A new more aggressive form of potato blight widely found in Europe could become established in the UK, resulting in much more difficult control strategies requiring more frequent treatments, a leading researcher is warning.

“The blight population in Europe has changed to a more aggressive one in the past 20 years,” says Huub Schepers, a plant pathologist at PPO Lelystad in the Netherlands.

The cause is the migration of a second mating strain, A2, into Europe from Central America probably on a batch of seed potatoes.

When in a population with the existing A1 strain it reproduces sexually to form oospores.

By itself the A1 strain can only reproduce asexually.

“When you make sexual spores there is a lot of variability, which makes it easier for the pathogen to adapt to any situation.”

Symptoms are similar, but control of blight developing from oospores is more difficult, he says.

“The pathogen grows faster – the latent period is three days – produces more spores and is more efficient at infecting its host.”

Oospores can also develop in a much wider range of temperatures than the less aggressive population.

“Infection can develop at below 10C or even at 30C.”

The spores also survive in the soil for up to four years providing another source of inoculum for epidemics, and lead to earlier attacks.

The more aggressive strain is already relatively common in some European countries, such as the Netherlands, Dr Schepers notes.

“It is more likely where there is intensive production, sandy soils or where potatoes are going for starch.”

Resistant varieties also help it spread.

“On susceptible varieties the leaf is killed by the blight more rapidly, so there is less time for A1/A2 reproduction.”

So far, there have been no confirmed cases of oospores in the UK, although it is suspected in a couple of instances.

But growers should not be complacent, he warns.

“It can take hold quickly.

For several years there wasn’t any A2 type in France, but now they are reporting huge numbers.”

Using newer fungicides is essential for controlling the aggressive form, it is no longer possible to control blight with just contact fungicides, he says.

“Spray strategies will have to change.

Make a mistake now and it will be punished much more quickly than before.”

Relying on scheduled treatment intervals could be dangerous, he warns.

“A seven-day spray schedule may have reduced control after day four, which, with this new aggressive form, may mean the next fungicide treatment is too late.”

Growers in Mexico, where the A2 type originated, have to spray every four days.

Future strategies will have to make better use of decision support systems, as well as specific characteristics of new blight fungicides.

“Decision support systems can provide more information.”

mike.abram@rbi.co.uk