THE COLD winter will not have made up for the fact that aphid pressure in key areas last autumn might already have restricted untreated rape crop performance this season, with even treated seed crops being swamped by disease-carrying aphids, experts have warned.

Tests for turnip yellows virus (TuYV) before Christmas confirmed that levels are high in a number of areas. However, according to Rothamsted Research’s Broom’s Barn crop protection leader, Mark Stevens, most of those who adopted second generation neonicitinoid-based seed treatments, such as Cruiser OSR or Modesto, should fare better.

But, even those seed treatments have, in places, been overwhelmed by high aphid numbers carrying the virus. Dr Stevens said that 30% of all myzus persicae (peach potato aphids) caught prior to Christmas, were carrying TuYV.

“Symptoms of TuYV infection were being picked up in crops at the end of November,” he says. “We’d not normally expect to see these until the spring, indicating that the aphids infected plants early in these crops and that autumn conditions were ideal for the virus to build-up in plants leading to symptom expression before Christmas.

“While the abnormally cold start to the year will have helped kill-off 99% of these aphids, unfortunately the damage has already been done and growers should expect to pick up symptoms in crops in the spring.”

He pointed out that unprotected crops could see yields cut by up to a quarter this year and in inoculated trials at Broom’s Barn in 2009 yield losses of up to 30% were recorded in some varieties.

Aphid counts at Broom’s Barn have revealed how important it is to select the right seed treatment. On the November 23, untreated crops of a conventional variety had a mean of 0.75 aphids/plant and Chinook treated plants 0.2/per plant. Cruiser OSR and Modesto treated plants were free of aphids, he said.

On hybrid varieties, levels were even higher – possibly due to reduced seed rate – with untreated crops having 1.6-1.7 aphids/plant and Chinook treated plots 1 aphid/plant. Cruiser OSR and Modesto respectively recorded just 0.1 and 0.2 aphids/plant.

Trials at Brooms Barn suggest that current oilseed rape varieties have little obvious in-built resistance to TuYV damage. “Growers need to be aware that while some varieties may express the symptoms in a slightly different way, any inter-veinal purpling or reddening of leaves may not be due to the cold winter, but TuYV.

Syngenta’s seed care technical manager, Michael Tait agreed with that assessment. “We feel that early-sown crops, that grew away fast, effectively diluting down the seed treatment benefit, are likely to exhibit some TuYV symptoms this year.

“Problems will be worse where follow-up foliar insecticides weren’t employed, particularly in areas where pressures were high,” he suggests.

Mr Tait also pointed out that those who used Cruiser OSR, had seen a wide range of other benefits. “Cruiser OSR has outstanding activity against other diseases, especially downy mildew. Crops, particularly those sown later into more stressful conditions, will also have benefitted from the vigour boost that the product provides.”