Staying ahead in the race against ToBRFV

Viruses evolve quickly – and so must breeding. In this interview, Frank Millenaar, Pre-Breeder Tomato, and Marco Mammella, Principal Phytopathologist, explain how the next generation of ToBRFV-resistant tomato varieties is being developed – and what it means for growers worldwide.

Frank, Marco – what is the major innovation growers can expect in the next generation of ToBRFV-resistant varieties?

Frank: The key innovation is the stacking of three resistance genes in one single variety. Until now, most commercial varieties have relied on one or two resistance genes. Moving to three represents a new level of genetic robustness. This level of resistance may represent a first in commercial tomato breeding for ToBRFV. It would position these varieties at the forefront of the next generation of genetic protection.

Marco: And what makes this particularly powerful is that these genes are functionally different. Each one contributes a distinct layer of defense, meaning the virus would need to overcome multiple independent mechanisms simultaneously. From a biological standpoint, that dramatically reduces the likelihood of resistance breakdown.

Frank: With a single gene, the virus can adapt relatively quickly. With two genes, the barrier becomes significantly higher. With three, we move into a level of durability where resistance is far more stable under diverse environmental and epidemiological conditions. For growers, this translates into a stronger safety buffer, especially in high-pressure situations and across varied production regions.  

Why is such a strong resistance needed? Haven’t resistant varieties already been introduced?

Marco: Resistance is never a final state. It is dynamic. RNA viruses such as ToBRFV have very high mutation rates, which allows them to continuously generate new variants. As soon as a resistant variety is widely grown, the virus population starts to adapt. That’s why we cannot rely on a single breakthrough. We need continuous innovation to stay ahead of the pathogen.

Frank: It’s an ongoing race. And today, that race is faster than ever due to global trade, propagation systems, and rapid movement of plant material. What emerges in one production region can spread internationally within a very short time frame.  

What are the risks for growers if breeding does not keep pace with these developments?

Frank: The main risk is loss of resistance effectiveness. Once that happens, plants become susceptible again, which can lead to yield loss, fruit quality issues, and rapid within-greenhouse spread.

And because ToBRFV is mechanically transmitted, control is extremely difficult once it is established. It spreads via tools, hands, and plant handling. That makes genetic resistance one of the most effective and reliable control measures available.

Are there additional factors that make resistance breeding more challenging today?

Frank: Temperature is an important factor. Some resistance mechanisms can become less effective under higher temperature conditions, particularly above 30 degrees Celsius. That’s another reason why stacking multiple genes is so relevant. It provides an additional safety margin under these challenging conditions.

Marco: Climate change adds complexity on both sides. Plants experience more abiotic stress, which can weaken defense responses, while viruses may also evolve more rapidly under changing environmental conditions.

Where do these resistance genes actually come from?

Frank: Most resistance genes originate from wild tomato species. These genetic resources have been collected and preserved over decades in germplasm collections.

Once a promising resistance source is identified, we validate its stability and characterize its genetic basis. This involves creating segregating populations, mapping the resistance to specific chromosomal regions, and developing molecular markers. This phase alone can take one to two years before practical breeding even begins.

And how do you transfer that into a commercial variety?

Frank: After identifying the gene, we introgress it into elite breeding material. We cross wild donor sources with high-performing commercial lines and then select offspring that combine resistance with agronomic and commercial traits such as yield, taste, shelf life, and fruit quality.

This process involves repeated backcrossing and selection. We aim to recover almost 100 percent of the original elite background, with only a small segment carrying the resistance. If the segment is too large, you risk negative effects on quality or yield.

Marco: In parallel, we validate resistance under controlled and real-world conditions, including exposure to different virus isolates and environmental scenarios, to ensure broad and stable effectiveness.

Given that this process takes several years, how do you keep up with the speed of virus evolution?

Marco: Breeding is not linear; it is continuous. While one generation moves toward commercialization, the next is already in development. At the same time, we actively monitor virus populations in the field. We analyze genetic sequences and track epidemiological trends across regions. This allows us to anticipate shifts in the virus population rather than simply react to them.

What role do modern technologies play in accelerating breeding?

Frank: Molecular markers are essential. Once we know the genetic location of a resistance gene, we can track it at the DNA level. That means we can select the right plants at the seedling stage, without waiting for full plant development or infection tests. This dramatically speeds up the breeding cycles and increases precision. Without molecular markers, stacking multiple genes efficiently would be almost impossible.

Will all growers need these new three-gene varieties?

Frank: Not necessarily. The need depends strongly on local disease pressure and production context. In regions with low or stable ToBRFV incidence, current varieties with one or two resistance genes may still provide sufficient protection and excellent performance. In other regions with higher pressure or greater epidemiological uncertainty, three-gene stacking can provide an additional layer of security and production stability.

Looking ahead, will resistance stacking go even further?

Frank: Yes, it will continue. Three genes are a major step, but not the endpoint. We are already working on additional combinations for the longer term.