Oilseed-rape growers need to protect crops against turnip yellows virus until at least the end of October to avoid damaging yield losses, according to the latest HGCA-funded research on the disease at Broom’s Barn Research Station.

That means growers who planted crops in August may need to check crops for Myzus persicae aphids in the coming weeks, even if seed was treated with Modesto (clothianidin + beta-cyfluthrin) or Cruiser OSR (thiamethoxam + fludioxonil + metalaxyl-M) insecticidal seed treatments, says Mark Stevens from Broom’s Barn,

“The seed treatments have around six to eight weeks’ persistence generally, although that can vary depending on temperature, rainfall, crop growth, etc,” he points out. “So we’re coming to the period where they will start to run out of steam.”

So far, aphid numbers have been low, he acknowledges. “But it is worth inspecting crops, and if you see wingless aphids then an application of a pyrethroid could be worthwhile.”

That’s particularly the case while still in October and early November, last year’s research suggests.

For the first time the yield penalty by infection date was quantified by sequentially inoculating plots of Castille with aphids carrying the virus each month from September.

“The plan was to inoculate each month through to March. September to November went according to plan, but then we hit the coldest winter for years and we couldn’t inoculate in December, while it was very cold in January and February, [which isn’t good for aphid survival],” Dr Stevens says.

Even so, the data from the first four months of inoculations show how important it is to keep virus out of the crop during those months.

Against a background level of 41% infection in the trial, the September and October plots reached infection levels of 81% and 96%, while November had 62% infection.

As expected the earlier the crop was infected the greater the yield penalty, with a 15% reduction in the September-inoculated crop, 13% in October and 3% in November.

“So it looks to be crucial to keep aphids out of the crop in the early window.”

Seed treatments should help achieve that, although they cannot stop primary infection as the aphid has to feed on the crop to pick up the insecticide, and in doing so can transmit the virus. “But they will stop secondary infections, and typically trials have shown a 0.4t/ha yield response from them. I would be willing to bet a lot of that response comes from turnip yellows virus control.”

This year’s trials should provide further evidence of their value. While no seed treatment was used in last year’s trial, this year’s trial will include a control plot treated with an insecticidal seed treatment.

But he admits that, because in some years up to 70% of Myzus persicae aphids have been carrying the virus, the value of seed treatments is harder to call than in sugar beet where typically less than 1% carry beet yellows virus.

“That’s why we need to find varieties that can cope with the virus.”

The second trial last season was attempting to do exactly that. All 49 varieties in Recommended List trials were sown in microplots of 6m by 1.5m. One replicate was inoculated with infected aphids, while the second was left as a control. The plots were separated by a block of commercial oilseed rape to help prevent the infected aphids moving onto the control varieties.

“On average, we had 88% infection in the inoculated area, against 22% natural infection.”

Early summer leaf testing for virus load showed there was a range of infection levels depending on variety, which corresponded well to a visual assessment in early June when symptoms are expressed most strongly.

“The most susceptible varieties were NK Grace, Hornet and Gloria, while the most resistant were DK Sequoia, Expert and WC0R07-1.”

Popular commercial variety DK Cabernet was in a group neither susceptible nor resistant, while Flash and Sesame were towards the more resistant end.

The trial is being partially repeated this season with 10 varieties representing a range of susceptibilities, with the intention to take it to yield, Dr Stevens says.