Within the framework of the Discovery Field Days for Leafy Vegetables & Open Field by Enza Zaden, the seed company held in December the event ‘Creating Value, Sharing Knowledge.’ This forum tackled two of the most pressing problems for lettuce producers under the heading: “Bremia and Fusarium, key elements in the redefinition of the fields.”
“We are experiencing a time of redefining the main paradigms,” Alba Redondo, Marketing & Customer Services at Enza Zaden, affirmed. Bremia Lactucae and Fusarium oxysporum f.sp. lactucae are spreading across the fields. In the case of Bremia, what was “a concerns in 2012-2014” and a “problem looming on the horizon,” according to the former research from IMIDA, Alfredo Lacasa, has become a growing reality due to the adaption of this microorganism to climate change. “In optimum conditions, explosive epidemics occur. The number of cases shoot up and propagate,” causing important damage.
Fusarium oxysporum f.sp. lactucae has appeared in South-eastern Spain more recently and it behaves in a different way. “Each strain of fusarium affects a specific crop, for example, tomato fusarium does not affect lettuces. But in all the cases, it causes soil fatigue.”
Although there is currently a clear predominance of the Fol:1 strain, there is an “emerging situation of the Fol:4 strain, which is becoming increasingly more important. It is essential to continue monitoring the fusarium problem,” explains Víctor Segura, Crop Breeding Manager at Enza Zaden.
“In the USA, a new variant has appeared, and we expect that, over time, if we don’t make an effort with resistances, another strain will appear here.” In the R&D phase of new varieties, Enza Zaden is working with several resistances to create ‘layers’ and to ensure that the resistance against the attacks lasts as long as possible.
Segura also details that “a gene with resistance might show resistance to a new strain. That is what happened in the USA, where varieties with high resistance to Fol:1 and Fol:4 are also resistant to the new isolated strain that has appeared in that country.”
Which direction is the lettuce crop taking?
Cayetano Fernández, Product Manager for West Europe at Enza Zaden, showed the evolution of Fusarium and Bremia around the world.
Fusarium is present in many areas, mainly in Japan and other Asian countries, where the Fol:2 and Fol:3 strains predominate, while in the rest of the world Fol:1 and Fol:4 are more common.
In South-eastern Spain, Fusarium appeared in 2005, over a 10 hectare area. Up until 2010 there were 200 has and in 2015, 700 has, when the Fol:4 strain also appeared. Over the next 5 years, by 2020, the figure shot up to 2,000 has. “It started to proliferate on more farms, companies and surface area, and during the period from 2020 to 2023 the surface area has quadrupled, reaching 8,000 has, and the first cases have appeared in the Campo de Cartagena.”
What strategies can be used against Fusarium?
There are 2 resistance levels: high (HR) and intermediate (IR). Until 5 years ago, the IR worked acceptably, but for the past 3 years, the situation has changed and it is becoming more and more complicated working with varieties with IR if the conditions are favourable for the fungus. “To date, varieties with HR are not having any problems.”
Another important aspect is its proliferation in months when previously it was not common. In 2010, it could be found in the fields from June to September; in 2020, from June to November and in 2023, only the months from January to March “are free”. Additionally, in many cases the Fol:1 and Fol:4 strains can now be found on the same farms.
Enza Zaden is tackling Fusarium with an extensive catalogue with HR and IR to one or both strains in different typologies. Amongst its challenges, is the development of an iceberg programme with HR to fusarium. Next summer, they will already have 4 varieties. “We are speeding up the process, and in 3 years we think that we will have a programme to cover from June to mid-December.” In Mini Romaine, Little Gem and Romaine, they already have a complete programme with resistance to Fol:1 and Fol:4
How can Bremia be beaten?
Bremia or lettuce mildew has been present in Europe for many years. There are strains that are not very stable over a period of time; they have an important capacity for mutation, as may be seen by looking back: in 2015 the BI-EU:1-15 strains disappeared and they were renamed the Bl-EU: 29, 30, 31. In 2016, the Bl-EU: 32 appeared; in 2017, the Bl-EU:33; in 2018, the Bl-EU: 34, 35 and in 2019, the Bl-EU: 36. The Bl-EU:37 strain appeared in 2021 and initially it caused a problem, but currently there is very little incidence. In 2023, the Bl-EU:16-28 disappeared and they were renamed the Bl-EU:38, 39, 40, with Bl-EU:38 and Bl-EU:40 in the lead at the moment.
At the last meeting of the IBEB, the existence of new isolated cases was put on the table, with a proposal for studying them.
The affection period of Bremia has increased over time and it can be found in the fields virtually all year long, except for the second half of July and the first half of August. Likewise, there might be combinations of more than one strain on the same plot of land.
A large part of Enza Zaden’s lettuce catalogue shows HR to the new Bl-EU:38, 39, 40 strains and many include complete resistance to Bl-EU:29-40.
