By Carol Miller, Editor, American Vegetable Grower
Crop breeding companies are in a perpetual dance with disease strains. Just as a new race of Fusarium steps forward, breeders quickly introduce a resistant variety, taking the lead from the disease.
It’s a remarkable feat, considering the complexity of breeding and the time it takes to develop, test, and market a new variety.
We spoke with Enza Zaden’s lettuce breeder, Juan Marroquin, about how he and his colleagues stay a step ahead.
MEISTER MEDIA WORLDWIDE: There's a lot of tension between how quickly diseases development the amount of time it takes breed crops. How do you, as a breeder, respond to that?
Juan Marroquin: Your question is very complex.
As we know, breeding takes time. When we are doing a cross between two parents, parent A and parent B, we call the cross created an F1. From that F1 we need to move it to F7, F8, F9.
Today, by using different techniques like marker assisted backcrossing (MABC), marker assisted selection (MAS), and rapid cycling, we can shorten breeding time from between eight to 10 years to a much shorter time — between four and six years. But it is still four to six years.
This is what used to happen many years ago: If we were doing only one cycle a year, that means it will take us 7, 8, 9 years to get to a fixed variety that we can bring to the market.
Potential varieties need to go through a thorough research and development process. Part of our development process involves testing by our product development teams alongside our channel partners and sometimes, even with some select growers to see how a variety performs in commercial conditions.
So on top of the time it takes to breed the variety, you will have to add another two or three years. A very complex, very long breeding process.
It's a lot of time when you think about it. And diseases are always coming.
MMW: It seems like breeders get varieties resistant to new strains to market quickly, though. How does that work?
Juan Marroquin: We as breeders spend a lot of time trying to anticipate the evolution of some diseases.
We monitor the pathogens present in the commercial fields, how the disease evolves geographically, through different varieties, etc. When we find a disease we believe will become commercially important, we then try to identify a resistant gene or genes.
When trying to identify new sources of resistance, it is important to have a diverse germplasm base. This base allows us to find new traits, whether we source these from wild types and/or elite lines.
One additional question we must keep in mind is: What else is needed in these varieties? Normally, resistance to only one disease is not enough and there are commercial/agronomic needs that must also be met.
MMW: One disease sending shock waves through the leafy green industry is impatiens necrotic spot virus (INSV). I’ve been told it used to be a 1% problem, meaning it affected so few, most breeders did not focus on it. Then it began wiping out crops mid-season in the Salinas Valley. How are you responding to it?
Juan Marroquin: We have been keeping close track of INSV evolution and we are actively breeding for resistance to INSV throughout our portfolio. One thing I do want to highlight is that we already have a variety in our portfolio that performs well under INSV pressure. This variety is called ‘Telluride,’ one of our iceberg varieties.
One additional question we must keep in mind is: What else is needed in these varieties? Normally, resistance to only one disease is not enough and there are commercial/agronomic needs that must also be met. We are working actively to fill these grower needs throughout our portfolio.
To illustrate further, in spring, we know that one of the resistances we need in our resistance package is bremia. Whereas in the summer, at a minimum, we must include a combination of verticillium, fusarium, and INSV. We are working actively to deliver a full-spectrum disease package to growers soon. In 2023, we already saw some lines with promising resistance to fusarium in both Salinas and in Yuma.
MMW: What is a marker and how you use it?
Juan Marroquin: A molecular marker is a genetic marker that allows us to identify particular traits that are of interest in an organism. The most common technology used is single nucleotide polymorphism (SNPs).
In the breeding process, we take leaf tissue samples for the purpose of DNA extraction. Once we have the extracted DNA in the lab, we then run the genetic markers and it will let us know the presence or absence of the marker associated with the specific trait of interest. These markers have already been mapped by our own private research or through published research papers.
In Enza Zaden, we have molecular breeder teams dedicated to the development of molecular markers for the various diseases that we have around the world.
Meister Media: Which diseases keep your attention?
Juan Marroquin Aco: This is a great question. We need to break it down first by type, location, seasonality and/or slot, to name a few of the variables. To give an example, for icebergs in Salinas, we look at the three main seasons, spring, summer, and fall, with the summer season being the largest in market size by far.
We as breeders cannot say we're only working on one disease. We need to look at the broader picture.
We need to break down disease by the season because every season has its own characteristics. We know in the spring there is more rain and more fog, making bremia pressure/incidence higher. So growers are going to need varieties with resistance to bremia.
Then we have the main slot, summer, which starts to be a little bit more complicated, because we have fusarium and verticillium. And now everyone talks about INSV.
So those are the three main targets that we are working on.
Finally, when we go to the fall, it's a mix of everything because we have fusarium, verticillium, INSV, pythium, and sclerotinia.
Moving on to Yuma, we have fall, winter, and spring markets. In the early fall we see the need for fusarium resistance. During winter, our focus is bremia resistance, but we need to look at the agronomic characteristics as well. Growers look for a plant type that has better frost tolerance. This is done by focusing on specific plant characteristics like plant structure, plant size, shape, leaf coverage within the plant to name a few examples.
