Outbreaks of soil-borne blight caused by oospores could be an emerging problem for UK growers, according to SAC potato pathologist Rhairidh Bain.
This type of blight is more unpredictable and difficult to control.
That’s why SEERAD and the British Potato Council are funding a research project at SCRI to monitor potential oospore outbreaks in Britain, which has yet to have a confirmed outbreak.
The programme will also investigate which fungicides work best to control this type of blight.
This year’s weather meant blight threat from sources in the crop – rather than external sources – would be comparatively greater, said Dr Bain.
“For sources outside the crop, you need warm, humid, wet conditions.
Where we have warm, dry conditions, blight spores die off very quickly.
“On the other hand, if blight is coming from the seed, some of it will get up on to the haulm and even in conditions like this – far from conducive to blight – you can get blight spreading up into the foliage and developing under the canopy.
“One of the key factors about sources in the crop – such as oospores or infected ground-keepers – is that the blight can develop even without Smith Periods.”
Oospores are formed when the A1 and A2 blight mating types reproduce.
In doing so, they produce a lot of genetic variation.
Before the introduction of the A2 mating type, blight was genetically very uniform.
“Recent research has shown that the presence of the A2 mating type has gone up to 35% in the UK, which means the risk of types A1 and A2 getting together has increased enormously,” explained Dr Bain.
“This means we are much more likely to get oospores that survive in the soil, so there is an increased risk of soil-borne blight.”
The oospore problem was particularly bad in countries such as Finland, which had very poor rotations, he pointed out.
Increasing rotation length was the key to controlling the problem.
“One effect is that a blight epidemic is likely to start earlier because the inoculum is there before you plant the crop,” he said.
“There will also be a lot more variation and so blight will be more unpredictable and more difficult to control.
“What is really characteristic of this type of blight is that right from the word go, a large area is affected and all the lesions seem to be concentrated on the lower leaves.”
Growers sign up to stop the rot
About 100 seed growers (25%) have signed up to the British Potato Council’s Safe Haven scheme, aimed at keeping ring rot out of the UK. So far there have been three outbreaks in England and Wales and all are suspected to have resulted from contaminated Dutch seed.
Dr Wale encouraged growers to be very critical of farmers who are importing seed from countries such as Holland. SEERAD is also offering a free testing service for Scottish growers who import seed (contact SEERAD by email at firstname.lastname@example.org at least 48 hours before seed arrival).
“If we introduce rot, it will have a very serious impact on livelihoods and our reputation as a seed-growing country will be badly damaged,” said Dr Wale.
Export markets were firming because Scotland had remained clear of disease, he noted.
Aphid traps save unnecessary sprays
Aphid traps can be an inexpensive tool to improve the targeting of aphicide use, according to Stuart Wale, head of crop services at SAC.
An aphid water trap set up in the field could be a valuable indicator of the level of aphid presence, he said. One trap per field would be sufficient, with higher aphid counts likely in traps located at the field edge.
The traps cost £18 each with lab analysis at £14 per sample. “That might sound expensive, but it could easily save you one aphid spray, which would easily cover the costs,” said Dr Wale. “Results can be turned round in 24 hours.
“Last year the number of fields where virus was detected was up – most notably potato leaf-roll virus. We still have the best virus health status in the world, but whenever we see an increase, we should be worried.”
Aphicide-resistant aphids have been found in Scottish potato and vegetable crops, so choice of aphicide is crucial to prevent resistant aphids transmitting virus.
Until now, aphid numbers have been low this growing season, but Dr Wale warned growers that aphid numbers were now on the rise.
He recommended the use of Plenum, Biscaya or Teppeki early in the aphid control programme (either first or second treatment) to target any aphicide-resistant aphids.
For more information on aphid numbers, go to www.potato.org.uk/aphids; www.rothamsted.ac.uk/insect-survey and www.sasa.gov.uk/seed_potatoes/aphids/bulletins/index.cfm