Split fungicide treatments could be worthwhile this autumn

WITH concern increasing over the spread South of light leaf spot, split fungicide treatments in autumn can be worthwhile, say experts.

Independent agronomist Richard Alderman, of Crop Management Partners, has become increasingly concerned by light leaf spot in recent seasons.

He says: “We have seen increased incidence here in Hampshire and the surrounding area in the last couple of years and I think it’s a combination of the weather and intensity of the crop in rotations. The disease continues to cycle even when it’s cold.”

This year Mr Alderman expects to use split sprays on a number of oilseed rape crops, and that could include late drilled ones. “Most of the rape went into the ground in good time this season so I think we can expect to see phoma a little earlier than in recent seasons. It will be too early for light leaf spot so we’ll have to come back with a second spray targeting both diseases.

Late drilled

“For later drilled crops it will all depend on the weather. If it is possible to apply a single application then I will – but I’m not going to take any chances. If a second application is needed then you don’t really have a choice. Having seen the severity of light leaf spot in recent seasons I will be keeping a particularly close watch on the more susceptible varieties,” he says.

Options include either flusilazole (Punch 25) or prothioconazole (Proline). “Plover [difenoconazole] is another option for phoma, but not quite as good on light leaf spot. If I’m splitting sprays it will be half doses of Punch 25 or Proline. However, where just a single application is to be applied it would have to be Proline, and at a more robust rate.”

Northern crops

Further north, in Humberside and Yorkshire, it is possible to get away with a single application in most situations, but light leaf spot incidence is likely to be high again this autumn, says independent agronomist Sean Sparling.

“This year we’ve had rape going in the ground while there’s still crop to harvest. The carryover this autumn could be significant. light leaf spot can decimate a crop and you simply cannot afford any complacency. At the first signs of the disease you must come in with a robust dose of an effective fungicide,” he says.

He agrees a split strategy can help, but says product choice is just as important. “Delay or use an inferior product and you simply won’t hold the disease down until the spring. With phoma you do have options but not with light leaf spot. Only prothioconazole and flusilazole offer effective activity so strategies need to be based on these.

“For a single application you will need a robust dose of either but going the split route does allow you to alternate. However, where both phoma and light leaf spot threaten prothioconazole is the stronger option,” he says.

Split treatments are unnecessary in the far north of England and Scotland, but can make sense in the South, says SRUC’s Dr Fiona Burnett.

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