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Consequences of alternaria still somewhat underestimated
Because the emphasis was on controlling phytophthora last year, some people have lost sight of alternaria. That is what Piet-Jan van der Eijk and Bauke Lettinga of Van Iperen say. "As a result, several crops in this region have died off somewhat faster than was necessary in retrospect."
With the designation of (new) NV areas – where a 20% lower nitrogen standard applies – both advisors expect that the influence of alternaria on potato cultivation will become even greater. "Nitrogen is the basis for a vital crop. If you give less, the chance of stress and therefore also of alternaria increases."
"So far, it's been a textbook spring. We were able to sow and plant everything nice and early, and there was also some rain in this corner on time. As a result, the soil herbicides worked perfectly. On April 30, we even tilled the last potatoes, which is three weeks earlier than last year," says Piet-Jan van der Eijk with a satisfied look on his face. In addition to being an advisor at Van Iperen, he is also a (part-time) arable farmer in Dronten. Although his parents Pieter and Janny still do a large part of the work, he is fully involved in the business operations. "Luckily, there are seven days in a week; so my parents don't have to do it all by themselves," he says with a smile.
Lessons from the 2024 season
This afternoon, Van der Eijk and colleague Bauke Lettinga will talk about disease control in potatoes – and in particular about the lessons that can be learned from the past season. "As far as phytophthora is concerned, farmers in our working area have come out of it reasonably well," Lettinga looks back on 2024 with some relief. According to him, arable farmers have taken up the urgent advice to alternate and combine agents well, although some still tried to 'extend the spraying interval riskily long'. "In my opinion, that is false economy and you are simply taking too many risks. With high disease pressure, you have to come back consistently, especially in a year with such high disease pressure," he says about this. Both advisors believe that the use of a BOS system has proven to be an important and valuable addition to the timing and approach of spraying. "Nine times out of ten, such a system provides very well-considered spraying advice. Together with the knowledge and experience that you have as a farmer – and with some extra input from an advisor – you will arrive at the best possible approach to the disease."
It is important that the BOS system is well 'fed' with information about the crop. "The more up-to-date and precise you do that, the better the advice is'', says Lettinga, who also puts forward Van Iperen's own module (in Agro Vision) in this context. "This is fed daily during the season, which ultimately leads to the best decision tree for the growers", is what he wants to say.
Alternaria sometimes underestimated
As for alternaria, both advisors still see some underestimation by growers. "Alternaria is a creeping disease. It can cause a lot of damage by prematurely dying off the foliage. Because last year the emphasis was strongly on controlling phytophthora, some people have lost sight of alternaria. Several crops here in the region have died off somewhat faster than was necessary in retrospect", says Lettinga.
A difficult question is always when the first spraying against alternaria should take place. According to both men, there is no ready-made answer to that. "We used to say that such a spraying should take place when the first petals fall off, but that rule of thumb has become outdated with the increased pressure. It is important to see how the crop is developing and whether it is vital and healthy. In the event of stress or damage due to, for example, stem breakage or sclerotinia, the chance of alternaria increases considerably. If something like that happens early in the season, then you will also have to add an agent against alternaria to the phytophthora control early on. That could be in July, but perhaps even at the end of June," says Lettinga. According to him, the variety also matters. The 'more difficult' it is to grow - Innovator is an example of this - the greater the risk of alternaria. Van der Eijk adds that due to the set NV areas, for which a 20% lower nitrogen standard applies, the chance of alternaria will only increase in the coming years. "Nitrogen forms the basis for a vital crop. If you give less, the chance of stress, and therefore also of alternaria, increases," he states. Especially in years with a lot of growth at the end of the season, alternaria can easily cause a 5% to 10% loss of yield. "As a farmer, you simply cannot afford to miss those kilos," Van der Eijk believes.
Getting Started with Propulse
As for the choice of remedies against alternaria, both men are quite definite: Propel is without a doubt the strongest remedy against the disease. Lettinga points to independent research by PPO, in which Propulse is the only one with four pluses. "Our advice is therefore to always start with Propulse. After a second spraying with a product from a different active group, the third spraying can be carried out with Propulse again. If necessary, a fourth spraying can follow with another product from a different group. With this sequence, you not only benefit from the power of Propulse, but you also ensure sufficient variation, which gives resistance formation as little chance as possible," concludes Lettinga.
