b'promising results on varieties that incorporates resistance to thisarea for winter outdoor lettuce, near Cartagena in Spain.bacterium.Garca adds that climate change has also caused someVisser notes that there is a lot of research being conducteddisease pathogens and insects to move to areas where they have to find the molecular keys to reduce oxidation of the polyphenol- never been found before. rich cells that are damaged or stressed around the cut surface.This makes it necessary to update the set of resistances On this note, Schut reports that a few years ago, Rijk Zwaan hasincluded in each variety, he says. By in-situ selections during the introduced a new trait, the so-called Knox trait, which causes alast eight years, we have obtained material adaptable to these delay in wound induced oxidation. This trait is the result of a non- extraordinary changes.expressed gene in the phenyl-propanoid pathway, he says, and it was first found in a butterhead lettuce plant 15 years ago. Diseases, Pests and More Visser adds that the use of different post-harvest handlingLettuce is a tender plant and is quite vulnerable, above all, to methodsreducing the temperature during processing and stor- fungal pathogens. Garca explains that due to market circum-age, for example, and the use of modified atmosphere packagingstances, most lettuce producers grow the same lettuce year (MAP)can make a big difference in reducing browning.after year on the same fieldand with damp weather favorable The final solution for the marketplace would be a combina- to disease, every subsequent crop is at higher risk of funguses tion of MAP and genetics, he says. The latter providing mostand other diseases/pests. benefit once the product reaches the consumer and the packag-Prakash explains that soilborne fungi like Fusarium oxyspo-ing is opened for the lettuce to be eaten over several days.rum f.sp. lactucae (Fusarium wilt) Sclerotinia spp. (lettuce drop) and Verticillium dahlia (Verticillium wilt) and are generally hard to Other Major Breeding Targetsand Challenges control once established in the soil. Agronomically, Zonneveld believes the main challenge of breed- Especially Fusarium wilt appears to be an increasing problem ing lettuce lies in ensuring varieties perform well under manyfor lettuce growers, and resistance breeding is complicated by environmental conditions. Screening in multiple environments inthe fact that two dominant races (Fol:1 and Fol:4) occur in some various places around the world is therefore very important forimportant lettuce markets, he says. But by far the most impor-lettuce breeding companies.tant disease of lettuce is Bremia. Its spread by wind, and resist- For his part, Schut considers breeding to address theance breeding is very complex due to the occurrence of many dynamics of pathogen development (for example, the continualdifferent races. Prakash reports that seed companies are col-emergence of new Bremia/downy mildew races) to be the mostlaborating via the International Bremia Evaluation Board (IBEB) challenging aspect of lettuce breeding. He also lists the majorto create more clarity about this disease for the seed industry challenge of combining a large number of desired traits in oneand its customers. variety in a short period of time.Schut explains that in response to the emergence of newBesides resistance to disease, other agronomic traits ofBremia strains, several breeding companies including Rijk Zwaan significance for lettuce are bolting tolerance, yield, color, tipburnhave continually introduced new resistance genes from wild let-resistance, core length and pink vein, as well as optimal planttuce species. Because of the increased range of available resist-size and shape for harvesting and packaging. And all of ourance genes, he says, there are less Bremia problems in growers breeder experts agree that climate change can potentially affectfields than 20 years ago. many of these traits. Besides Bremia and Fusarium, Bejo focuses on breeding Yes, says Roel Veenstra, crop research manager of Bejofor resistance to bacterial diseases; Meridiem on pink rib, a virus Zaden, in every area we see that the weather is less predictabledisease called Lettuce Big Vein and the currant-lettuce aphid compared to the past, and we need to make varieties which can(Nasonovia ribisnigri); Rijk Zwaan on Lettuce Mosaic Virus (LMV), adapt to different circumstances.Nasonovia bacterial leaf spot and Big Vein. Zaccaria echoes the same thoughts. BASF and Syngenta focus on all of these diseases as well as Response to climate is not a negligible detail during theNasonovia. Prakash notes that although Nasonovia causes the selection process, she says. We are creating new varieties ablemost damage in the lettuce sector, multiple aphid species can to perform well during the during the whole cultivation cycle,attack the crop. even if sudden weather changes occur. And its extremely impor- The root-infecting aphid Pemphigus bursarius can cause sig-tant to have varieties with uniform high-temperature germinationnificant losses in some lettuce markets, he explains. And, besides as the springs are getting warmer, as well as strong tolerance tocausing direct damage to the lettuce plant, aphids can spread bolting, with a thick texture and tolerance to Fusarium.numerous Potyviruses such as LMV. LMV can also be transmittedZonneveld notes that much warmer conditions are beingfrom infected seeds into a new crop, and the presence of aphids seen in spring in Northwest Europe, so Syngenta now has varie- can quickly multiply the number of infected plants.ties that can handle this. Rijk Zwaan is also using parents suited to Prakash points to another group of viruses that can impact the Mediterranean for crossings in its Northern European lettucecertain lettuce markets, the Tospoviruses, specifically Tomato breeding programs. Spotted Wilt Virus (TSWV) and Impatiens Necrotic Spot VirusPart of the breeding for climate change is to create plants with(INSV). higher drought, heat and salt tolerance. Schut notes that salty con- Although Tospoviruses are not seed-transmitted, they can ditions are a long-lasting issue in the main European productionbe transmitted by thrips insects, he says. Among the bacte-58/ SEEDWORLD.COMFEBRUARY 2021'