Growers experience last year highlighted the need to maintain shorter intervals between blight fungicide applications when crops are under intense blight pressure. The impacts of changing weather patterns and more aggressive blight strains look set to make seven-day intervals standard industry practice.

Whilst blight forecasting can be a useful aid in assessing the current and impending risk of infection, in practice the need to maintain high levels of protection on the leaf demands regular applications, according to leading blight researcher, Dr John Keer of Agrochemex. Particularly where larger acreages require several days to get around with the sprayer and growers can not afford to miss application opportunities.

Dr Keer advocates growers should always maintain the spray interval and make their fungicide choice according to the level of disease pressure. “During periods of intense pressure growers need to be using the most effective blight fungicides possible, where they can be confident of good results and the highest levels of persistence between applications. If disease pressure is low, however, then it may be possible to safely incorporate lower cost options in the programme.”

New blight chemistry could help growers tackle aggressive blight strains in the future. Fungicides that are inherently more active can provide effective protection against new infection for longer and at lower concentrations on the leaf surface.

Research for the EU Eucablight programme at the Wageningen Applied Plant Research centre in the Netherlands has shown new products can provide protection of an expanding leaf area. Citing the Dutch trials with Revus, Dr Huub Schepers reported: “We have seen from the flow characteristics of mandipropamid that it will continue to redistribute within the leaf. Its intrinsic activity and persistence appears to give effective protection, even when there is only a very thin layer of molecules left at the end of a spray interval.”

New research into today’s evolving blight strains suggest the pathogen is able to develop at lower temperatures and may cause infection to break out in conditions growers had, hitherto, considered safe.

Blight investigations at Bangor University, for example, demonstrated blight will cycle at temperatures as low as 4ºC, with the optimum range at 10.5 to 11.7ºC, warns Eric Anderson of Scottish Agronomy. Smith Periods remain a useful guide, but growers must be aware that several isolates will sporulate readily at less than 10ºC. He also points out that, under a leafy crop canopy, conditions could be far more hot and humid, allowing blight to develop.

More importantly, the period between infection and production of spores of the new blight genotype is far shorter than previously encountered - typically four and a half days for the new A2 blight, compared to six or seven

Blight fungicides need to achieve good coverage of foliage and stems to protect from infection. It is especially important with contact activity fungicides, but even systemic products work most effectively moving up the plant.

Syngenta application research has shown significantly improved coverage of stems and lower leaves by using angled nozzles to achieve better penetration of dense potato crop canopies. Typically operators should achieve over 25% greater spray deposition on lower leaves and stems with the Syngenta Potato Nozzle, compared to conventional flat fan nozzles.

Syngenta Application Specialist, Tom Robinson, advises growers to maintain water volumes of at least 200 litres per hectare to give good coverage, using higher water volumes in vigorous leafy crops. The Syngenta Potato Nozzles are now available as an 04, applying 200 l/ha at 7 to 9 Km/hr, or an 05 for faster application at 10 to 12 km/hr.

“Trials have also shown that fitting angled nozzles alternated to face forwards and backwards along the spray boom gives better all round coverage of the plant, and significantly reduces the risk of spray drift,” reports Mr Robinson.