FAST DESSICATION of potato crops will be essential for potato growers this year to minimise blight infection spread to tubers, a leading potato specialist had warned.

Douglas Dyas, Syngenta's field technical manager for the crop, told growers at the recent Potatoes In Practice event, in Blairgowrie, that shortening blight spray intervals and greater use of the highest performance fungicides, such as Revus, had mitigated the spread of infection – particularly in the south and eastern counties of England, where weather conditions had dried up.

“However, blight pressure has remained extremely high in the north of England and Scotland, compounded by high winds and rain interrupting spray programmes," he said.

“Wherever there is, or has been, blight lesions in the crop, there is now an increased risk of zoospore spread to infect tubers. As we move into harvest of second earlies and towards maincrop varieties, many destined for storage, effective desiccation will be crucial to protect tubers.”

He advocated a two-spray Reglone programmeas the most efficient and cost-effective route to prepare crops for lifting and assure clean tubers. “Achieving a rapid removal of green leaf deprives blight of its host material. Reglone has been shown to be faster and more reliable than some other diquat products or alternative desiccants,” he added.

Best results have been achieved in a dense or actively growing crops from using a water volume of 400 l/ha and angled spray nozzles to achieve better penetration of the leaf canopy. The first spray is aimed at removing as much leaf as possible to open up the crop for a second application hitting the stems.

“Desiccation trials, and growers’ experience, has demonstrated the importance of leaving sufficient time for the crop to open up between sprays,” he advised. “Typically, that would be five to seven days, but depending on conditions and the crop, it could be up to 10 days.

“During that time, the crop will have been triggered into senescence and the initiation of skin set, so there is no delay in harvest from waiting to apply the second treatment,” he added. The second application can usually be made at a lower 200-300 l/ha water volume, again using angled Syngenta potato nozzles, he advised.

Though research has shown that Reglone has a direct effect on blight spores, with this season’s high pressure he advised that a tank-mix with the blight-zoospore active, Shirlan, at each desiccant spray timing, would add extra insurance.

In Scottish trials, reported at Potatoes In Practice, efficient desiccation was also shown to minimise the risk of blackleg spread to tubers, particularly where crops are being flailed prior to lifting and compounds the risk of disease spore spread.