USA
February 12, 2025
When it comes to disease resistance, there are two classifications: HR (high resistance) and IR (intermediate resistance). These definitions were created by the International Seed Federation, but, in practice, what do they actually mean?
“Don’t get me wrong,” said Marcel Prins, Global Head of Product Strategy. “Of course, resistance is important in the fight against the ToBRFV virus. It is not without reason that we at Syngenta are working on expanding our portfolio with resistant varieties every day. And I will always advise a grower to grow a resistant variety. But in conversations with growers we notice that misunderstandings easily arise on this subject.”
There’s an immediate assumption that high resistance (HR) means no chance of disease breakthrough, and that intermediate resistance (IR) is not as effective. However, this might not be a correct assumption.
“The most important reason is that there is no official standard protocol to measure the effectiveness of resistance,” said Prins. “The boundaries between sensitive, intermediate and highly resistant are also not clearly defined. When a breeding company wants to determine the level of resistance of a new variety, it will do so under the conditions for which the variety was bred. But if that variety is then grown under different conditions in practice, consciously or unconsciously, the effectiveness of the resistance can be different.”
The genetic source or sources of the resistance also have an influence.
“I can talk about that for a day,” laughs Prins, “But it quickly becomes too complicated. I sometimes hear people say that you need multiple genes for high resistance. From a purely breeding technical point of view, that statement is incorrect. But when two or more resistances from different sources have been crossed into a variety, the resistance will probably be stronger and will be broken less quickly than with a single resistance. But even that is not a law of the Medes and Persians because there are known examples in which an IR variety performs better than an HR variety. And an HR variety can also contain virus after an infection, certainly with the sensitive Q PCR detection. All these things make it difficult to make a good comparison.”
Understanding Intermediate ToBRFV Resistance at Syngenta
Syngenta is always on the cautious side with sharing resistance type for ToBRFV, says Prins.
“We currently have more than a dozen commercially available varieties with ToBRFV resistance and dozens of varieties with that resistance in trials. These are single resistances and we characterize them as IR. At the end of 2024, our first variety will be on the market with two resistances, which will then be HR. Because we have breeding stations all over the world, we can test our varieties in many climates and under different conditions and thus prevent surprises.”
According to Prins, it is especially important for growers to focus on maintaining the effectiveness of a resistance and to assess a variety on how it performs in the greenhouse.
“Ultimately, it is all about the result for the grower. A resistance is nice, but it is especially important to test that resistance as little as possible because then you will benefit from it the longest.”
That means following integrated pest management strategies, and keeping greenhouses free of ToBRFV are imperative.
Proper Greenhouse Hygiene is Critical
“It will not be news, but hygiene is crucial to keep infections at bay,” Prins said. “In that respect, we have a head start in the Netherlands because we grow in closed systems and on substrate. It is then up to the grower to regularly disinfect machines, materials and clothing and to minimize the number of human-plant contacts. So not too many different people in the greenhouse, good personal hygiene and trying to limit the number of crop operations. Good zoning in the greenhouse can also contribute. And keep your irrigation water clean.”
Resistance to the ToBRFV virus is a common topic in discussions between tomato growers and their advisors. Breeder and plant virus Prins sees that misunderstandings can easily arise and is happy to contribute to the discussion from his professional knowledge. “Hygiene measures are and will remain important.”
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