Breeding for bremia
Natural resistance in salad lettuce varieties is key to effective management of downy mildew (Bremia spp) within integrated disease management programmes. Understanding the diversity of bremia populations demonstrates the challenges for breeders.
UK lettuce growers are being urged to gather any samples of downy mildew outbreaks this summer, in an initiative to breed ever more resistant varieties that could pre-empt future issues.
Unique and new isolates of downy mildew are identified from many countries across Europe each year from disease outbreaks, with France by far the biggest contributor. UK growers, however, typically only report one, or no instances each season for further testing and genetic analysis.
Syngenta lettuce breeding specialist, Miguel Roca, highlights it is extremely valuable to find and assess evolving genotypes of bremia at the earliest possible opportunity.
He points out that whilst most varieties are extremely robust against all the official races declared by the International Bremia Evaluation Board (IBEB), in reality the variation in the background environment can be much higher.
There are possibly hundreds of local races and early incidences of evolving genotypes, which may, or may not, go on to become significant issues and an official race.
In testing in 2019, for example, of the 239 isolates tested, there were 86 unique isolates identified, representing 36% of samples. In the past decade there has been as many as 224 unique isolates identified in a season, or nearly three quarters of the samples assessed. Yet there are only 36 official races recognised by the IBEB.
Mica (above) pointed out bremia races are only declared ‘official’ once, among other criteria, they have become stable and widespread, after several years of successive occurrence across several areas. The isolate type also has to be breaking relevant resistance genes.
In the interim, new races and localised issues could cause significant losses to growers. Without widespread track and trace of the genotypes, they may never get recognised.
The search for new isolates of the disease gives breeders and the industry the ability to react more quickly to what’s occurring in the field.
Mr Roca outlined wild lettuce germplasm that often has very strong natural resistance to bremia is still a rich source of resistant genes against existing and evolving strains of the disease.
However, it is currently a complicated process to pull the desirable resistance genes across into commercially viable varieties, with multiple successive back-crosses to clean out unwanted traits – a process that could typically take eight to 10 years.
“Genetic analysis can indicate the combining ability of new sources of resistance,” he advocated. “The use of molecular markers is critical to enable breeders to combine and stack different resistance genes into new varieties.”
Ideally breeders look to utilise different combinations of natural bremia resistance genes, since if one combination is broken it could become susceptible to a whole group of other bremia races.
“For example, you could get a completely new variety that offers full resistance to all official races, but is susceptible to 80% of new races,” he reported. “That variety is extremely likely to get hit by infection in the field within a short period.
“Conversely, a variety with gaps in the resistance spectrum to the official existing catalogue could still have 95% resistance to new isolates, and actually remain robustly resistant for longer in commercial growing.”
“To create superior genetic combinations our target is full resistance to all official races, and as many new isolates as possible," added Mr Roca.
“We select and test against a large number of these new isolates, considering their importance. That includes their frequency of occurrence, aggressiveness and geographic distribution of the isolate.”
Hence the importance of keeping track of new or local races, he reiterated. “Along with the ability to predict how they might evolve in the future and the likelihood of them becoming more widespread, or even official races.
“With really effective early identification and predictive breeding programmes, we can look to get ahead of evolving Bremia, before it becomes a problem for growers.”
Syngenta salad crop specialist, Rosemary Atwood (above), reported a proactive campaign to collect bremia samples from UK field crops this season. With a significantly increased programme of on-farm variety testing and evaluation with UK growers this season, she believes the company is well placed to help with the information gathering.
“The effects of bremia can be devastating in the field, particularly with babyleaf varieties,” she warned.
“As growers and agronomists look to develop Integrated Crop Management programmes resistance is a primary factor for variety selection.
“We are keen to hear from growers who suspect bremia outbreaks in any of their crops to get in touch, especially in varieties with full mildew resistance,” she urged. “The more samples that can be collected from growers, the better the breeding programme can deliver varieties that have resistance built-in for the future.”
Download the Syngenta Vegetable variety brochure here, for all the info on bremia resistant salad crops