A closer look at our favourite salad ingredient, its breeding and its challenges – Part 1: Classical Breeding
In each issue, European Seed takes a close look at one of the many crops that European plant breeders are working on. In both this issue and the February issue, we’ve put lettuce in the spotlight. Below, leading breeders comment about lettuce markets, breeding goals and challenges. In the February issue, they will focus on regulatory challenges, access to genetics and innovation. Our industry experts are: Olaf Zonneveld, Global R&D Crop Lead Salads, Syngenta Seeds; Roel Veenstra, Crop Research Manager, Bejo Zaden; Alfonso García, Lettuce Breeder, Meridiem Seeds; Johan Schut and Jair Haanstra, Lettuce Breeding Managers, Rijk Zwaan; Lisa Zaccaria, Lettuce Breeder, Maraldi; Gowtham Prakash, Pre-Breeder leafies and Peter Visser, R&D Crop Technical Lead for leafies, artichoke, okra, asparagus and celeriac, BASF Vegetable Seeds.
Origins and Market Evolution
There is evidence that the cultivation of lettuce (Lactuca sativa) began as far back as 2680 BCE. It has most often been used for food, but like many plants, it also has historic significance in the areas of religion and medicine. It was originally farmed in ancient Egypt for its seed oils and also for the edible leaves. Lettuce was considered a sacred plant of Min, god of reproduction. It was carried during festivals, used in religious ceremonies and its image was added to tombs and wall paintings. As time went on, the Greeks and Romans grew lettuce and by 50 CE, there were many different varieties. From the 1500s to the 1700s in Europe, more varieties were developed, and by about 1750, cultivars had been created that still survive today. The crop also became popular in North America, but by the late 1900s, lettuce was grown around the world. It is reported that in 2017, China produced over half the lettuce being grown on Earth. Lettuce contains high levels of antioxidants (vitamin C and polyphenols) and fibre.
Range of Varieties
Maraldi currently breeds only babyleaf lettuce and Bejo a few of the main types. Rijk Zwaan, Syngenta and BASF offer most or all of the main varieties, such as iceberg, romaine, Batavia, butterhead, babyleaf, crunchy, gem, incised leaf, multileaf, crystal, lollo, and oakleaf lettuce, some in conventional/organic, indoor/outdoor, and hydroponics/soil-suited varieties. For Meridiem, iceberg lettuce is important, and it markets varieties of this lettuce for many different areas/growing seasons and uses. “Also, in the romaine and little gem typologies, there is a continuous demand for more competitive varieties,” says García. “The ‘specialties’ and ‘baby leaf’ type also have importance in our breeding program.”
Main Breeding Targets: Yield
As with the breeding of all crops, both agronomic and market-oriented traits are critical to each lettuce variety’s success. The market for this vegetable is extensive, and Visser explains that because it’s affected by many factors (cultural preferences, uses, requirements for different production systems, climates and seasons), “this necessitates, for every type, specific genetics and screening conditions. Therefore, the investment needed to breed for one type, or another greatly differs.”
Yield of any crop matters, but Zonneveld is of the view that yield in ton/ha is less important for lettuce than in other crops. “A large part of the lettuce is sold by piece and not by weight,” he says. “For the fresh market, yield exists mainly in quality traits like no visible diseases on the head, no internal disorders like tipburn and a high percentage of harvestable plants per ha. For segments where yield is important, we are measuring yield in our trials.” Schut notes that in general, lettuce yield has increased over time due to a reduced impact of factors that lead to a bad harvest, such as susceptibility to abiotic stresses, diseases and pests.
Prakash explains that whilst yield is important in all lettuce market segments, it’s especially important in those segments where the crop is cultivated in poor growing conditions, such as winter. The yield is less related to region but very much influenced by maturity stage, season slot, soil type and growing conditions. “In the early stages of variety development, we generally measure the size of the heads at their harvesting maturity stage,” he says. “Later, when varieties make it to the precommercial stage, we collect data on yield per plant and square meter, depending on the type of product: whole head-hand harvest or leaves-mechanical harvest.”
Yield of Maraldi’s babyleaf lettuce is evaluated by both leaf count and leaf weight at maturity. “Also, the suitability of our varieties for multiple cuts increases their profitability,” says Zaccaria. She adds that the main differences in varieties of their babyleaf lettuce is whether it’s grown in the field or greenhouse. With the latter, the varieties grow more quickly, and the leaves are more tender, “so, in our breeding program,” she says, “we focus a lot on leaf texture.”
Another Breeding Target: Fresh Cut Discolouration
Susceptibility to browning or pinking at the cut surfaces is a factor that affects purchase choice at the grocery store and one with which restaurants also have to contend. Browning or pinking after lettuce is cut is related to the oxidation of leaf tissue. “Most pinking problems start before harvesting and extend to bagging for fresh produce or cutting for processing,” García explains. “The bacteria Pseudomonas marginalis is directly related to this condition and Meridiem is working with very promising results on varieties that incorporates resistance to this bacterium.”
Visser notes that there is a lot of research being conducted to find the molecular keys to reduce oxidation of the polyphenol-rich cells that are damaged or stressed around the cut surface. On this note, Schut reports that a few years ago, Rijk Zwaan has introduced a new trait, the so-called ‘Knox’ trait, which causes a delay in wound induced oxidation. This trait is the result of a non-expressed gene in the phenyl-propanoid pathway, he says, and it was first found in a butterhead lettuce plant 15 years ago.
Visser adds that the use of different post-harvest handling methods – reducing the temperature during processing and storage, for example, and the use of modified atmosphere packaging (MAP) – can make a big difference in reducing browning. “The final solution for the marketplace would be a combination of MAP and genetics,” he says. “The latter providing most benefit once the product reaches the consumer and the packaging is opened for the lettuce to be eaten over several days.”
Other Major Breeding Targets – And Challenges
Agronomically, Zonneveld believes the main challenge of breeding lettuce lies in ensuring varieties perform well under many environmental conditions. Screening in multiple environments in various places around the world is therefore very important for lettuce breeding companies.
For his part, Schut considers breeding to address the dynamics of pathogen development (for example, the continual emergence of new Bremia/downy mildew races) to be the most challenging aspect of lettuce breeding. He also lists the major challenge of combining a large number of desired traits in one variety in a short period of time.
Besides resistance to disease, other agronomic traits of significance for lettuce are bolting tolerance, yield, colour, tipburn resistance, core length and ‘pink vein,’ as well as optimal plant size and shape for harvesting and packaging. And all of our breeder experts agree that climate change can potentially affect many of these traits. “Yes,” says Veenstra, “in every area we see that the weather is less predictable compared to the past, and we need to make varieties which can adapt to different circumstances.” Zaccaria echoes the same thoughts. “Response to climate is not a negligible detail during the selection process,” she says. “We are creating new varieties able to perform well during the during the whole cultivation cycle, even if sudden weather changes occur. And it’s extremely important to have varieties with uniform high-temperature germination as the springs are getting warmer, as well as strong tolerance to bolting, with a thick texture and tolerance to Fusarium.”
Zonneveld notes that much warmer conditions are being seen in spring in Northwest Europe, so Syngenta now has varieties that can handle this. Rijk Zwaan is also using parents suited to the Mediterranean for crossings in its Northern European lettuce breeding programs.
Part of the breeding for climate change is to create plants with higher drought, heat and salt tolerance. Schut notes that salty conditions are a long-lasting issue in the main European production area for winter outdoor lettuce, near Cartagena in Spain.
García adds that climate change has also caused some disease pathogens and insects to move to areas where they have never been found before. “This makes it necessary to update the set of resistances included in each variety,” he says. “By in-situ selections during the last eight years, we have obtained material adaptable to these extraordinary changes.”
Diseases, Pests and More
Lettuce is a tender plant and is quite vulnerable, above all, to fungal pathogens. García explains that due to market circumstances, most lettuce producers grow the same lettuce year after year on the same field – and with damp weather favourable to disease, every subsequent crop is at higher risk of funguses and other diseases/pests.
Prakash explains that soilborne fungi like Fusarium oxysporum f.sp. lactucae (Fusarium wilt) Sclerotiniaspp. (lettuce drop) and Verticillium dahlia (Verticillium wilt) and are generally hard to control once established in the soil. “Especially Fusarium wilt appears to be an increasing problem for lettuce growers, and resistance breeding is complicated by the fact that two dominant races (Fol:1 and Fol:4) occur in some important lettuce markets,” he says. “But by far the most important disease of lettuce is Bremia. It’s spread by wind, and resistance breeding is very complex due to the occurrence of many different races.” Prakash reports that seed companies are collaborating via the International Bremia Evaluation Board (IBEB) to create more clarity about this disease for the seed industry and its customers.
Schut explains that in response to the emergence of new Bremia strains, several breeding companies including Rijk Zwaan have continually introduced new resistance genes from wild lettuce species. “Because of the increased range of available resistance genes,” he says, “there are less Bremia problems in growers’ fields than 20 years ago.”
Besides Bremia and Fusarium, Bejo focuses on breeding for resistance to bacterial diseases; Meridiem on pink rib, a virus disease called Lettuce Big Vein and the currant-lettuce aphid (Nasonovia ribisnigri); Rijk Zwaan on Lettuce Mosaic Virus (LMV), Nasonovia bacterial leaf spot and Big Vein.
BASF and Syngenta focus on all of these diseases as well as Nasonovia. Prakash notes that althoughNasonovia causes the most damage in the lettuce sector, multiple aphid species can attack the crop. “The root-infecting aphid Pemphigus bursarius can cause significant losses in some lettuce markets,” he explains. “And, besides causing direct damage to the lettuce plant, aphids can spread numerous Potyviruses such as LMV. LMV can also be transmitted from infected seeds into a new crop, and the presence of aphids can quickly multiply the number of infected plants.”
Prakash points to another group of viruses that can impact certain lettuce markets, the Tospoviruses, specifically Tomato Spotted Wilt Virus (TSWV) and Impatiens Necrotic Spot Virus (INSV). “Although Tospoviruses are not seed-transmitted, they can be transmitted by thrips insects,” he says. “Among the bacterial pathogens of lettuce, Pectobacterium carotovorum and Xanthomonas axonopodis pv. vitians can be considered relatively important. Whereas the first one can cause head rot or soft rot, X.a.vitians is the causal agent of leaf spot disease in lettuce. Splashing water from rain or overhead irrigation facilitate disease spread and development.”
“Maraldi seeds is a medium-small company, with quite young research and in the last 10 years we worked hard to be competitive and offer varieties with a strong package of resistances,” says Zaccaria. Her company focused a lot on the introgression of resistance genes for the current main pathogens for baby lettuce, Bremia lactucae and Fusarium. “Today Maraldi offers a wide range of baby lettuces with a full Bremia resistance (actually Bl:16-36EU) and we are working to increase the level of Fusarium tolerance in our materials both for race 1 and for the new one 4. Looking at the future surely, we want to improve and expand our resistances package,” she adds.
New threats to lettuce come from both new diseases and evolving forms of existing pathogens.
On the Fusarium front, Schut explains that the well-known Race 1 has spread more widely during the last two decades and is still the main race worldwide. “However, Race 4 was found five years ago in the Netherlands,” he says. “It has spread, also to the neighbouring countries, and is causing problems mainly in indoor lettuce. We are introducing more and more varieties with resistance against these races.”
And, although resistance to Nasonovia was introduced in lettuce varieties by Rijk Zwaan in 1997, in 2007 a new variant (biotype 1) appeared in Europe and broke through the resistance. “This biotype is also causing problems in Australia and California,” Schut reports. “Resistance to this new biotype was found in wild lettuce but is not yet available in cultivated lettuce.”
Although new races of existing diseases or the movement of existing diseases to new geographic areas do happen with lettuce, Prakash says appearance of completely new diseases is very uncommon. “But it does happen,” he says. “For example, in 2011 a new virus disease was discovered in lettuce which was named Lettuce Necrotic Leaf Curl Virus (LNLCV) of the genus Torradovirus. Until then, no Torradoviruses that can infect lettuce were known. Although this disease is not of major importance, it is important to continuously monitor for new disease occurrences. The impact of a disease in a crop can change as a result of new cultivation practices, increasing crop pressure and reducing crop area, limitations in the use of crop protection products such as seed treatments and fungicide sprays, and changing climate conditions. It’s therefore important to have new disease symptoms always diagnosed by a phytopathology specialist.”
Zaccaria says to address new pathogens/pests or new forms of existing ones, the research of new germplasm as source of new resistance genes is very important. “Also, the research for molecular markers to speed up the selection of new resistance genotypes plays an important role,” she notes.
There will be more information on how new techniques and technologies are being used in lettuce breeding in the February issue.