Competing in nature's rat race
When Rijk Zwaan came up with a solution to the troublesome lettuce aphid in 1997, it marked a turning point for growers. The resistant varieties brought an end to their concerns about lettuce harvests. Similarly, controlling downy mildew is an ongoing and equally essential process for customers. “We cross in new resistances before new variants have even emerged, while looking for broader resistance at the same time,” says Johan Schut, Breeding Manager Lettuce.
Large breeding team
Johan has been leading the breeding team for this crop for 25 years in total, including together with colleague Jair Haanstra in the past few years. When Johan started, there were around 20 people in the team, whereas that figure is now closer to 70. But that’s not surprising, since today’s assortment includes more than 500 commercial varieties across 12 lettuce types: for both outdoor and indoor cultivation, for soil-based and hydroponic systems, and for different countries, seasons and applications.
Resistant varieties are increasingly important
The search for resistances has been a priority within the breeding team since the 1960s, but its importance still continues to grow every day. For Johan, the reasons are clear: “Diseases and pests cause major crop losses. Lettuce aphids can make a grower’s entire crop unsaleable. The same goes for an outbreak of downy mildew (Bremia). And Fusarium in the soil can even force growers to switch to a different crop or cultivation system.”
According to the Breeding Manager, stricter regulations – especially in Europe – are another reason: “These increasingly restrict the use of chemicals. But even if growers may still use them, they would rather choose a resistant variety than spend time and money on spraying. Not to mention the high value that consumers place on food safety.”
Resistance against aphids gives certainty
Lettuce varieties with resistance to the lettuce aphid Nasonovia ribisnigri (Nr:0) made a big impact when they were introduced in 1997. That was just before Johan’s time at Rijk Zwaan, but he remembers that it caused a stir. “Varieties with that resistance were very successful because they greatly reduced the need for growers to spray against the aphid. Moreover, the varieties gave them certainty. Within no time, we had crossed the trait into 90% of our lettuce types to allow all markets to benefit from it. In the 25 years since then, we still haven’t discovered precisely why the resistance works so well. The aphid pierces the lettuce once, but then doesn’t feed on it. It doesn’t die, but it just doesn’t seem to like the lettuce anymore.”
New variant caused a setback
The discovery of a new variant of the lettuce aphid in Germany and France in 2007 caused a major setback. The new variant does like the taste of the resistant lettuce variety and has spread quickly across Europe.
“I’ve visited the lettuce fields myself and talked to growers,” says Johan. “The consequences are dramatic. Lettuce aphids get into the heads, making them unsaleable. Growers have been using systemic insecticides to control the aphids so far, but the active ingredients are either already banned in Europe or soon will be. The European rules are much stricter than in the rest of the world, so growers, processing companies and supermarkets are very concerned. Fortunately, we have found a resistance in wild lettuce and we’re in the process of crossing it in. So a solution is in sight.”
Resistances in preparation for new variants
In the case of Bremia, which is the major disease in lettuce worldwide, it is much more common for new variants to breach a resistance. Thanks to alert growers who send affected lettuces to Rijk Zwaan for analysis, a breach can be detected quickly. The breeding team is prepared for anything, states Johan. “Using DNA markers, we are constantly looking for new resistances, and we cross them into existing varieties in advance. This means we have resistances ready if there is a breach. We just don’t know for sure whether they will work against future variants, but they often do. That’s what enables us to introduce new varieties so quickly.”
A colleague once asked Johan how we could prevent resistance breaches. “We simply can’t. The Bremia fungus adapts to resistances, just as the coronavirus does in the human immune system. That’s nature’s rat race.”
Giving growers worldwide peace of mind
To offer growers a structural solution in the future, the breeding team is now diving even deeper into the genetics to look for resistances to Bremia. “We found that some hundred-year-old varieties have broader resistance. It’s quite extraordinary. It’s harder to pinpoint exactly why, because it involves multiple genes with relatively small effects. But we’re applying ever-more advanced research techniques to understand this.”
The team is also working on resistances to the extremely harmful soilborne fungus Fusarium, as well as on solutions to problems encountered in specific production regions. Johan: “In the Salinas Valley in the USA, we successfully introduced two INSV-resistant varieties in 2023. This is one example of how we try to give lettuce growers worldwide peace of mind, both now and in the future – just as we have been doing for the past 100 years.”