31 March 2000

Outflanking eyespot – the resurgent foe

By Andrew Swallow

TRADITIONAL eyespot treatment thresholds could be allowing severe disease to develop untreated, warns the SAC. That could be particularly costly in Scotland this season as many crops fall into a high risk category, warns senior pathologist Simon Oxley.

"There are a lot of early drilled crops and we have had a mild, wet winter. You can almost count the frosts weve hand on the fingers of one hand," he comments.

Such factors are key to the development of eyespot disease and diagnostic techniques such as PCR show that it is already present in some cereals. However, the absence of disease at GS30-32 can no longer be assumed to mean crops are at low risk, he warns.

"Ten years ago the eyespot was all wheat strain. Now, the rye strain dominates and it is more cunning in that its rapid development phase is delayed – sometimes not until flag leaf emergence or even later."

Despite that late development, yield losses in wheat can reach 1t/ha (0.4t/acre) if eyespot fungicides are not applied at stem extension, and even worse if stem lodging occurs.

Typical yield losses attributable to untreated eyespot are 0.3-0.5t/ha (2-4cwt/acre), he says.

The widespread shift in the pathogen population means the old ADAS treatment threshold of 20% of stems showing lesions at T1 is no longer reliable.

If any visible lesions are present, treatment is warranted, he maintains. Even if no lesions are visible, if the risk factors add up to put the crop into a high risk category, include an eyespot control at T1, he says. For most wheat growers in Scotland, that should be a GS31-32 dose of Unix (cyprodinil) at 0.5kg/ha, mixed with another product for foliar diseases such as Opus (epoxiconazole). Sportak (prochloraz) is not as effective on the R-strain, and has to be applied earlier for optimum control, he adds.

Landmarks (epoxiconazole + kresoxim-methyl) eyespot control at T1 is useful and has the edge over the Unix/Opus mix for foliar disease, he acknowledges. But growers should stick to FRAC guidelines and the two strobilurin applications are best deployed at flag leaf and heading in Scotland. &#42

EYESPOT RISK RAISERS

&#8226 Early drilling.

&#8226 Any second cereal.

&#8226 1st cereals after 1 year break, unless late sown.

&#8226 Any of the above? = Eyespot alert, even if no symptoms.

Beating stem-base blotch in barley

While less is known about eyespot in barley, control is less of a dilemma, as Unix brings broad-spectrum T1 foliar disease control too, says Dr Oxley. "The good news here is that Unix controls rhynchosporium, mildew and early net blotch, therefore it is easier to justify." However, growers should avoid cutting the dose rate below 0.5kg/ha where eyespot is a concern, or from going too early, he warns. "A low rate of Unix and Corbel might be needed pre-T1 to clean up mildew and rhyncho, but dont be tempted into thinking that will control the eyespot."

No eyespot at T1 may not mean no eyespot by ear emergence, in winter wheat or barley, warns SACs Simon Oxley.