The International Bremia Evaluation Board Europe (IBEB-EU) has collected more than 300 Bremia isolates in European lettuce in 2018 and compared them with the 500 Isolates found in 2017. Most isolates in 2018 belonged to local races. However, one race already spotted in 2016 and 2017 appeared to spread over large distances and was isolated in various centres of lettuce production. The IBEB-EU decided to denominate this isolate as new race Bl:36EU. The Board emphasizes the importance of chemical control and hygiene measures in addition to the use of resistant varieties to prevent the development 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 met in November 2018 in Paris to evaluate a total of 800 Bremia lactucae isolates found in Europe in 2017 and 2018. In 2018 the most widely spread races were the 3 recently denominated races Bl: 33EU, Bl:34EU and Bl:35EU. These 3 races were isolated in 27 % of the samples. More than 50 % of the isolates encountered in 2017 and 2018, were new races. Most of these new races were isolated only once or a few times in the same region, indicating epidemic outbreaks with probably only local significance. However, one isolate already found in previous years appeared to spread as a pandemic over an increasing area. This race represented 16 % of the isolates.
A formal evaluation process started in November 2018. In April 2019 the IBEB-EU concluded that the selected candidate isolate gave consistent test results.
This isolate is denominated as the new race Bl: 36EU (EU-C sextet code: 55-15-01; DE1620). Bl:36EU is widely spread in Europe and is found a.o. in France, Germany, the Netherlands, Italy, Austria, Switzerland and Scandinavia. This race maybe described as a more virulent variant of Bl:34EU, able to break one additional resistance source.
Occasionally, isolates with only regional significance are causing serious problems. This type of isolate diversity complicates the official denomination of all important Bremia isolates. The trend towards more local epidemics may be related to the successful introduction of an increasingly wide range of resistance genes by breeding companies. Therefore, the board emphasizes that, although breeding companies will supply growers with lettuce varieties possessing resistance to the nominated Bl:16-36EU races, this is not a full insurance against Bremia.
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, cleaning of farm equipment and prevention of prolonged periods of leaf wetness, will reduce the spread of Bremia in lettuce crops.
Information on the differential set and sextet codes is available through the ISF website: http://www.worldseed.org/our-work/plant-health/other-initiatives/ibeb/
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).
Tests from Enza Zaden confirm that a large part of their lettuce portfolio is resistant to this new race. Many of their varieties even are resistant to the full range Bl:16-36EU. They have created a leaflet with an overview of all their lettuce varieties that have this new Bremia resistances. Also, the production section on the breeder's local websites is updated with this new information.
Gerhard Voelkel, Bejo’s Crop Manager for lettuce, says: “We are happy that all our lettuce varieties with resistance Bl:16-35EU, have been proven to be resistant to Bl:16-36EU. Our assortment in Iceberg, Batavia, Butterhead and Babyleaf in general demonstrates strong resistances. We will continue breeding for the best possible Bremia resistances as a main focus for the next generation of lettuce varieties.”
Rijk Zwaan’s lettuce program has a range of varieties resistant to Bl:36EU, other official strains, and in many cases to other local isolates. For the most up to date information on varietal performance and downy mildew, they recommend that growers speak to their local sales representative.
Rijk Zwaan advocates an integrated approach to disease control; resistant varieties are a large part of this, but crop protection agents and hygiene measures are also important to help prevent the development of new downy mildew races. Proper hygiene practices, such as removal of debris and diseased plants, will reduce the spread of downy mildew in crops.