A new race of Bremia Lactucae, BL: 37EU identified and named in Europe

In 2019 and 2020, the International Bremia Evaluation Board Europe (IBEB-EU) collected more than 500 Bremia isolates from European lettuce. Most of these isolates are local variants. However, according to a press release, a particular isolate, which initially spread mainly in France, has recently been detected in Spain, Portugal and Italy. 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 stresses the importance of combining chemical control and hygiene measures with the use of resistant varieties to prevent the emergence of new races.

Bremia lactucaethe causative agent of lettuce downy mildew, is genetically highly variable. Even within the same lettuce production field, several races may be present.

The IBEB-UE met 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, occurring in 20% of samples. More than 60% of isolates showed novel virulence profiles. Most of these novel profiles were found only once or a few times in the same region, suggesting outbreaks of likely local importance. However, variants with a particular virulence profile, which had been detected in previous years, showed signs of spreading as a pandemic over a growing area. This virulence profile was present in 20% of the 500 isolates tested.

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

The statement states that the mentioned sextet code is based on an adjusted differential set. The new official differential set is now set D. Set C will no longer be used. In the new set D, three new differentials are added in the third sextet group on positions 4, 5 and 6: Fenston, Bataille and RYZ20007. More information on the differential set and sextet codes can be found on the ISF website: http://www.worldseed.org/our-work/plant-health/other-initiatives/ibeb/

The Committee also stresses the importance of combining chemical control and hygiene measures with plant resistance. Application of fungicides, particularly at the young plant stage, provides additional protection to resistant lettuce crops, helping to prevent the emergence of new races of Bremia. Proper hygiene practices, such as removal of debris and diseased plants, cleaning of farm equipment and prevention of prolonged leaf wetness, will reduce the spread of Bremia in lettuce crops.

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

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