Consider the limitations of the Smith Period in predicting blight infection

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The last few seasons have seen some real changes in the way we need to think about potato blight says Darryl Shailes, Hutchinsons root crop technical manager.

“2007 was the first real blight epidemic to get the industry thinking that things might be changing. If we cast our minds back, blight came in earlier than normal with severe infections widespread by about mid-June. The disease pressure did eventually calm down as the weather changed, but for a while no-one really knew how to react and blight product supply was getting very tight.”

He says that during the peak of the epidemic in 2007 many fields were being sprayed every 4-5 days with multiple tank-mixes and still the blight was only just being held. “Luckily the weather eventually intervened – it became hot and dry and the blight stopped almost in its tracks.”

There was a lot of R&D commissioned by The Potato Council as a result of the problems we encountered in 2007 and the ‘Blight Scout’ scheme was set up. This was probably the first time that the different strains of blight were brought to the attention of anybody beyond the academic research institutes.

“Along with fears that we were getting sexual recombination between the two mating types that were identified (A1 and A2), it was first muted that they were forming resting spores that could overwinter. Since then, thanks to all the research that has gone on, we have come to understand a little more.

Mr Shailes notes that the dominant strains over the last few years have been A2 Blue 13 and A1 Pink 6. And that the evidence for overwinter resting spores has not been found, although some of the strains are able to sexually recombine in the laboratory.

He says that these newer stains of blight are however more aggressive, able to operate over a wider range of climatic conditions and some are resistant to blight fungicides such as Metalaxyl.

“Since 2007 we have had 2 other very significant blight epidemics – 2012 and 2014. Both of these epidemics, in common with 2007, started very early in the season, with many crops being infected almost as soon as they came through the ground.”

So what does this mean for 2015?

No one can predict whether or not we will have a bad blight year, although what we can say is that modern potato blight does not seem to have heard of a ‘Smith Period’ says Mr Shailes.

A full Smith Period occurs if, on each of 2 consecutive days, the minimum air temperature is at least 10°C and there are a minimum of 11 hours with a relative humidity of at least 90%.

“This was the infection criteria defined by Mr Smith who worked for the Agricultural department of the Met office in the 1970’s. We know that the modern blight strains can operate outside of these parameters, but the Smith Period has yet to be re-defined.”

“Unfortunately, most of the freely available forecasting systems such as ‘Blight Watch’ still use Smith criteria. However, we do know that in dry weather blight pressure will be low, as leaf wetness is a critical factor that is not even mentioned in a Smith Period. Also, by its very nature, a Smith Period is historic and a reaction to Blight pressure, whereas we need to have a more predictive model if we are not just to set a programme at the start of the season and spray every 7 days.”

“In recent years in trials and in the field, Hutchinsons have been able to see that when blight is very active, it can be difficult to control and single actives are usually not enough.”

“In our trials conducted by Dr John Keer of ‘Richard Austin Agriculture’ and also in the Agrisearch trials sponsored by the manufacturers, it has been the products or mixtures that contain contact or protectant actives, mixed with others that offer translaminar or kickback activity, that have been the most effective. “

A good example and one that has done very well in Hutchinsons trials in recent years is ‘Hubble’, a mixture of dimethomorph, a translaminar active with some kick back activity and fluazinam, a contact protectant active.

Mr Shailes adds that when the epidemic occurs early in the season this is combined with very rapid canopy expansion and, as has already been mentioned, Metalaxyl, the only very systemic active currently available for blight control in the UK, does not control Blue 13 – which is completely resistant.

“Under high early season pressure, to counteract the lack of truly systemic products and the increased aggressiveness of the new blight strains able to operate in cooler weather, we must be prepared to spray at shorter intervals (sometimes less than 7 days), with mixtures of actives or products that contain kickback and contact materials.”

“So for 2015 if we consider what we know and are prepared to react accordingly, as many did in 2014, then we should still be able to effectively control blight. However, if we sit back and wait for a Smith Period to come along, then do not be surprised to find blight in crops if the weather is conducive to infection.”

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