Arable News

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Dont let early blight get out of sight

Dominic Kilburn gets an update on blight forecasting developments

With blight coming into some potato crops last season ahead of Smith Period warnings, talk of updated forecasting systems will be welcome news for growers and agronomists alike. Dominic Kilburn gets an update on blight forecasting developments as well as strategies in the field for managing early blight.

A specialist potato adviser believes that the long-established Smith Period blight warning system for assisting growers and agronomists in planning their potato crop protection campaigns is becoming outdated as new strains of blight develop.
According to SPUD Agronomy’s John Sarup (left) , it’s already known that the now dominant Pink 6 and Blue 13 potato blight strains in the UK develop in crops at lower temperatures than Smith Period calculations account for, and he welcomes on-going research looking into a more precise system of forecasting. “A system that encompasses wider temperature parameters would give growers a much better idea of when the disease is likely to enter a crop as well as enabling them to tailor the different blight products available to the scale of the threat,” he comments, adding that it’s a lot more costly to deal with blight once it is in the crop rather than protect against it in the first place.
“One of the biggest issues is how the climate varies from field-to-field,” continues Mr Sarup. “Some fields are surrounded by trees, for example, others will be irrigated during the season, and these kind of variable conditions will all have a part to play in the climate of individual fields and the level of blight likely to be present.
“And with Smith Period forecasts based on postcodes, blight pressure can be very different within that area,” he adds.
The AHDB Potato Council’s head of communications and knowledge transfer, Dr Philip Burgess (left) agrees, to a certain extent, that the Smith Period has become outdated because of the changes in blight pathogen now faced by UK crops, and changes in agriculture as a whole, since the system was first developed in the 1950s.
He says that the Potato Council is actively working towards developing a new blight forecasting system while highlighting that the Smith Period continues to have a very important role to play. “It still offers a very resilient forecast whereby if you have inoculum in your area when a Smith Period is declared, then you will certainly get blight in your crops unless preventative action has been taken.
“It’s a very good predictor and it will tell you when there is a problem,” he adds.
He says, however, that there are occasions when there will be ‘near misses’ in terms of a Smith Period, and blight infection gets into the crop without warning, and so a good balance is needed between what works well now for forecasting, and what can be developed for the future.
Dr Burgess points out that the Potato Council’s successful Fight Against Blight’ (FAB) service continues in conjunction with the Blightwatch map alerts sent to subscribers when Smith Periods occur and blight risks are identified in their area.
“We’ve learned more about the different types of blight strains UK growers have had to deal with over the past few years through our national network of Blight Scouts, who provide intelligence they’ve gathered to Fera and The James Hutton Institute for further analysis.
“The James Hutton Institute has taken the work further forward to determine growth characteristics of blight strains; the latent period of spores and temperature and humidity factors, for example, and researchers are building a much more detailed picture of changing blight populations.
“Sophisticated modelling work to help understand how epidemics evolve and spread are being developed and we should be able to use this information to better forecast when blight is likely to be a threat in the future,” Dr Burgess concludes.

New criteria
According to crop protection company Syngenta, its popular BlightCast five-day blight forecasting service, updated hourly on the company’s website and delivered direct to growers by email, will now feature a ‘New Criteria’ forecast, alongside a traditional Smith Period forecast. 
The ‘New Criteria’ pinpoints forecast temperature to hit over 8*C and more than 11 hours at 90 per cent humidity over two consecutive days to trigger a Blight Period, or a ‘Near Miss’ where conditions occur for a shorter period. Syngenta potato manager, Andrew Curtis, highlighted that leading independent blight researchers are currently assessing these new cooler parameters as the conditions where they believe today’s blight strains may already be active.
“While it is still subject to evaluation and further research, we wanted to make growers easily and quickly aware of these conditions, when hitherto risk was considered low but, in fact, blight may be infecting crops,” he said. “It will enable them to time sprays more effectively and make better informed decisions on appropriate fungicide choice to prevent infection.”
The existing recognised Smith Period five-day forecast remains the primary information on the BlightCast website, with the secondary ‘New Criteria’ forecast and a five-day spray application window forecast – all based on the grower’s local postcode. “Using these tools in combination, growers can now assess the potential risk facing their crops and alter product choice of spray scheduling accordingly to ensure effective protection is in place,” advised Mr Curtis. 

Blight strategy
Any products used very early last year did well in controlling blight, however each season is different and lessons can be learned, continues John Sarup. “Conditions were ideally suited for blight at the start of last season. It had been a mild winter, there were plenty of volunteers while warm, humid weather coincided with rapid crop growth,” he points out.
“The blight came in early which caught people out, and then they went in with the wrong products – protective rather than curative – and, by the time the curative products went on, blight was already on plant stems and very difficult to control,” he explains. “Generally speaking disease was then managed for the remainder of the season, but it wasn’t got rid of and plenty of rain in August washed the spores off the stems and onto the tubers causing more cases of tuber blight than usual,” he says.
Although it’s been a similarly mild winter for this season, spring weather has been a little cooler than last year, points out Mr Sarup, who stresses that his main concern at this time of the season is the management of potato dumps which are a major cause of spreading blight. “As well as low value discards, dumps can include those that are left on one side for stock feed and so aren’t necessarily the main priority of a stockman, and, put simply, it’s critical that they are used before growth occurs – it’s fundamental for preventing the spread of blight.

“It is also worth remembering that FYM from stock-fed potatoes can transfer diseases and viruses such as powdery scab and mop-top virus (PMTV),” adds Mr Sarup.
He emphasises that growers need to set out on a prophylactic, 7-day interval spray programme for effective control of blight this season. Blight fungicide products should be used according to the conditions and pressure at the time, but it’s the frequency of those applications that is key to whether a successful strategy is achieved, he advises.
“Get started with 7-day spray intervals and then, if there is less blight pressure, tailor the products accordingly,” he says, adding that it’s about understanding which products are available and what we are trying to do with them.
First applications are usually at the rosette stage of the crop, he points out, highlighting that protection at that very early stage will buy growers time for when crops begin to grow away very quickly.
He recommends starting with a fluazinam-based product such as Shirlan because of its activity on the soil. “You will be spraying as much as 50 per cent of the application on the soil at that stage of the season and so it may as well be a product that is able to give protection to the tubers.
“This time last year with the high pressure it would have needed a straight cymoxanil in the mix such as Option or C50 for strong curative activity,” he adds.
Available for the first time this spring is a new product from Syngenta for early blight programmes called Carial Flex (cymoxanil + mandipropamid) and this, suggests Mr Sarup, could be applied as the second application of the programme offering both preventative and curative activity. The advice from Syngenta is that Carial Flex should be applied before blight enters the crop however applications one day after an infection event has occurred have shown to give good levels of control.
If things are growing slowly then Mr Sarup suggests that Shirlan + cymoxanil or Tanos (cymoxanil + famoxadone) will also provide preventative and curative activity, adding that the latter is often tank mixed with herbicide Titus (rimsulfuron) when applied to late-emerging cleavers, couch and volunteer OSR.
“If I really want to hot things up I’d use Curzate, or a similar product, containing cymoxanil + mancozeb. This gives superior control of blight and the use of multi-site fungicide mancozeb helps with an anti-resistance strategy.
“Others to consider early in the programme include Invader (dimethomorph + mancozeb) and Hubble (dimethomorph + fluazinam) which, when pressure was high early last season, worked very well,” he concludes.

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