Salinas, California, USA
January 1, 2026

The International Bremia Evaluation Board, Unites States (IBEB-US) is a collaborative effort of private seed companies, informed by researchers from public institutions, that collects and characterizes field isolates and nominates new races of Bremia lactucae, causal agent of downy mildew in lettuce. This information is vital for lettuce growers and breeders. IBEB-US is a regional association that, along with IBEB-EU in Europe, is coordinated by the IBEB Global Coordinating Body (IBEB-G).

Previously, ten races have been denominated within the United States. However, the first six of these are no longer relevant for US lettuce production, as they have not been found in nature for over a decade. As these six races are no longer present and the lettuce resistance genes defeated by these races are also broken by the races from Bl: 7-10US, any claims of resistance to Bl: 1-6US are redundant and unnecessary. For these reasons, IBEB-US has voted to limit Bremia lactucae resistance claims to only Bl: 7-10US.

Although breeding companies supply growers with lettuce varieties possessing resistance to the denominated Bl: 7 to 10US races, this resistance is not a guarantee against downy mildew. The declared resistance gives growers protection against these races. However, downy mildew disease may be caused by rarer pathotypes with novel virulence characteristics that have yet to be denominated as races. Also, resistance is defined as the ability of a plant variety to restrict the growth and development of a specified pest or pathogen and/or the damage they cause when compared to susceptible plant varieties under similar environmental conditions and pest or pathogen pressure. Resistant varieties may exhibit some disease symptoms or damage under heavy pest or pathogen pressure.

IBEB-US 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 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.

Website: http://www.worldseed.org/isf/ibeb.html

Published: January 5, 2026