TESTING FOR ring rot in the UK is woefully inadequate and the proposed supply chain safe haven scheme will not work unless testing levels is increased and there is 100% participation from seed producers.

Speaking at last week’s BPC Seed Potato conference, US ring rot expert Neil Gudmestad said that while the scheme might work in theory, it would not keep the UK free from ring rot unless there was 100% participation. “You need that buy-in.”

More tubers needed testing from stocks within the scheme to be sure infection had not occurred. “Testing 200 tubers makes no sense whatsoever. You might as well not bother. If you are serious about keeping ring rot out then numbers need to increase, and increase dramatically,” Dr Gudmestad warned.

That also applied to seed stocks imported from countries within the EU known to have ring rot, he added. “The UK should, at a bare minimum, require tuber testing levels from other EU countries known to have ring rot, at the level the EU requires of Canada, which is 2000 tubers.”

Dr Gudmestad believed the UK should actually go further and ban the re-planting of seed stocks originating from outside the UK.

“I would also exclude ware potatoes from countries where ring rot is endemic, although my limited understanding of EU trade agreements suggests this is not likely to happen.”

impossible to eradicate

Ring rot in the US was a big problem, he admitted. “Once it is established, it is virtually impossible to eradicate.”

Losses from ring rot were likely to be close to 8m ($15m) this year in the USA, with Washington State and Idaho both having serious outbreaks.

In Canada, ring rot costs the seed potato industry 0.9m (C$2m), he said. “That is mostly the costs associated with post-harvesting testing of tubers.”

Typically, if seed was transported to another farm, 400 tubers/stock were tested, unless it was exported to an EU country, when it was 2000 tubers/stock, said Dr Gudmestad.

detection problems?

Ring rot infections could go unnoticed in the field, Dr Gudmestad warned. “It can go through three generations in the field before symptoms start to show.”

Latent infections might occur because inoculum levels were low, the potato variety was tolerant and did not show foliar symptoms strongly or early enough, or because of the strain of ring rot. “There are two strains of ring rot bacteria – one that produces lots of slime and one that doesn”t. The latter type is much more difficult to detect.”

Worryingly, Dr Gudmestad also believed ring rot, at least in North America, could survive using sugar beet as an alternative host. “It is not totally accepted, but we can recover ring rot bacteria from sugar beet varieties in the US, although not in Europe.”

And it could survive outside a host as “dried slime” on contaminated surfaces for long periods, perhaps more than five years. “That makes it very difficult to control.”