THE loss of mancozeb has left a big hole in blight programmes, both for late blight and Alternaria control, and for resistance management, and trials by The Potato Partnership (TPP) continue to be carried out to fill the gap with biological and plant health products, rather than more of the same systemic chemistry.
Those involved in the trials say they are seeing some positive signs in the work they’re carrying out with biostimulants and phosphonates.
Technical Manager for non-combinable crops at Agrii, Don Pendergrast, said: “We are seeing a lot of potassium phosphonate-based products coming through, so there’s a lot of focus on these. We also looked at some novel biocontrol options alongside plant health stimulators like Innocul8, which we have looked at for some years now.”
The 2025 trial was at the Eurofins site in Derbyshire, using the variety Melody, inoculated with EU36 and EU37, although the conditions were so dry that little blight was seen until September, by which time the dominant strain was the oxathiapiprolin (OXTP) resistant EU46.
“Last year’s trials built on our 2024 work, which used the biostimulant Innocul8 at the beginning of the programme. Alongside fungicides, it seemed to indicate a way of managing the loss of mancozeb,” notes Don.
The conventional systemic fungicide programme, with Innocul8 replacing mancozeb, outperformed the programme including mancozeb in terms of green leaf area and late blight infection levels by the end of the season.
“The plant health effects of Innocul8 were underpinning the fungicide programme,” Don said.
Because of these results and the fact that mancozeb is no longer on the market, all the 2025 programmes tested were based on a programme using early Innocul8 alongside the standard fungicide programme, which included several new products that are close to launch, submitted by crop protection manufacturers. Don says they wanted to move away from mancozeb always being in the conversation and compare a series of alternative products that represent a different approach.
Mixed into this programme, they compared different treatments using a range of potassium phosphonate products, micronutrition, orange oil and plant extracts.
“The first major infection event happened on August 14th, and differences were clearly visible by the middle of September, just prior to crop desiccation.
“There was a clear benefit to adding a third application of potassium phosphonate in the programme. Three applications of Privest (ametoctradin + potassium phosphonate) clearly looked like the best plot in the trial. This is partly because it was the only programme that excluded oxathiapiprolin or an equivalent mode of action fungicide, given that the dominant strain in the trial was OXTP resistant EU46.
“The orange oil treatment looked noticeably different in the field. The leaves were more waxy, and they had a sheen to them, which is no surprise considering we were applying something oil-based. The plant extract produced a similar sheening, and the crop looked healthy throughout,” Don said.
Considering the number of phosphonate products applied, they checked residues in the tubers at the end of the season, and they were well below the maximum residue limit (MRL). Don said the team will continue to monitor this, because in a different season, up to six applications might be used which is close to the limit.
Last year’s work shows that there are some alternative product options to support a standard blight programme, he added. Additional applications of potassium phosphonate products, orange oil products and micronutrition all offered benefits in the programme. The aim of the 2026 TPP blight trials will be to determine the most suitable timing for these.
THE confirmed presence of both resistant EU43 and EU46 strains in the UK, combined with the loss of the primary multisite in fungicide armouries, means that growers are right to be concerned about blight fungicide resistance this season, says Don.
So, can some of the biological products tested by TPP offer some degree of protection for the systemic chemistry?
“We have to draw partly on experiences in other crops,” Don said “But, if you think of mechanisms like eliciting the plant’s own defence mechanisms, which is what Innocul8 does, there is evidence that it disrupts resistant strains as well as susceptible ones. Products like orange oil have a physical mode of action, so they work similarly to a multisite.
“The more things you can add in a programme, the more potential you have to disrupt the overall flow of resistance development, particularly in high pressure situations. However, biologicals aren’t the answer to blight resistance, especially when they act as biostimulants.”
Ed Maule, lead Potato Technical Advisor for Agrii, stressed that a lot of Alternaria control is about keeping the crop healthy throughout the growing period with good base nutrition to support specific Alternaria fungicides in a preventative, protective manner.
It is because of this that the TPP trials also examined how plant health elicitors and micronutrition can enhance the efficacy of conventional fungicides against Alternaria. The loss of mancozeb will also affect Alternaria control, another issue the industry will have to get to grips with in the coming years.
“The trial showed that Innocul8 delayed the ingress of Alternaria into the trial. The best treatment was a fungicide programme supported by Innocul8 early, followed in later applications by a biostimulant containing copper and zinc,” said Ed.
“This supports green leaf area retention. We know there are some varieties that can lose a lot of green leaf area quite early. If we can take those through to later in the season with this approach, we can help it to achieve its yield potential.”
