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 2020 to evaluate a total of 500 Bremia lactucae isolates found in Europe in 2019 and 2020. The most widely spread races are the recently nominated Bl: 34EU, Bl: 35EU and Bl: 36EU. These three appear in 20 % of the samples. More than 60 % of the isolates had new virulence patterns. Most of these new patterns were isolated only once
or a few times in the same region, indicating outbreaks with probably only local significance.
However, variants with one particular virulence pattern, which were already found in previous years, appeared to spread as a pandemic over an increasing area. This virulence pattern was found 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 isolate gave consistent test results. The IBEB-EU decided to denominate this variant as race Bl: 37EU.
The type isolate of the new race Bl: 37EU is FR19064. The sextet code is D 46-15-14.
New code sets
Please note 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.
Bl: 37EU is widely spread in France but is also present in Spain, Portugal and Italy. This race has not yet been found in the northern European centers of lettuce production. This race may be described as a more virulent variant of Bl: 36EU, as it is able to break one additional resistance factor.
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: 16-37EU 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.
Response of Enza Zaden:
"The IBEB-EU recently announced a new race of Bremia, BI:37EU. Our tests confirm that a large part of the Enza Zaden lettuce portfolio is resistant to this new race. Many of our varieties even are resistant to the full range Bl:16-37EU. We have created a leaflet with an overview of all our lettuce varieties that have this new Bremia resistances. Also, the product section on our local websites is updated with this new information."
Response of Bejo:
Gerhard Voelkel, Bejo’s Crop Manager for lettuce, says: “Our current varieties, which are already on the market for over five years, are resistant against this new Bremia race. We are confident that the reliability of our varieties offers growers the best available protection for the future. Our assortment in Iceberg, Batavia, Butterhead and Babyleaf in general demonstrate strong resistances. We will continue breeding for the best possible Bremia resistances as a main focus for the next generation of lettuce varieties.”
Response of Gautier Semences:
"Gautier Semences already offers on the market a range of resistant varieties to Bremia Bl:37EU, of course, but also varieties resistant to Fusarium Fol:1 and/or Fol:4, for maximum safety. Aware of the major stake represented by this new Bl:37EU strain for crop safety, Gautier Semences has been mobilized for several months in a race against time against the new generation of the fungus and is already offering a panel of lettuce resistant to Bremia Bl:37EU, to help lettuce growers fight against this threat. Fusarium wilt is also a growing concern in lettuce crops. Gautier Semencesis developing research programs to effectively control this disease and offers a selection of resistant varieties Fol:1 and/or Fol:4 to go even further in securing lettuce growers’ next campaign."
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Bejo Zaden
Gautier Seeds