When a new variant of the black currant-lettuce aphid Nasonovia ribisnigri emerged in 2007, Johan Schut saw with his own eyes how much damage it caused to lettuce crops. The breeding manager lettuce made a promise to lettuce growers on behalf of Rijk Zwaan: “We will find a solution for this”. Now, 18 years later, the company has finally fulfilled that promise by launching two new Nr:0,1-resistant iceberg lettuce varieties.  

What was your main driver in the quest for a new resistance? 

“I hadn’t been working at Rijk Zwaan for very long when the new biotype 1 of the black currant-lettuce aphid broke through the Nr:0 resistance in our lettuce varieties. My German colleagues asked me to visit them in Stuttgart, where the aphid had first been discovered. As the plane came in to land, I had a view of the lettuce fields from the air. Soon afterwards, I saw with my own eyes just how badly the fields were affected. On the evening of my arrival, I stood in a roomful of growers and their disappointment was tangible. This was a huge setback for them, on a major scale. That’s when I promised them we would try to solve it.” 

Why has it taken so long? 

“Finding and crossing in a new resistance is a lengthy process. First, we had to look for a resistance. We finally discovered it in a wild lettuce that a colleague found in a verge somewhere in 2013. That wild lettuce is rather unattractive; it has spines and looks a bit like a dandelion. We started by performing crosses to test how easily the trait could be passed on, and which specific genes were involved. Once we saw how strong the resistance was, we had to cross it back numerous times into today’s cultivated lettuce varieties, including all the different types of lettuce for the various seasons and continents. That process accelerated when colleagues developed a DNA marker enabling lettuce plants to be tested for the trait when they are still young.” 

When did you realise that a solution was within reach? 

“We had the feeling that we were on the right track in 2018, but we knew there could still be a hitch. After all, a resistance can be accompanied by undesirable traits that aren’t immediately apparent. We needed to grow a few generations in the field to rule that out. The varieties also needed favourable agronomic properties such as a good head shape, an attractive appearance and no tipburn.” 

How strong is the resistance in the new varieties? And how is that determined? 

“The varieties have high resistance (HR) against biotypes 0 and 1 of Nasonovia ribisnigri. The International Seed Federation (ISF) defines high resistance as the ability to significantly limit the pest’s growth and/or development. To test this, we conducted field trials to directly compare susceptible and resistant varieties. When there was high aphid pressure in Spain in the spring of this year, the difference was extremely clear: the susceptible varieties were covered in aphids, while we only found the occasional aphid in the resistant varieties.” 

Which varieties are you introducing right now? 

“The problems are most urgent for iceberg lettuce growers in the European Union (EU), so we’re introducing two iceberg lettuce varieties for temperate climates in Europe: Happinas RZ and Friendlinas RZ. The dense head of iceberg lettuce makes the black currant-lettuce aphid difficult to control with pesticides and natural enemies. Moreover, from October 2025 onwards, growers in the EU won’t be allowed to use the systemic agents that are effective.” 

What can growers expect in the future? 

“Our team is busy crossing Nr:1 resistance into other lettuce types for each market. We’re conducting trials of those varieties together with growers. Furthermore, our goal is to combine Nr:1 resistance with the complete package of resistances against Bremia races, including the latest ones – Bl:38EU, Bl:39EU, Bl:40EU and Bl:41EU. We’re also working on combining it with resistance to the soilborne fungus Fusarium, which is a problem in southern Europe. So growers can expect many more new varieties from us.” 

What can growers do to prevent the emergence of a new variant of the black currant-lettuce aphid? 

“To reduce the risk of a new biotype emerging, one of the things we advise growers to do is to spray against aphids – especially at the end of the season, to prevent a new variant of the aphid from surviving the winter. Spraying also helps against other aphid species that can sometimes still be found on outer lettuce leaves.” 

Is Rijk Zwaan already preparing for a new biotype? 

“We hope this resistance will last for a long time, of course. But we will continue working on and crossing in other resistances against this aphid – just as we do with Bremia, which regularly sees the emergence of new races. In fact, in the longer term, we would like to offer double resistance protection against Nasonovia ribisnigri.” 

Does it feel good to have fulfilled your promise? 

“Definitely. It was only recently that my German colleagues reminded me of that promise I made to growers in 2007. I’m very pleased that we can help growers with our Nr-resistant varieties, especially now that the other options are so limited. Even though they had a long wait for these Nr:0,1-resistant varieties, I hope they are happy with them.”