Lettuce is consumed globally; its nutritional value makes it an excellent food for weight loss and for maintaining optimal health. The farming origin of this crop, Lactuca sativa, goes back nearly 5000 years when the Egyptians started farming it. Through the millennia, humans have improved the crop by breeding wild types to obtain the different types of lettuce that we know today. Like lettuce’s genetic base, the cultivation of lettuce has vastly evolved over the past decades, and we are now able to farm this crop thousands of miles above ground and even below sea level. However, in the 21stcentury, growers are facing the greatest disease pressures in the history of lettuce farming. These disease pressures are a consequence of land restriction, and tight rotations, that have led to the intensification of lettuce farming. Nowadays, intensive lettuce farming is occurring in a context where the choice of effective crop protection products to control disease spread are being reduced. It is a fact that agricultural fungicide choices are narrowing, as the permitted maximum residue levels are being reduced and more resistant pathogens are appearing. Additionally, growers need to meet strict farming requirements to be able to compete in a market demanding food certification. Undeniably, growers are in need of a tool that can inform them about the disease pressures in their systems and guide the right treatment to use for effective control of disease pressures.
When did we start eating lettuce and why is it so good for you?
Farming common lettuce, Lactuca sativa, begun in the Middle East by the ancient Egyptians about 2,700 B.C. The ancient type of lettuce, which was cultivated by the Egyptians was less appetizing with bitter leaves than the lettuce that we know today. Originally this crop was mainly used for its oil extracted from the seeds, it was used for cooking, medical and mummification purposes. With time, the wild type of lettuce was genetically engineered by breeding different types through generations to produce a tastier and easier to farm lettuce. Later the Greeks learnt from their Egyptians contemporaries how to cultivate lettuce and used it as a sedative medicine, they also added it into salads as starter for their meals. With the rise of the Roman empire, the Romans conquered the crop, and named it “lactuca” meaning milk, due to its white sap, this word will later evolve into the English word lettuce. The Romans also managed to further develop the plant to obtain tastier varieties and extend its consumption and farming around the empire. During the last millennia, our predecessors further evolved the crop into the varieties that we know today through selective breeding. (1)
Even though today there are more than 100 different varieties of Lactuca, there are only 5 main types of lettuce that are cultivated worldwide. The varieties are categorised by leaf type and head formation for example leaf, cos or romain, crisphead, butterhead and stem or asparagus lettuce. The most popular is leaf lettuce, with crisp leaves loosely arranged on the stalk. Cos or romain grow upright in a spoon shape, with thick ribs and tightly folded leaves. Crisphead or iceberg lettuce is the most popular type consumed in the US, is low in flavour and nutrient content due to its rich water content . Butterhead is a delicate type, generally small with tender leaves that has a sweet flavour. Stem or asparagus lettuce produce enlarged seedstalk, and is a popular ingredient used in Asian cuisine. (2)
Lettuce became very popular in our diet due to its nutritional value, as it is ideal for weight loss and for maintaining good health. In fact, lettuce has really high content of water (95-96% water), therefore, it has a very low caloric content (19 kCal/100 g). So what is the nutritional value of lettuce that makes it so good for us? It is a very good source of dietary fibre, vitamin A, C, and K, folate and manganese, and a good source of thiamin, vitamin B6, iron and potassium. The added benefits to our health come from its rich beta-carotene content, which is metabolised into vitamin A in the body, and it helps with iron absorption, for the skin, mucous membranes, vision, immune system and for growth. Furthermore, lettuce is low in sodium, and very low in saturated fat and cholesterol. (3)
What are the general requirements needed for lettuce cultivation?
According to the Food and Agriculture Organisation (FAO), In 2017 the total global production of lettuce (combined with chicory) was 27 million metric tonnes. The top 10 country producers were China, United States, India, Spain, Italy, Japan, Iran, Turkey, Mexico and Germany. The two largest being China (56%) and USA (14%) of the total world production. Interestingly, Spain where only 4% of the global production of lettuce is cultivated, is the world’s largest exporter of lettuce, and 70% of the Spanish production comes from the region of Murcia. (Data taken from FAOSTAT data for 2017).
What are the general requirements in lettuce cultivation? This crop grows best in full sun, in a wide range of soil types, but prefers light to medium loams, with good drainage conditions, requires nitrogen and a pH 6.5 to 6.8. Lettuce grows best in temperatures 16-18°C, grows poorly above 24°C and can tolerate temperatures as low as 7°C. In commercial production, lettuce is generally grown on a bed system, so precision farming activities can be used. The spacing is determined by different parameters (head size, fertility levels, disease and weed control considerations) and the recommended density ranges from 95,000 plants per ha for crisphead or iceberg lettuce to >230,000 for little gem or small cos type of lettuce. Lettuce cultivation demands a good irrigation system, the planting modules must be moist at planting, and the soil condition should be close to field capacity before planting. On the other hand, it does not stand over watering, especially a week before harvest, as it can reduce the shelf life and damage leaves. Harvesting can occur as quick as 7-14 days after planting, and as with many crops, the harvesting time is crucial, as cold and warm spells during the crop growth can damage the crop and limit its marketability. (4)
In UK, lettuce is best grown when the temperatures are mild and even, so the season generally begins around mid-May and finishes at the end of October. However, lettuce can also be grown under glass or polytunnels the whole year-round, especially winter varieties that are adapted to grow under short days and low heat. Outdoor lettuce is generally grown on long rotations, whereas when is grown protected, in glasshouses or polytunnels, it is often grown intensively with 6-8 crops per year on the same land. As a consequence, disease pressures increase in the growing system, and the right measures must be put in place to avoid spread of diseases. (5)
What are the main disease threats for lettuce and how can they be controlled?
Lettuce crops are an attractive target for fungal and oomycetal diseases, being the most common downy mildew, grey mould and sclerotinia. Other very damaging diseases include ring spot, powdery mildew, pythium root rot and fusarium wilt. Some diseases, such as downy mildew and powdery mildew, can spread very quickly with the potential to damage the entire crop, others, such as sclerotinia and ring spot tend to be localised, and some pathogens, such as Fusarium oxysporum can remain in the soil for many years.
The disease risks and mitigation strategies largely depend on the type of cultivation. For example, if a soil borne disease establishes in a soil in a protected environment, soil sterilisation is recommended. Generally, fungal diseases in a protected crop, are more dangerous than in an outdoor crop, as in a protected environment, the moisture can remain in the leaves for longer periods, increasing the susceptibility of the plant to fungal diseases. Hydroponic systems can offer protection against soil-borne diseases (i.e. Fusarium, Sclerotinia or Rhizoctonia), yet, it offers an opportunity for other fungal like pathogens to thrive, such as Phytophthora and Pythium species. Not surprisingly, season and weather greatly affect the likelihood of disease. In autumn, downy mildew is more likely to strike due to great differences in temperature between day and night that result in leaves condensation. During persistent wet weather, grey mould is the biggest threat, whereas Fusarium wilt requires high soil temperatures, so in the UK there is a greater risk of Fusarium wilt during late summer to early autumn.
Below are some instructions for limiting the spread of diseases taken from the Agriculture and Horticulture Development Board publication ‘Diseases of lettuce crops’, complied by the expert Tim O’Neil from ADAS (info taken from (6)) and how FungiAlert services can minimise the impact of disease outbreaks in lettuce fields:
1. As many fungal diseases can develop and spread very quickly, a preventative fungicide programme for diseases that tend to occur on a site should implemented. FungiAlert’s soil health analyses can describe the diseases threats that occur in a site, this information helps to better decide what active ingredients should be included in a preventative fungicide programme.
2. In protected crops, hygiene should be closely monitored to reduce the incidence of diseases. Hygiene measures include the removal of crop debris, disinfection of the surfaces in the production site, flush using a disinfectant of the whole irrigation system or UV sterilisation methods of the irrigation system. The SporSenZ screening analyses the irrigation system to check if it is free of diseases, or if the disinfection methods are eliminating disease threats that spread through water.
3. Check with your seed supplier the health status of seed batches and investigate the treatments applied during the propagation of plants. If you are suspicious of a problem before starting, it is better not to plant that batch of plants. FungiAlert can test seeds to find out if a batch is infected.
4. Soil disinfection by sterilisation may be able to reduce the inoculum of soil-borne pathogens before planting. Conversely, this also has the potential to reduce the inoculum of beneficial microorganisms in the soil and can ultimately pose a higher risk for the spread of diseases. If you wish to investigate the effect of soil sterilisation in the soil microbial community, FungiAlert offers tailored studies of the soil microbial communities to understand the effects of soil sterilisation.
5. Be cautious when selecting fungicides to ensure their maximum efficacy, cost-effectiveness, and minimal side effects on natural enemies and to avoid nasty deposits, crop damage, or measurable residues above maximum residue levels (MRL) at harvest. Our Fungicide Tolerance Service has proven to be extremely efficient selecting the right fungicides. This service investigates the level of resistance of the fungal or fungal like pathogens to the active ingredients of fungicides.
6. In organic production sites, the management of fungal diseases is challenging due to the lack of effective biological options. Hence, cultural control is key, including use of more tolerant varieties, crop rotation and less-intensive cropping. FungiAlert can also test the susceptibility of pathogens to commercially available bio-fungicides.
What is the future of the lettuce crop?
Currently, researchers are investigating new varieties that are more resistant to pathogens, climate effects and grow better in new cultivation systems. The consortium LettuceKnow is mapping the properties of 500 lettuce lines with the aim of developing a database that will help breed better suited leafy crops. For example, the plant architecture is one of the features being mapped by LettuceKnow consortium in order to obtain a variety that has a well-shaped head and a good root system adapted to the changing climate conditions. Moreover, this project will also map the activity of 30,000 genes in order to identify those that contribute to pathogen resistance, salt stress and response to other environmental factors. The data collected in this consortium will help identify the best candidates to design a super lettuce variety that would be more resistant to diseases and adapted to challenging climatic conditions. (7)
Besides the nature of lettuce, its nurture is also rapidly evolving, as novel ways of farming lettuce using hydroponic or aeroponic systems will soon convert the flat way this crop is farmed into a vertical system that will be sustainable and more efficient. Vertical farming is based on soil-less cultivation employing hydroponic or aeroponic set ups that sustain the growth of the crop from seed to harvest. Vertical farming is more sustainable because it can help overcome the limitations of conventional farming, such as the demanding and inefficient use of water, soil degradation, high concentrations of pesticides and fertilisers in the runoff, etc. A recent study proved that hydroponic systems use 92% less water to produce 1kg of lettuce than traditional agriculture (8). Vertical farming of lettuce also has the potential to increase the crop yield harvested in the same surface by more than a 10-fold. Due to the fact that vertical farms can be set up in any empty building, then the total amount of greenhouse emissions resulting from farming, can be largely reduced. (9)
Lettuce farming using traditional methods in the field is limited by seasonality and weather conditions. Vertical farming offers the opportunity of cultivating crops all year round in remote and extreme environmental conditions. For example, NASA astronauts have already tasted red romaine lettuce grown in a microgravity environment in the International Space Station (10) . Vertical farming not only offers the opportunity to farm lettuce in the outer space, but also underground. An innovative company in London is farming lettuce and other crops in a WW2 bunker in Clapham.
Besides the cultivation and the genetic base of lettuce, an immediate issue in lettuce farming that needs to be addressed is the increasing disease pressure combined with the reducing availability of chemical crop protection products. As fungicides are commonly applied to prevent disease outbreaks, fungal pathogens evolve resistance to active ingredients, furthermore, due to the intensive farming of this crop, new and more aggressive strains of pathogens are continuously evolving. These fungal pathogens are problematic especially during seed emergence, causing root wilt during growth and decay in storage. It is now widely accepted that our gut microbiome determines our health and its alteration can result in serious illnesses or, on the contrary, can solve metabolic disorders. FungiAlert believes that a similar approach, which focuses on the “farm” microbiome, can be used in farming to prevent serious disease outbreaks. In order to achieve this farming approach, we need to first understand the beneficial microorganisms living in the crop environment and encourage these to prevent the entrance and spread of devastating pathogens. The SporSenZ screening analysis is beginning to describe the microbiome in lettuce farming “guts”. Although we are only at the infancy of revealing the picture of the network of pathogenic and beneficial microorganisms in lettuce farming, we have proven that knowing the principal microorganisms in a crop system helps fighting disease pressures. FungiAlert’s vision is to contribute towards the sustainability goal in lettuce farming by understanding and encouraging the presence of beneficial microorganisms to reduce the presence of pathogenic microorganisms that cause disease and are often untreatable.
For more information, check our flyer for salad leafy crops here.
Angela de Manzanos Guinot, CTO & Co-Founder
3rd September 2019