostic major resistance genes (see Table 1) and on the denomination of new races of Bremia lactucae<\/em> (see Table 2). A binary coding system for the description of reaction patterns is used and denominated All isolates have the prefix \u201cBl:\u201d followed by a space and the race number: European isolates have the suffix \u201cEU\u201d and American isolates have the suffix \u201cUS\u201d. Note that with this nomenclature for American races the prefix \u201cCA\u201d is replaced by the suffix \u201cUS\u201d and no longer Roman numbers are being used. The numbering of for the two continents is independent; contemporary race denominations are currently less than 10 for the USA and over 15 for Europe.<\/p>\n Table 1. Differential sets to characterize isolates of Bremia lactucae<\/em> as defined by IBEB<\/a> The set of differential lines used to test the virulence of isolates of Bremia lactucae has evolved over time as more resistance genes become available and others have become non-diagnostic. The IBEB C set of 15 differential varieties consists of two groups of six varieties (sextets) and one group of three varieties. The position of a differential within the group determines the sextet value of that differential. Sextet values are ascending powers of 2 (1, 2, 4, 8, 16 or 32). The sextet code of an isolate is the sum of the sextet values of the differentials that are susceptible, as indicated by + or (+) in the table. Archive: Reactions of Bremia<\/em> races Bl:1-31EU to the B set of differentials and of Bl: 1-8US to the A and B set of differentials<\/a><\/p>\n [separator-plain]<\/p>\n A common observation scale is needed for harmonization between labs. In 2015, a common scale was developed during a workshop at GEVES. A summary is given in Figure 1. This scale is the basis for revisions of UPOV and CPVO protocols.<\/p>\n Figure 1. Observation scale for the interpretation of Bremia<\/em> reaction phenotypes on lettuce seedlings<\/a><\/p>\n [separator-plain]<\/p>\n Thousands of isolates have been collected by Bremia<\/em> workers whenever or wherever there was a reason to expect the presence of a novel or resistance-breaking race. A total of 6530 isolates (Table 3) were tested in the period 2002-2018, mostly from the northwestern part of Europe. Isolates also have continously been tested in the western US since 1982; their reactions can be viewed at http:\/\/bremia.ucdavis.edu\/<\/a>. All these isolates were tested on the differential set in order to characterize their reaction pattern and to identify any novel ones.<\/p>\n Table 3. Number of isolates obtained and analysed by IBEB in the period 2002-2018<\/a> When a specific reaction pattern appears repeatedly in several regions and over several years, IBEB identifies isolates with this reaction pattern as a threat to the lettuce industry, defines a new race and selects an isolate as the representative for this new race. Besides the occurrence in several regions and years, other elements play a role in the decision process to nominate a new race. It should overcome the most used resistance genes and the combination of genes. Preferably it is found in several seasons. The isolate should be stable in reproducing and give robust results. One or more races are denominated irregularly as and when necessary, depending on the outcome of the annual review of the threat represented by recently collected isolates.<\/p>\n IBEB presents the denomination of a new race to the press. Press publications can be found at https:\/\/www.plantum.nl\/pers<\/a> with \u2018IBEB\u2019 in the title of the publication IBEB\u2019s activities have gained wide acceptance and have been the basis for important updates in the regulatory framework for variety description defined by CPVO (the Community Plant Variety Office) within Europe and globally by UPOV (the International Union for the Protection of New Varieties of Plants). A test on Distinctness, Uniformity and Stability (DUS) is performed to get a variety listed on the EU common catalogue or to have it protected by Plant Breeders\u2019 Rights anywhere. In the EU DUS tests are executed according to the protocols of the CPVO. In non-EU countries, UPOV guidelines are used. A summary table of obligatory and facultative isolates for variety registration is given in Table 4. This table is based on CPVO Technical Protocol TP013\/5 rev. 2 and UPOV Technical Guideline TG\/13\/10 rev. 2, and on information about revisions that are currently submitted.<\/p>\n Table 4. Present status and provisional status updates of Bremia isolates in CPVO and UPOV protocols for DUS tests.<\/a> Validated differentials and isolates may be requested at Naktuinbouw in the Netherlands (Resistentie<\/a>) or GEVES in France (MATREF<\/a>) by Bremia<\/em> workers all over the world. Bremia<\/em> workers from the US may contact Enza USA (Nicki Phillips<\/a>) or Rijk Zwaan USA (Ryan McMillen<\/a>) for seeds of the differential set or UC-Davis (Richard Michelmore<\/a>) for isolates. IBEB expects that users of the differential set are willing to contribute to the ongoing battle with Bremia<\/em> by sharing their data with IBEB.<\/p>\n
\n[separator-plain]<\/p>\nA joint initiative<\/h2>\n
\n[separator-plain]<\/p>\nDifferentials and races<\/h2>\n
\nTable 2. Reactions of Bremia races Bl: 5-9US and Bl: 16-36EU to the IBEB C set of differentials<\/a><\/p>\n
\nSome races are omitted in Table 2. The races Bl:1-15EU are no longer supported by IBEB-EU, because they are no longer important in the field. The other omitted races were either lost or do not give consistent results in seedling tests. Similarly, Bl: 1-4US are no longer detected in the western US and therefore not considered important.<\/p>\nObservation Scale<\/h2>\n
Monitoring<\/h2>\n
\n[separator-plain]<\/p>\nRace denomination<\/h2>\n
\n[separator-plain]<\/p>\nThe significance of IBEB’s work<\/h2>\n
\n[separator-plain]<\/p>\nServices<\/h2>\n