Increased phoma risk in oilseed rape

Late drilling, small plants, unsprayed rape stubbles with lots of volunteers, and perfect conditions for spore spread: It all adds up to a high-risk season for phoma in oilseed rape.

And for growers in the north, and possibly further south too, light leaf spot is also likely to be a problem.

Phoma infections have already been found in emerged crops in Norfolk, says Chris Cooksley of Bayer CropScience. That is not surprising. Virtually everywhere will have had more than the 20 days with rain, which ADAS plant pathologist Peter Gladders says primes phoma spores for release. Alternative dry and wet days after that trigger spore release from stem cankers, and with many growers struggling to spray off rape stubbles, early phoma infection looks almost inevitable.

Don’t ignore the issue because plants are small and you can’t see lesions, says Patrick Stephenson, an independent agronomist from North Yorkshire. “The risk is big because the ability to get into the stem [and cause stem cankers] from a leaf infection is greater.”

Russell McKenzie

Cambs grower Russell McKenzie is likely to adopt a two-spray strategy for phoma this season to protect small plants

An early infection period will probably need a split fungicide treatment to achieve the longevity of control likely to be required, says Mr Cooksley. That approach has paid dividends in previous trials, although not in all trials last season, he admits. “Last season it generally looked like an early disease season to begin with, but then it went dry and cold before we saw considerable amounts of phoma in crops in December and January. But by then, crops were bigger and there was a lower impact of stem canker in the spring.”

Russell McKenzie, who grows 200ha of oilseed rape near Huntingdon, is not afraid to use the two-spray approach, having sprayed twice for phoma in the autumn twice in the past four seasons. This year may require the same approach, he says. “I’ll be amazed if we don’t get early phoma this year, so we need to be prepared to go earlier, especially with smaller plants.”

His first spray is likely to be prothioconazole-based – “It gives two weeks’ extra protection” – before returning with an alternative second time around.

Mr Stephenson will also be advising prothioconazole as the first spray, but for slightly different reasons. In his region, light leaf spot is also an issue, particularly because the loss of sugar beet in the Vale of York has increased the prevalence of oilseed rape. And although some alternative fungicides are good against phoma or light leaf spot, he wants a product that covers both diseases. “I wouldn’t do an autumn treatment that ignores light leaf spot,” he says. “Proline offers some protection against both.”

But it is not always easy to persuade growers to spray twice against phoma in the autumn, he admits. The first one is almost pre-programmed with a graminicide. Going back four to six weeks later is a specific management decision, which coincides with Atlantis sprays in wheat, the close-down of the season, and shooting, he says. “With Atlantis, you also have to make sure you clean out the spray tank thoroughly.”

Light leaf spot also need two sprays, he adds. “But usually that is one either side of Christmas.”

Sclerotinia threat

Two more fungicide sprays are likely to be applied for sclerotinia control by Mr McKenzie. “We’ve moved from a one full-rate Compass or Filan strategy to two sprays,” he says. “You only need to look at what happened in 2007 when it was dry in April, and yet we still had significant sclerotinia, to see why.”

Last year was even worse, with the highest-recorded incidence of sclerotinia since records have been taken by CSL, says Mr Cooksley. “Even rape grown on virgin land was decimated by the disease.”

That means more growers are likely to go down the two-spray route, says Mr Stephenson. “We will be sclerotinia-dominated for the next two to three years [after two bad seasons],” he adds.

Timing is important for sclerotinia control, says Nigel Godley of Bayer CropScience. “Ideally, you need to get on within a few days of the infection period.”

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