Seed World

A New Race of Bremia Lactucae, BL: 37EU Identified and Denominated in Europe

The International Bremia Evaluation Board Europe (IBEB-EU) collected over 500 Bremia isolates from European lettuce in 2019 and 2020. Most of these isolates are local variants. However, one particular isolate, which initially spread predominantly in France, has recently been detected in Spain, Portugal, and Italy, according to a press release. This variant is expected to spread to other major lettuce production areas in Europe. The IBEB-EU has designated this variant as race Bl: 37EU. The Board emphasizes the importance of combining chemical control and hygiene measures with the use of resistant varieties to prevent the emergence of new races.

Bremia lactucae, the causal agent of downy mildew in lettuce, is genetically very variable. Even within one lettuce production field, several races may be present.

The IBEB-EU convened in November 2020 to evaluate 500 Bremia lactucae isolates collected across Europe in 2019 and 2020. The most prevalent races identified were Bl: 34EU, Bl: 35EU, and Bl: 36EU, appearing in 20% of the samples. Over 60% of the isolates exhibited new virulence patterns. Most of these new patterns were found only once or a few times within the same region, suggesting outbreaks of likely local significance. However, variants with one particular virulence pattern, which had been detected in previous years, showed signs of spreading as a pandemic over a growing area. This virulence pattern was present in 20% of the 500 isolates tested.

A formal evaluation process started in November 2020. In April 2021, the IBEB-EU concluded that the selected type of isolate gave consistent test results. The IBEB-EU decided to denominate this variant as race Bl: 37EU. The type of isolate of the new race Bl: 37EU is FR19064. The sextet code is D 46-15-14.

The release notes that the mentioned sextet code is based on an adjusted differential set. The new official differential set is now the D-set. The C-set will not be used anymore.  In the new D-set three new differentials are added in the third sextet group on position 4, 5 and 6: Fenston, Bataille and RYZ20007. Further information on the differential set and sextet codes is available through the ISF website:

The board also highlights the importance of combining chemical control and hygiene measures with plant resistance. Fungicide application, especially at the young plant stage, offers additional protection to resistant lettuce crops, helping to prevent new Bremia races. Proper hygiene practices, such as removing debris and diseased plants, cleaning farm equipment, and preventing prolonged leaf wetness, will reduce Bremia spread in lettuce crops.

IBEB-EU consists of representatives of the Dutch and French seed business associations Plantum NL and UFS, and the independent organisations GEVES/SNES and Naktuinbouw. IBEB is supported by several Bremia researchers across Europe. Lettuce breeders of Bejo, Enza Zaden, GAUTIER Semences , BASF Vegetable Seeds, Rijk Zwaan, Bayer, Syngenta and Vilmorin represent the business associations.

All denominated isolates and seeds of the differential set are available at GEVES/SNES (France) and at Naktuinbouw (The Netherlands).