15 May 1998

Potato growers are advised to spray early against blight

BLIGHT spraying must start earlier than ever this season to protect late emerging potatoes from rapidly rising disease pressure.

Outbreaks have already been spotted on potato dumps and first earlies, so growers waiting for plants to touch down the row before spraying could be caught out, experts advise.

"Take prompt action and spray as soon as there is 20% ground cover," recommends Ang-lian Produces Norfolk-based agronomist Simon Bowen. Many of last years outbreaks were caused by late first sprays, he believes.

"It should be possible to get away with a contact material. But if the risk is high a semi-systemic such as Curzate or Invader would be better," Mr Bowen notes. "Warm weather alone does not constitute high risk. You need to be in an early area to really justify a semi-systemic."

Risk is particularly high in Cornwall, where several outbreaks have been confirmed. "We have also seen ground keeper-induced blight in Lancashire as polythene covers are removed," Mr Bowen notes.

Cheshire-based agronomist Mark Palmer echoes his advice. "Heavy blight pressure last year, a high grade-out from stores this spring and increased use of home-saved seed adds up to more blight pressure this season." The disease has already been found on first earlies under fleece in Cheshire, he notes.

"Second earlies and maincrop are between three and five weeks late, and blight has come in particularly early. Normally we do not expect to see it on maincrops until July," advises the Profarma consultant.

"A contact-acting fungicide such as Dithane, Shirlan or Brestan, should go on at 100% emergence. Crops are not growing fast enough to justify the use of systemics, which should be saved for later in the season when they will do more good."


Growers wanting to use systemic fungicides later should apply a contact-acting material, provided foliage is growing slowly, advises Novartis. "This is when protection of newly emerging foliage is not too critical," says blight fungicide specialist Ben Miles. "But once plants start growing rapidly they need a systemic, like Fubol, to protect emerging growth." Systemics also increase management flexibility as the timing of the follow-up spray is less critical than where a non-systemic is applied, he notes.