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• Biology and control of Bremia lactucae
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The International Bremia Evaluation Board (IBEB)

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. IBEB has denominated a total of 12 new isolates in the period from 1999 to 2011 starting with Bl: 17.

The EU-B set of 24 differential varieties consists of four groups of six varieties (sextets) (see Table 3). The position of a differential within the sextet 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 63 because all differentials are susceptible and 1 + 2 + 4 + 8 + 16 + 32 = 63, and the fourth sextet code is 1 + 2 + 16 = 19. The virulence pattern of Bl:27 on the EU-B set is completely described by the codes of the four sextets as 63-63-13-19.

Note that race 8 and 9 do not exist.

A table with important isolates described outside the scope of IBEB is presented below.

Monitoring

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 4331 isolates (Table 5) were tested in the period 2002-2012, mostly from the northwestern part of Europe (Figure 1). All these isolates were tested on the differential set in order to confirm the novelty of their reaction pattern. 

Race denomination

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. One isolate was denominated in the following years: 2000, 2001, 2002, 2004, 2008, 2010 and 2011.

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 6.

Services

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.

References

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.

Disclaimer

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. 

ISF World Seed Congress

The ISF World Seed Congress 2013 will take place in Athens (GR) on 27-29 May 2013.

Registration closes on
9 May 2013 at 13.00 h GMT. More information at www.worldseed2013.com.

ISF Secretariat - Chemin du Reposoir 7 - 1260 Nyon Switzerland

tel +41 22 365 44 20 - fax +41 22 365 44 21 - isf@worldseed.org