Virus yellows

Virus yellows in this season’s sugar beet crops, which can cut yield by up to 50% and at one stage looked set to be a serious problem, has turned out to be no more troublesome than usual, thanks to seed treatments and a timely insecticide Specific Off-Label Approval.

But with more beet mosaic virus about in this year’s crops, growers will need to be on their guard next season especially if the winter is mild, warned Broom’s Barn’s Mark Stevens. Although not a yellowing virus, its leaf mottling symptoms can trim output by nearly 10% if it infects crops early, he noted.

After the mild January and February, up to 80% of this year’s UK crop was expected to be hit by virus diseases unless aphids bearing them were controlled.

“Even in crops from treated seed we were expecting 4-6% infection,” said Dr Stevens.

Concern grew during April’s hot spell as aphids multiplied rapidly on some crops.

The numbers of Myzus persicae caught in water traps was three times higher than normal, and initial tests showed 1-2% were carrying beet mild yellowing virus in the early part of the season.

“On several treated crops we were finding second instar progeny of M. persicae raising fears that they might be resisting neonicotinoid seed treatments.”

However, once rain came the numbers began to tail off and only about 0.5% were found to be potentially infective.

“But even then, with such high numbers around we’d have expected to be getting a lot of virus.”

Also, about 70% were already MACE-resistant making pirimicarb-based aphicide sprays (the only alternative option available for beet) ineffective.

Fortunately emergency off-label approval for Biscaya (thiacloprid) was granted just in time (as yet for this season only) to allow clear-up spray treatments to be applied where necessary.

The rain also eventually helped plants take up the seed treatments to save the day where growers held their nerve and held off with those sprays.

“So the virus yellows impact won’t be as great as we feared,” said Dr Stevens.