Double trouble for oilseed rape

Oilseed rape growers are being urged to keep on top of diseases to give small plants every chance of surviving winter and pigeon predation.


The advice, from several sources, comes as crops have already been lost to a combination of poor emergence and slug damage, while the survival of plenty of others is in the balance.


Although phoma remains the main concern, downy mildew is a particular problem this autumn in later sowings. The snag is that no single fungicide controls both, warns United Oilseeds’ Richard Elsdon.


“There are huge variations between crops, even within the same parish, but also within fields. In some there are plants the size of dinner plates but also far too many only at the cotyledon stage.”


That creates a dilemma over whether individual fields with lots of bare soil still showing justify spraying.


TAG’s David Parish estimates that about 5% of the crops he oversees in the Bedfordshirearea have been removed for re-cropping. “Perhaps another 15-20% are still vulnerable and downy mildew is obvious,” he adds.


“Slugs are still the number one problem,” says ADAS’s Peter Gladders. “But we’re getting reports of more and more downy mildew. There’s still a battle going on, and I think some people may regret that they’ve pulled out crops too early.”


With both diseases early treatment is important, he stresses.


“Many small crops haven’t reached phoma treatment threshold (10-20% plants affected) yet, and the bigger ones are getting sprayed. But with downy mildew it really is a lost cause once you see the cotyledons going yellow.”


Mr Elsdon fears that given cooler, damper conditions downy mildew could become “rampant“.


Spray timing is critical, agrees Mr Parish.


“People may be reluctant to spend unless they are sure the crop is going to make it. But if you’ve decided to leave it you must control phoma early – it can go from 10% to 80% in a week.”


The difficulty with downy mildew is that there are no curative treatments available, so it needs applying as soon as symptoms appear.


“We only have mancozeb which has no eradicant activity, although it’s reasonably cheap.”


 

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