New strains cause disruption - Potato Review - John Keer

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Dr John Keer, of Richard Austin Associates, who has many years' experience of running blight trials, told delegates new strains of blight have disrupted the way growers and agronomists put their fungicide programmes together, not least following the failure of fluazinam to control the so-called dark green' blight strain, otherwise known as 37_A2.

"I don't believe we should be using fluazinam, he said. "Anyone who went to the Eurofins trials in 2017 would have seen Shirlan plots pretty much failing to control 37_A2. In some ways, it is fortunate that we're only dealing with one blight species. While a lot of its reproduction is asexual, which means that we don't quite have the genetic mix that there is in other parts of the world, because there are so many cycles of blight in one season, there are a lot of chances for mutations to occur.”

Most mutations don't last from one season to another, but occasionally one will out compete the genotypes that dominate the population, and we hasten this natural selection pressure by applying fungicides every seven days or so. 37_A2 is able to survive over winter and there has been an increase of 36_A2. "I think these two strains are set to dominate and they look as though they are capable of pushing out genotypes which we previously thought of a being aggressive,” said John.

Produce Solutions' agronomist Keith Chappell said he had seen 36 A2 near the North Norfolk coast while 37_A2 is known to have caused problems in Shropshire. He wondered whether the fact that growers in those areas are positively looking out for the new genotypes might be skewing the population data. John said the picture would only become clearer if more people got involved with AHDB's Fight Against Blight campaign and kept sending in samples wherever the disease was found.

“The stacking of blight products is really the sole reason we have these difficult genotypes to deal with. All of them had their origins in the Benelux countries, they then spread to France and Germany and eventually we imported them. I don't know any of these genotypes originated in the UK which says something about the way we control blight here," he said.

"It does appear that 37_A2 is more likely to infect tubers and I suspect that this strain is more adept at producing zoospores. It has been tested in the lab on the Continent and you can see that it is capable of growing faster and producing more spores.

36_A2 responds to different fungicides but is the most aggressive strain so far. It appears to be localised in Kent, around The Wash and in North Norfolk, he said. 36_A2 is more than adequately controlled by fluazinam.

How has the appearance of 37_A2 influenced John Keer's treatment choices? "It would be easy to say we should just ditch Shirlan but over the last five to 10 years it has been a real cornerstone to my blight programmes. It has proven zoospore activity and it has one or two unique attributes that we can't find in other material. No other fungicide has good residual soil activity and nothing else mixes safely with desiccants as well as fluazinam does,” John said.

So how do we best replace the protectant properties provided by fluazinam? John suggested mancozeb, a cheaper option but with much less foliar activity. In terms of efficacy, Revus is probably the best match but more costly. He said Ranman has slightly less foliar activity than Shirlan and is more costly.

“The disruption caused by the loss of fluazinam is unfortunately not remedied by tank mixing it with a curative material like cymoxanil or propamocarb,” he warned. “Both of these are short acting so you're not going to get enough activity to make Shirlan work again. We need to think about which products will best mirror its zoospore activity and for my money, it's only Infinito and Ranman Top. Electis has some zoospore activity but I don't think it should be considered in the same light.”

He stressed the need to start a treatment programme before blight has a chance to infect new growth. "I am absolutely convinced that it's those early sprays and the protection of what ends up being the bottom of the canopy that's important,” he said. "Others will have a different view on how to control blight at this time and other products would include Valbon (always a good performer in trials) or Revus plus or minus a curative. Curzate still fits in well under lower disease pressure.”

Up until Zorvec, nothing else controlled blight on foliage that hadn't been hit by the sprayer and while Zorvec is a genuine 10-day material, he said Corteva had made a mistake by not bringing to the market a seven-day option.

“Logistically, it is easier for a grower to fit in with that and to use a robust rate of another fungicide to protect Zorvec from resistance,” John said.