Blight remains a key threat to potato crops. Loss of actives, resistance concerns and decision support systems were among the topics under discussion at the biennial EuroBlight conference held in York.
Loss of herbicides in vegetable crops, lack of quality in spraying and over-reliance on a small number of actives are a threat to potato late blight control.
Speaking at the EuroBlight workshop held in York, Denis Buckley of Highfield Lodge Agronomy said: “We are losing herbicides at such a rate of knots that we cannot reliably control volunteers in many vegetable crops anymore and these can be important sources of blight, particularly later in the season.”
Ensuring the whole crop is treated with fungicide and avoiding spray misses is also important, he said. “With GPS-controlled sprayers you need to watch out for row starts as there is usually not enough pressure in the boom to instantly and consistently have the spray quality you want.”
While tools such as the Hutton Criteria on BlightWatch were important in gaining a handle on blight risk, Mr Buckley said at the moment weather forecasting was not accurate enough to allow a decision to apply fewer sprays, particularly on large farms. “Small growers could go out and spray within less than a day but bigger farmers who are spending most days a week spraying cannot say they will delay spraying for a week.”
The new aggressive blight strains – EU36-A2 and EU37-A2 have significantly increased the cost of blight control programmes, said Mr Buckley. He gave an example of a programme in 2015 before the strains were detected, based on fluazinam, mancozeb and Ranman Top (cyazofamid), costing £186/ha.
For this year, a more diverse programme containing Invader (dimethomorph + mancozeb) or Valbon (benthiavalicarb-isopropyl + mancozeb), Zorvec (oxathiapiprolin), Percos (ametoctradin + dimethomorph), Ranman Top, Infinito (fluopicolide + propamocarb hydrochloride), Rhapsody/Curzate (cymoxanil + mancozeb), mancozeb and fluazinam costs £370.
Mr Buckley said: “EU36-A2 and EU37-A2 are cropping up over the whole country. These are nightmare strains.
“I find it very scary that some people are using, for example, only Ranman or only Revus in blocks. This is very risky in resistance terms. Mixture with mancozeb should surely form part of any resistance strategy.”
Providing agronomy services to more than 1,000 potato growers in northern Belgium, Pieter Vanhaverbeke of PCA wanted to extend his offering by developing a decision support service for early blight (Alternaria solani).
Working with the University of Ghent, 622 potato fields were monitored for early blight and 1,386 lesions analysed, of which 28 per cent were infected with Altenaria solani. Various factors were recorded to determine their effect on development of early blight to develop a computer model capable of predicting disease risk.
“The weather is important for early blight and you need inoculum, but in our climate, the physiological resistance of the crop plays an equally important role. Leaf wetness was the most important factor for epidemics. Crop rotation also affected disease level and for an epidemic you needed a certain level of senescence.
“Once in a crop, early blight spreads more slowly than late blight but in 8-10 days it can destroy the potato plant.”
In 2017 trials, August 28 turned out to be the critical date for starting treatment to protect potatoes against early blight; the model recommended a first spray between August 17 and 23. In trials the following year, the critical date was August 23, with a first spray advised on August 17. Starting treatment earlier did not add anything in terms of crop protection, said Mr Vanhaverbeke. “A lot of sprays are done in June and July but do not add anything to protection at the end of the season. They should be scheduled towards the end of August.”
Mr Vanhaverbeke said using the model could enable early blight sprays to be cut from 6+ to 2-3. He adds that it is important to alternate fungicides
He is continuing to validate the model which can also take plant stress into account.
The chemical armoury for preventing early blight, in particular, is under threat, with mutations in early blight populations insensitive to key fungicide modes of action.
Albert Schirring of Bayer said the two ‘big’ modes of action for early blight – Qi’s and SDHI’s were under resistance pressure and potential loss or restricted use of mancozeb is a concern. “It is the last multi-site available for resistance management and we do not know whether approval will be renewed. If it survives, we are likely to be much restricted in how we can use it.”
Since the 1107/2009 European Union pesticide regulation had been implemented in 2011, 50 actives lost their registration in Europe and there had been a significant increase in emergency authorisations, in particular for insect control, said Mr Schirring.
Since the implementation of the new legislation only one active substance for use on potatoes had been approved while in the same period in the US, 12 new active substances had been approved, he said.
He contrasted the European system with the US system where it is clear what must be submitted in a dossier for a new active substance approval application and no additional requirements are added later in the process. Companies and research organisations are provided with a clear framework on the regulatory studies required and the associated investment costs and they receive feedback within a year after submission of the regulatory dossier, he said. “This clarity is not available for the industry in Europe.”
EuroBlight is a European network of scientists and other specialists working on potato early and late blight that meet every second year.
This year the workshop was hosted by ADAS in York, with more than 100 people attending from 20 countries from all over the world.