"It also means that when the pressure is on, the more robust Revus chemistry will give all varieties the necessary level of protection. Growers should also be looking to use the higher 0.4l/ha rate of Shirlan if there is a perceived higher risk of infection," he advises.

Mr Williams added that if an infection period has recently occurred prior to application, growers should also consider that Revus has been shown to give some kick back activity, or to utilise a tank-mix with some cymoxonil.

"The new variety resistance scores may also influence priority for spray programmes," he adds. "If difficult weather conditions are stretching spray intervals, it will be imperative to treat the more susceptible varieties first and to use a robust programme that will prevent blight getting in."

Gary Collins at the Potato Council endorses Stephen Williams' comments. "It is important to review your blight strategy for the whole crop, bearing in mind blight pressure from known outbreaks, Smith Periods, the new resistance ratings and choice of chemistry. It will take more planning if field order is changed, but the objective is clean crops." Details of the full list of varieties is available on the Potato Council website blight hub, at www.potato.org.uk/blight

SAC also estimates potential costs of yield loss at around £650/ha where haulm destruction is required two weeks ahead of schedule, incurring a 5t/ha yield penalty.

Whilst the physical loss of grading out 2% of tubers as a result of blight infection might be worth less than £60/ha, the practical implication for growers may be the need to sell stock early – resulting in a massive price penalty. Furthermore, there is the practicality of identifying problems, when blighted tubers may not be apparent for several weeks after harvest.

Added to that, Dr Bain highlights the extra time and manpower required when grading out blighted samples, which could double costs to £15/tonne. "Growers also have to be aware that with any grading process there is the high risk of spreading problems, especially if secondary soft rot occurred." And if the decision is made to hold onto blighted stocks in store, continuous positive ventilation for an additional two weeks could add costs equivalent to £200/ha, followed by the need for expensive refrigeration to remove moisture.

"Evaluating losses due to blight is very subjective," he adds. "A few pounds spent in preventing foliar and tuber blight, by increasing blight applications or upgrading to a more effective product when an outbreak occurs, can save a large cost in the store."

"It is imperative that the industry remains vigilant for any suspected cases of early late-blight infections occurring from oospore sources," advocates Mr Collins. Most of the distinctive features of an oospore infection are similar to seed-borne blight, which include many small lesions per leaf, although a seed infection will be isolated to a single plant. Early identification is essential to pick up the symptoms of potential oospore initiated attack. Mr Collins also points out that oospores may be formed from blight infections on volunteers and outgrade piles, which makes their continued control so important. "The best method of minimising the risk of oospore derived outbreaks is to ensure that rotations between potato crops are long, and that all growing crops are kept clean," he advises.

"Growers and agronomists should be registered as blight scouts. A sample form any suspected ooospore outbreak can then be sent to a lab for diagnosis. If positive, the mating type can be identified, which will help to better our understanding of oospore infections in Britain." Mr Collins highlights there is further information on oospore blight infection on the Potato Council website, where growers can also register as a Blight Scouts.

Seed source
Blight infected seed stocks have been responsible for nearly 40% of early disease outbreaks, according to surveys in Holland. Although there is no national recording of blight infection on UK seed stocks, even very low levels could result in the development of infected plants.

The key concern with seed-derived infection is that the initial hot-spots occur within the crop – making spread less dependent on warm, humid weather, warns the Potato Council.

New hard-wearing Syngenta Potato Nozzles have been shown to give an optimum compromise of sufficient velocity to penetrate the boundary layer of large-leaved crops, but minimising the risk of droplets bouncing or rolling off the potato leaf.

Rapid response to stop spread
Potato growers face an increasing risk of Alternaria, which appears to be attacking some varieties earlier and more aggressively than before. In Norfolk, agronomist Simon Alexander has reported successive years of early infection.

After witnessing devastating attacks in previous seasons, Simon Alexander is planning a fast response strategy this season to treat high risk crops as part of on ongoing programme

"In the past we have seen Alternaria coming into the crop late in the season, typically as it is beginning to senesce and hitting older lower leaves," he says. "But more recently infection occurred far earlier in the season and affected larger patches of the crop. It quickly spread across the whole field and led to early leaf loss." Alternaria infection can lead to yield loss, smaller tubers, variable dry matter and potential skin damage reducing marketability.

He warns that the earlier attacks mean growers face extended periods of high risk, when all the control options come into play. "Alternaria is now a major factor in agronomy decisions on susceptible varieties over a period of months. We are looking far more at proactive risk assessment to pinpoint high risk periods when we can prioritise applications to offer protection," he advises. "The aim is to protect clean upper leaves and to stop further spread across the field."

Now available in the dedicated potato agronomy area of the company's new look web site – www.syngenta-crop.co.uk - BlightCAST provides 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.

"Until now the use of Smith Periods has been largely historical and an indication of when blight may already have infected the crop," reports Mr Bullen. "Now growers can be more proactive in product selection and application timing to protect crops and keep ahead of blight."

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, he advises. "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."

The BlightCAST system was used by over 400 growers and agronomists last season. 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 and which significantly increases the risk for other local crops.

Potato growers and advisors can register now for the free Syngenta BlightCAST service at the website www.syngenta-crop.co.uk

5) The estimated cost per ha of haulm loss to blight two weeks ahead of schedule is:

• £300
• £450
• £650