More aggressive late blight infections could push spray intervals tighter and mean earlier starts for spray programmes, Huub Schepers (pictured), a Dutch researcher from Wageningen University told growers at a Certis-organised blight seminar.

There was little difference in the aggressiveness of A1 and A2 strains of blight, but both strains had become more aggressive over the past 20 years, he said.

And the formation of oospores through sexual recombination of the two strains, increasing the genetic variability of the fungus, could easily make it even more aggressive, he believed.

Oospore-derived outbreaks had been seen in Holland and Scandinavia, although they were not common. “They cause early infections in the field.” And unlike seed tubers, which started in individual plants, oospore infections could infect lots of plants at the same time, he said.

Rain played a key role. “It’s is important for oospores to germinate and infect plants. It is why you can have almost all plants infected at the same time.”

More aggressive blight could also potentially adapt to infect plants at higher and lower temperatures, to cycle quicker, produce more spores and survive better on tubers, he said.

The consequences for spray programmes could mean an earlier start, reducing spray intervals, protecting new growth as a priority, using stronger blight materials, and more emphasis on tuber blight control. “In practice this could mean changing standard spray schedules. Fixed seven-day intervals can be too short, if the crop is growing quickly or in high risk weather.

“Product choice is getting more important. too. Select better products to use when conditions are critical,” he advised. The EuroBlight project website www.euroblight.net was a good place to compare product strengths.

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