The significant change in recent years has been the rapid rise in the proportion of A2 Blue 13 blight, which has increased from around 10% to virtually 80% of blight samples in the past three seasons. It is now the dominant blight strain across Britain.

“For growers, the key issue with Blue 13 is that it is present right from the start of the season and completes its life cycle in the crop faster. This has a serious implication for spray intervals when conditions are conducive to blight.

“Early indications are that Blue 13 is more active at lower temperatures than other types of blight, so growers must remain vigilant even in conditions hitherto believed safe. Blight lesions tend to be larger, leading to rapid leaf loss and faster potential disease spread,” he adds.  The Potato Council have just agreed to fund studies to look into the way Blue 13 responds to temperature and humidity in greater detail.

“We know that oospores can survive in the soil over several seasons; the risk can also be perpetuated on volunteers that go untreated in other crops in the rotation,” warns Dr David Cooke of SCRI. “This creates the potential for early infection from the soil-borne sources and for strains of blight we have not encountered before.”

If a rapid increase in the number of blight genotypes were to be found, it would indicate sexual reproduction had occurred between different blight strains, termed recombination. However, Dr Cooke acknowledges that Potato Council funded research, led by SCRI, has seen very little evidence of such recombination in the field in the UK, with the population still dominated by a limited number of clonal lineages.

“Growers must remain vigilant to the threat of a blight shift that threatens to compromise the natural blight resistance of varieties such as Lady Balfour and Setanta, along with the efficacy of some fungicide options,” he adds.

Tom Whitworth of Syngenta reports that testing of blight fungicides against the current strains of the disease found in the UK crop show no resistance among Blue 13 or Pink 6 strains to either Revus or Shirlan. “Using a combination of the products in a robust programme attacks the blight pathogen at various points in its life-cycle, which significantly reduces the risk of an evolving strain developing resistance,” he adds.

Tom Whitworth of Syngenta reports that Revus achieved the highest rating of all the fungicides evaluated for both protection of new growth and leaf blight. It also achieved the highest available three star rating for ‘very good effect’ as a protectant fungicide, along with the same three stars for rainfastness.

Black rain clouds indicate high potential blight risk - and the need for rainfast REVUS to protect crops in difficult conditions.

The company’s web-based BlightCAST provides registered growers and agronomists with a five-day forecast of localised weather conditions that are likely to give rise to Smith Periods and high blight risk. A simple colour-coded warning system alerts growers to the blight risk on any given day, along with a five-day spray window forecast.

“If growers know that there is a period of sustained high blight risk coming, then it is imperative that they are using the most robust fungicides possible, such as Revus, to counter the threat,” advises Mr Bullen. “The spray window forecast will ensure they can make the applications at the most timely opportunity to give maximum protection through the high risk periods.

“If rain is coming, they can be confident that rainfast Revus will provide high levels of protection, when other fungicides may be washed off and leave the crop exposed at a time of the highest blight pressure.”

Developed from a blight model tailored specifically for UK conditions, the Syngenta BlightCAST system has been trialed extensively by growers and agronomists over the past two seasons. Mr Bullen advocates growers use BlightCAST in conjunction with the Potato Council Fight Against Blight website, which indicates where blight has been found in an area – which significantly increases the risk for other local crops.

• Dumps

The Potato Council reports in sources of blight infection outbreaks attributed to dumps doubled in 2008; grading out of blight infected tubers after last year’s high levels of infection could give rise to massive early sources of inoculum spread from dumps this season. Growers are being urged to take extra care this year to stop a further increase, with advice to cover dumps or regularly burn off green material with glyphosate.

• Volunteers

After last year’s wet and difficult harvesting conditions many ground keepers were left in the soil, creating potential sources of early blight when infected tubers emerge this year. Control of volunteers in other crops will be an important strategy.

• Blight protection needs to start as soon as the crop is out of the ground, if conditions are conducive to infection
• Select fungicides with good zoospore activity at rosette stage, such as SHIRLAN, to stop blight movement in soil moisture and protect emerged growth

1) What proportion of the blight population is Blue 13?

• 50%
• 70%
• 80%
• 95%

2) Why are mixed populations of A1 and A2 blight a threat?

• One attacks leaves, the other stems
• Increased chance of creating new blight strains
• It allows one blight strain to dominate
• Tuber blight infection is increased

3) Early season blight fungicides need good activity on:

• Tuber blight
• Zoospores
• Sporulating blight
• Blight lesions