An early start, tight spray intervals and choosing the right fungicide type at that stage of the programme are three key components of successful potato blight control, says one Dutch expert.
Potato blight is one of the most devastating crop diseases in agriculture and costs UK growers £20m/ year in fungicides with new aggressive strains making control more difficult.
While there are subtle differences in the make-up of blight populations across EU member states, the more aggressive strains of late blight that now dominate the region all have some common features, says Huub Schepers of Wageningen University and Research.
“First and foremost, they have a shorter lifecycle of three to four days, compared with the six to seven days that were typical of previous strains,” he reveals.
They also have a wider temperature range in which they can develop, making them more suited to lower temperatures, as well as the capacity to produce more spores, adds Dr Schepers, who is also a co-ordinator of the Euro-Blight network.
The UK situation
The two aggressive potato blight genotypes that dominate the UK population have now been around for a few years.
The good news is that there was very little diversity among the samples received from crops across England during 2014, reveals David Cooke of the James Hutton Research Institute, with both the Pink 6 and Blue 13 strains being found in almost equal numbers.
“That has been the case now for the past two years,” he says. “And while it remains like that, there is no reason to believe that sexual reproduction between the two different mating types is taking place.”
That is important, he adds, because if it should start to occur and oospores are generated, the blight population could change very rapidly.
“Added to this, the success rate of the spores is higher.”
His final point about these strains is that they can break down the resistance genes of potato varieties.
“So you get more late blight on resistant varieties where the strain known as Blue 13 is present, for example.”
Monitoring shows that the dominant strains in the UK are both Blue 13 and Pink 6, but in the Netherlands it’s a different picture.
Some 44% of the blight population is Blue 13, the rest is made up of a number of small groups, some of which are new, reports Dr Schepers.
“We have an additional source of diversity here,” he explains.
“Sexual reproduction between different mating types has occurred, so there is more variation and we have strains such as Green 33 and isolate Orange, as well as others.
“It means that for Dutch potato growers, soil-borne oospores are another potential source of late blight infections.”
That’s why it is important to keep monitoring potato blight, he stresses. “Given the current situation, it’s difficult to predict what will happen next.
“The EuroBlight network aims to keep a close watch on the situation across Europe, so that we can get an early warning of any changes.”
Looking ahead to the new season, blight control strategies have to address the ability of these strains to build up very rapidly in the right conditions, advises Dr Schepers.
“Protection is critical when the plants are emerging,” he advises.
“Although certain conditions are required for infection to occur, the pathogen can complete its lifecycle in a very short time. The new leaves must be protected.”
While contact fungicides such as mancozeb can do a good job, application timing with these is essential, notes Dr Schepers.
“Newer fungicides give a bit more flexibility, because some have anti-sporulant properties, as well as some kick-back or curative activity.”
Anti-sporulants act to prevent spores forming, so helping to control an epidemic, he reports.
“That’s different to sporicides, which kill the spores produced. And all fungicides do that, but some are clearly better at it than others.”
To help with fungicide selection, he refers growers to the EuroBlight tables, which rank the different blight fungicides according to the type of control that they offer. Other properties, such as rainfastness, are also given a score.
“The good news is that we do have the armoury to control blight, providing the most appropriate products are used at different stages of the programme at the right spray timings,” comments Dr Schepers.
Mistakes with blight control occur for three main reasons, he adds.
“Starting too late, failing to keep to tight spray intervals and choosing the wrong product. These are all areas where things can go wrong.”
The mild 2014-15 winter may have allowed the pathogen to survive, meaning that inoculum is present at the outset this spring, explains Dr Schepers.
“In this situation, volunteers and tubers on dumps aren’t killed and the green bridge remains. But of course the weather at the start of the season is also important – if we have a dry spring, there won’t be any blight.”
Knowing which blight strains are present makes it easier to choose the most appropriate fungicides for different stages of the crop’s development, says Albert Pineda of Adama.
“Strains that produce more spores and germinate faster in colder conditions mean that the disease can suddenly explode in crops,” he points out.
Therefore, an understanding of what to expect from fungicides with different modes of action is essential, he believes. “Look for products that combine protectant activity with the ability to reduce spore numbers. Remember that tuber blight incidence is linked to spore production.”
Adama’s Hubble (dimethomorph + fluazinam) has two modes of action, making it suitable for both the rapid and the stable canopy phases, he points out. Fluazinam remains on the leaf surface, while dimethomorph has some mobility within the plant.
“There’s also a synergistic combination of antisporulant and sporicide activity not available in other blight products.
As a result, Hubble protects new growth, provides some kick-back activity and reduces sporulation rates.”
Trials commissioned by Adama and carried out at Wageningen University showed that Hubble gave the best control of a range of aggressive blight strains.
“The results are relevant to the UK because we have very similar disease pressure challenges to the Netherlands.”
He also highlights the latest EuroBlight fungicide table. “It shows how different products perform in commercial conditions. Hubble has very good scores for both leaf and tuber blight.”