Bremia: a common enemy
Bremia lactucae, 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 is essential for a successful and durable disease control strategy.
A joint initiative
The International Bremia Evaluation Board (IBEB) is a joint initiative of lettuce breeding companies in USA, France and the Netherlands, University of California-Davis, the Dutch inspection service (Naktuinbouw) and the French National Seed Station (GEVES). IBEB’s mission is to identify new races of Bremia lactucae that pose a significant threat to the American and European lettuce industry and promote the use of standardized race names in communications with growers.
To this end IBEB utilizes a well-defined and internationally agreed system of race denomination. Companies in IBEB provide information on the genetic background of lettuce varieties while UC-Davis, Naktuinbouw and GEVES serve as independent testing facilities to facilitate the process of identifying new races of Bremia lactucae. Information about the evolution of the pathogen in response to resistance in lettuce varieties is updated regularly.
Because there is little movement of Bremia between the USA and Europe, race nomination specific to each continent is done by regional IBEB committees: IBEB-US for the USA, and IBEB-EU for Europe. A global coordinating body (IBEB-G) coordinates the use of a common set of host differentials, common test protocols, and communications on Bremia races. IBEB-G also advises on scientific denomination of new Bremia resistance genes.
Differentials and races
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 (see Table 2). A binary coding system for the description of reaction patterns is used and denominated All isolates have the prefix “Bl:” followed by a space and the race number: European isolates have the suffix “EU” and American isolates have the suffix “US”. Note that with this nomenclature for American races the prefix “CA” is replaced by the suffix “US” 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.
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.
Some 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.
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.
Thousands of isolates have been collected by Bremia workers whenever or wherever there was a reason to expect the presence of a novel or resistance-breaking race. A total of 5770 isolates (Table 3) were tested in the period 2002-2016, mostly from the northwestern part of Europe. A total of 2042 isolates have been tested in the western US since 1982; their reactions can be viewed at http://bremia.ucdavis.edu/. All these isolates were tested on the differential set in order to characterize their reaction pattern and to identify any novel ones.
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 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.
Significance of IBEB’s work
IBEB’s 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’ 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.
Validated differentials and isolates may be requested at Naktuinbouw in the Netherlands (Resistentie) or GEVES in France (MATREF) by Bremia workers all over the world. Bremia workers from US may contact Enza USA (Nicki Phillips) or Rijk Zwaan USA (Ryan McMillen) for seeds of the differential set or UC-Davis (Richard Michelmore) for isolates. IBEB expects that users of the differential set are willing to contribute to the ongoing battle with Bremia by sharing their data with IBEB.
Van Ettekoven, C. and A. van der Arend (1999) Identification and denomination of “new” races of Bremia lactucae. Pages 105-107 in Proceedings of Eucarpia meeting on Leafy Vegetables Genetics and Breeding. Olomuc, Czech Republic.
Maisonneuve, B. (2011) Amélioration des hôtes différentiels Bremia : obtention de lignées de laitue à résistance monogénique. Innovations Agronomiques, 15, 9-14.
Smilde, D. and A. van Dijk-Veldhuizen (2015) A streamlined differential set for Bremia lactucae. In: Proceedings of Eucarpia meeting on Leafy Vegetables, 14-17 April 2015, Murcia, Spain.
Perrot, S., M. Buffard and V. Grimault (2015) European harmonization of evaluation of resistance of lettuce to Bremia Lactucae. In: Proceedings Eucarpia Leafy Vegetables Congress, 14-17 April 2015, Murcia, Spain.
Parra, L., Maisonneuve, B., Lebeda, A., Schut, J., Christopoulou, M., Jeuken, M., McHale, L., Truco, M-J, Crute, I. and Michelmore, R.(2016) Rationalization of genes for resistance to Bremia lactucae in lettuce. Euphytica August 2016, Volume 210, Issue 3, pp 309–326.
Descriptions, recommendations and illustrations are based as closely as possible on experiences in tests and in practice. IBEB and ISF accept no liability on the use of this information.