“We’ve had the perfect autumn for aphid build up. While there few Myzus persicae seen through September, once we got into October and November numbers started to increase.
“From inspections in crops, we saw quite large numbers of Myzus persicae in oilseed rape crops, and often these crops had been treated with a seed treatment,” he told agronomists at the AICC conference.
“I think what has happened is these aphids have come into crops after the efficacy from the seed treatments has run out, and they are able to reproduce and move within the crop. In one crop we saw levels of 10-20 per plant.
“So I think we have an issue. If we don’t get some cold weather, not only will we see quite a lot of turnip yellows virus in the crops we have at the moment, but in other crops, such as sugar beet and potatoes, we could face some problems.”
In trials, yield losses had been more severe from earlier infections, Dr Stevens said. Plots inoculated with TuYV in late September or late October lost around 0.5t/ha compared with a control treatment, while the November inoculated treatment lost less than 0.1t/ha.
Most varieties seemed susceptible too, he said. In 2009 trials, nine out of 10 varieties lost 12-30% of yield from TUYV infection, with a similar result in a 2011 comparison.
The latter trial also showed some varieties lost 1-2% in oil content too.
Insecticidal seed treatments showed in trials that they could limit aphid build up in crops and reduce TuYV infection levels, he added. “We’ve seen yield benefits of 0.7-0.8t/ha from using seed treatments, and on average 0.3-0.4t/ha over the standard Chinook seed treatment.”
Seed treatments gave control of secondary infections for about six to eight weeks after planting, but Dr Stevens pointed out for very early-drilled crops that this might not be long enough to get through the risk period.