A new race of Bremia lactucae — the pathogen causing downy mildew in lettuce — has been identified in the western United States. Designated Bl: 10US, the new race highlights the increasing genetic variability of the disease in commercial lettuce crops.

The International Bremia Evaluation Board – United States (IBEB-US) confirmed the race after monitoring isolates across multiple sites in California and Arizona. The newly denominated isolate, VP-302 from San Benito County, CA, represents a group of field isolates all sharing the sextet code IBEB-D 45-7-2. These isolates have been observed breaking through commonly used resistance genes, including Dm25, Dm38, and Dm56.

Bl: 10US becomes the tenth official race recognised in the U.S. lettuce industry, signalling an urgent need for continued monitoring and resistance breeding. Although seed companies offer varieties resistant to known races, growers are warned that emerging pathotypes not yet classified may still cause outbreaks.

IBEB-US — a collaboration between private seed companies and public researchers — underscores that plant resistance alone is not sufficient. Fungicide applications, particularly during early growth stages, can enhance protection. Additionally, strict hygiene practices such as removing plant debris, disinfecting tools, and reducing leaf wetness are essential to limit disease spread.

The ongoing emergence of new Bremia races underscores the need for integrated disease management combining genetic resistance, chemical support, and cultural controls.

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