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 France and the Netherlands, 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 European lettuce industry and promote the use of commonly accepted race names in communication with growers.
To this end IBEB maintains a well-defined and internationally agreed system of race denomination. Companies in IBEB provide information on the genetic background of lettuce varieties while Naktuinbouw and GEVES serve as independent testing facilities to streamline 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 annually.
Differentials and races
IBEB decides on the definition of a set of host differentials with distinct 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 (Van Ettekoven and Van der Arend, 1999) and denominated isolates have the prefix “Bl:” followed by a space and the race number. All denominated 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 this nomenclature of American races replaces the use of a Roman race pathotype number. The numbering of races for the two continents is independent; contemporary race denominations are currently less than 10 for the USA and over 15 for Europe.
The EU-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. For example, the first sextet code of Bl: 27 is 47 because 1 + 2 + 4 + 8 + 32 = 47. The virulence pattern of Bl: 27 on the EU-C set is completely described by the codes of the three sextets 47-38-00.
Some races are omitted in Table 2. The races 1-15 are no longer supported by IBEB, 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.
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 5381 isolates (Table 3) were tested in the period 2002-2016, mostly from the northwestern part of Europe. All these isolates were tested on the differential set in order to investigate the novelty of their reaction pattern.
When a specific reaction pattern appears repeatedly in several countries and over several years, IBEB identifies isolates with this pattern as a threat to the lettuce industry, defines a new race and denominates an isolate for this new race. In 1999 – the second year of IBEB’s existence – four new isolates were denominated. Since then, one isolate was denominated every irregularly, every one or two years, depending on the outcome of the annual discussion about the recently collected isolates collected.
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. In the EU DUS tests are executed according to the rules and 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 and UPOV Technical Guideline TG/13/10, and on information about revisions that are currently submitted.
Validated differentials and isolates may be requested at Naktuinbouw in the Netherlands or GEVES in France by Bremia workers all over the world. IBEB expects that users of the differential set would be 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.
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.