A new race, race 9, of the lettuce downy mildew pathogen (Bremia lactucae) has been confirmed and named in 2018. Here are some miscellaneous notes regarding the identification and naming of new races of this lettuce pathogen.
Bremia lactucae is a genetically diverse pathogen and is made up of distinct, separate sub-groups or sub-populations called races (or pathotypes).
Different races can overcome different disease resistance genes present in lettuce cultivars and breeding lines. Therefore, a particular lettuce cultivar that is labeled as “resistant” to certain numbered downy mildew races will not develop disease if exposed to the races listed but can develop disease if exposed to a different race.
Races are characterized by inoculating a series (called a differential set) of lettuce cultivars and breeding lines. For a particular race, some of the lettuce will become infected and other lettuce will be resistant. Depending on the pattern of susceptible/resistant reactions, a defining pattern or “fingerprint” will be created that will be characteristic of that race.
In California and the western USA a coalition of university and seed company researchers, called the International Bremia Evaluation Board for the US (IBEB-US), monitors the lettuce downy mildew situation and looks for evidence of new races. The IBEB-US serves as a useful means of standardizing information on lettuce downy mildew races and communicating such information to the lettuce industry. A similar group (IBEB-EU) operates in Europe and uses the same differential set for worldwide comparisons of the lettuce downy mildew pathogen.
A downy mildew outbreak on previously resistant cultivars is one bit of evidence that could signal the development of a new Bremia race. The IBEB-US will then determine if the isolate causing such outbreaks meets three criteria: (1) this isolate is causing disease in multiple lettuce growing regions; (2) this isolate continues to be found over multiple years; (3) this isolate is genetically stable.
After assessing the lettuce downy mildew situation from 2015 to 2017, in 2018 the IBEB-US determined that a Bremia isolate met the new-race criteria and therefore announced that race 9 had been designated.
Implications for growers, pest control advisors, and other field professionals:
Resistant cultivars that previously showed no downy mildew could develop severe disease if race 9 is introduced into the area and environmental conditions favor development of downy mildew.
However, occurrence of severe downy mildew on resistant cultivars will not always mean that the newly named race 9 is responsible. Research surveys continue to find the presence of new and unique isolates that do not match the genetic fingerprints of the known races (mostly races 5, 6, 7, 8, and now 9). These unique isolates are called “novels” and in the future could become or contribute to the development of yet additional races.
Over the past few years the pesticide industry has developed and registered a number of fungicides that are very effective against lettuce downy mildew. At this point there is no evidence in California that any of the races, including the newly named race 9, has developed resistance (or “insensitivity”) to these fungicides.
Growers, PCAs, and other field personnel are encouraged to take note of unusual downy mildew developments on resistant lettuce cultivars. Notify the Richard Michelmore lab at UC Davis or the CA Leafy Greens Research Board office in Salinas if aggressive outbreaks occur.
TriCal Diagnostics also works with lettuce downy mildew pathotypes and provides screening services to document whether breeding lines and new cultivars are resistant or susceptible to the known Bremia pathotypes.
Steven Koike, TriCal Diagnostics
Cayla Tsuchida, Arcadia Biosciences
Photo 1. Lettuce downy mildew is readily recognized by the white sporulation on the undersides of leaves.
Photo 2. The races of the downy mildew pathogen are identified by their reactions when inoculated onto a set of differential lettuce breeding lines and cultivars.
Photo 3. In using the differentials to identify Bremia races, a susceptible lettuce breeding line or cultivar will exhibit the characteristic white growth of the downy mildew pathogen.