Revus has been shown to move with the growth and effectively protect new leaf area from blight infection, making it an ideal option from rapid canopy growth right through to canopy complete.

“In some areas, such as western Scotland, the deep penetrating frosts will undoubtedly have eliminated most ground keepers. But in many other areas where there was substantial snow cover, the ground was effectively insulated from deep frosts, and the majority of volunteers will have survived.” Trials have shown that temperatures of -1.5 to -1.9 °C at the surface of buried tubers cause some tuber death, but temperatures of -2.8 °C or below are required for extensive tuber destruction. The extent of tuber damage is greater the longer the period of exposure.

Potato growers must remain vigilant for signs of blight infection, especially during rapid canopy growth when any sources can rapidly infect other plants.

Dr Bain reminds growers to remain vigilant for early sources of infection, highlighting that just a few isolated infected plants can pose a significant risk for surrounding crops from early emergence. “The same applies to dumps, where any stocks graded out since the January cold weather will need to be dealt with to prevent infection sources being created,” he advises.

One of the UK representatives on the Euroblight group, Dr Ruairidh Bain of SAC, believes the system is the most relevant for UK growers, as it tests products under a wider range of northern European environmental conditions and, since no two seasons are ever the same, its results should give confidence that products will perform under extremes of disease pressure.

Revus has an outstanding rating for both protection of new growth and rainfastness.

Over recent years the Euroblight rating scale for the control of leaf blight has evolved to a decimal rating between 2.0 and 5.0, instead of a symbolic guide of + to +++. This is claimed to give greater scope for differences in performance of newer fungicides introduced to the market. The decimal point provides greater discrimination than the former system based on a much more limited number of categories.

“The highest possible rating is 5.0, but that can only be achieved when no late blight occurs in any of the experiments, which is obviously almost impossible to achieve,” points out Dr Bain. The rating of 4.0 for Revus (Table 1) is the highest foliar blight score yet achieved.

The Euroblight analysis also now charts a Standardised Area Under the Disease Progress Curve (StAUDPC), which essentially identifies the efficacy of a product in preventing the development of blight throughout the whole season, rather than a simple snapshot in time of conventional assessments. As a guide, the lower the StAUDPC figure achieved, the more efficacious the fungicide.

West-Midlands based agronomist, Mark Taplin, who advises growers of around 1000 hectares of processing crops, welcomes the fact that BlightCAST is easily interpreted at a glance, and provides a five-day forecast of blight risk to help forward planning and decision making.

Last year, for example, he recalls that BlightCAST gave growers greater reassurance that blight pressure was low early in the season, but when the pressure came later in the year they were adequately warned and able to step up the blight programmes accordingly.

Mr Taplin highlights growers particularly appreciate the forecast of forthcoming spray window opportunities alongside the blight risk prediction, which lets them know if and when they will be able to get sprays on. “Where we can see a high risk period coming and there are limited spraying opportunities, for example, then it’s imperative to get on some strong protection with good rainfast properties, such as Revus.”

He points out that whilst reducing blight fungicide rates is rarely an option, with products such as Shirlan that has two recommended rates available, BlightCAST can give the confidence to use the lower rate if risk is relatively low, or indicate when the higher 0.4 l/ha rate may be more appropriate.

Generally Mr Taplin does not consider that BlightCAST will reduce the number of blight sprays over the course of the season, “but it does help us to make more proactive decisions and achieve better application timing with the most appropriate product choices, which ultimately should give us more effective blight control.”

BlightCAST is available free to all growers and agronomists registered on the Syngenta website: www.syngenta-crop.co.uk

• 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.
• Potato crops can double their leaf area in a week during periods of rapid growth. Blight protection needs to move with the growth to prevent infection. Revus is recognised as a leading blight fungicide for protection of new growth.