3 April 1998

Danger signs as early eyespot is rife

EYESPOT could cause significant yield loss this year if vulnerable crops are left untreated and conditions remain favourable.

Preliminary results of a nationwide survey by AgrEvo suggest half the wheat crop could be infected, some already at severe levels.

But opinion is divided on the appropriate treatment threshold.

Even at low levels control will be worthwhile, advises AgrEvos John Burgess. "Dont assume no visual symptoms means no eyespot," he warns. "The cool, damp conditions we have had in recent weeks are ideal for low levels of disease to develop into damaging outbreaks."

Bill Clark of ADAS agrees risk is high. "We are set for a bad eyespot year, unless conditions turn very dry." But he applies a higher treatment threshold. If more than 20% of plants have penetrating lesions then spray, he advises. For susceptible Brigadier and Soissons use a lower threshold of 15% of plants, he adds.

But other diseases may need taking into account, says John Innes researcher Dr Paul Nicholson.

"You must control all components of the stem base disease complex or you may fail to see an agronomic benefit."

That raises questions about new specialist eyespot killer cyprodinil (Unix), believes Mr Burgess.

Neil Waddingham of Novartis disagrees. "Fusarium and michrodochium are weak pathogens. They ride on the back of eyespot to enter the plant. We are very confident cyprodinil doesnt increase other diseases."

Mr Clark agrees. "There is always competition between diseases for the stem base site, but eyespot is much the most damaging."

Results from Du Ponts ELISA test confirm the high eyespot risk. "Of samples received to date, 4% already have a high level of infection warranting a spray, and 59% have moderate levels," says product manager Andy Selley.

However, samples drawn by distributors tend to over state the case, he warns.

His advice is to make foliar diseases the main target, and suppress eyespot where it is a concern.

&#8226 AgrEvos survey samples are being checked by Adgen Diagnostic Systems using a PCR technique which can identify the eyespot strain and amount of inoculum. The R-strain has appeared in 52% of samples, and w-strain in less than 1%.

Barely visible levels of eyespot (brown specks) can develop into damaging disease,

warns John Burgess of AgrEvo.

EYESPOT ANGST

&#8226 Eyespot incidence high.

&#8226 Spray threshold disputed.

&#8226 Effect of stem base complex?

&#8226 AgrEvo yield loss forecast from diagnostics next year.