Bremia lactucae, or downy mildew in lettuce, is genetically very variable. Even within one lettuce production field several races may be present. In November 2014, the International Bremia Evaluation Board (IBEB) met in Paris to evaluate the Bremia lactucae isolates found in Europe in 2014. A total of 411 isolates were on the gross list and discussed. About 62 Isolates (16%) of all 411 concerned were known and already nominated Bl: races. Bl:29 was mostly found. This race is widely spread over Europe. Also Bl:24, Bl:30 and Bl:31 were found (“Bl:” is the official code for races of Bremia lactucae).
Most of the isolates encountered in 2014 were not found before or were only locally spread. However, two isolates were known from earlier years and appeared to have spread more widely. They became candidate for nomination and a formal evaluation process started in November 2014. In April 2015 IBEB concluded that one candidate, although widely spread across Europe, gave inconsistent test results making it impossible to nominate.
The 2nd isolate, with sextet code 61-31-03-00, gave consistent results and is nominated as Bl:32.
Bl:32 was found in the past few years in France, Germany, the Netherlands, UK, Austria, Switzerland , and Belgium. Recently it also spread to Portugal and North Spain.
The board notices that several local isolates, next to the nominated Bl:32, are causing serious problems in some regions in Europe. However, a clear trend is visible where local isolates appear which spread less fast than races nominated in the past as a result of the successful introduction of an increasingly wide range of resistance genes by breeding companies. This isolate diversity increasingly complicates the official nomination of European Bremia races.
The board emphasizes that, although breeding companies supply growers with lettuce varieties possessing resistance to the nominated Bl: 16-32 races, this is not a full insurance against Bremia. Occasionally problems may occur due to a local new isolate that does not survive over years and does not spread to other areas.
The board also emphasizes the importance of chemical control and hygiene measures in addition to plant resistance. Fungicide application, especially in a young plant stage, gives additional protection to resistant lettuce crops, which will help prevent the development of new Bremia races. Proper hygiene practices, such as removal of debris and diseased plants, will reduce the spread of Bremia in lettuce crops.
IBEB consists of representatives of the Dutch and French seed business associations Plantum NL and UFS, and the organisations of GEVES and Naktuinbouw. IBEB is supported by several Bremia investigators across Europe. The business associations were represented by lettuce breeders of Agrisemen, Enza Zaden, Gautier, Bayer CropScience Vegetable Seeds, Rijk Zwaan, Monsanto Seminis, Syngenta and Vilmorin.
For testing and breeding purposes all nominated races and the EU-B determination variety set are available at GEVES/SNES (France) and at Naktuinbouw (The Netherlands).
