Beware ‘green bridge’ when spraying off stale seedbeds

Growers are reminded to avoid short gaps between spraying off and drilling, which can leave a green bridge for aphids to survive and spread barley yellow dwarf virus into new crops.

Cereal volunteers and some weed species can host grain aphids and bird cherry-oat aphids – both of which are vectors of BYDV. But infection rates are lower when there is a nine-day gap compared to just two days, according to Bayer trials.

“When there is a short interval between spray off and drilling these aphids can survive on the dying volunteers and weeds, then quickly infect the new crop as it emerges,” explains Bayer’s Matt Siggs.

Optimal interval

The Bayer trials team set out to find the optimal interval between green bridge destruction and drilling. Infected bird cherry-oat aphids were released into plots of cereal volunteers. Plots were then sprayed off, with winter barley drilled after a two-day or nine-day gap.

Barley yields were significantly higher in plots treated with glyphosate nine days before sowing. But aphid survival was much higher in plots where there was a two-day gap – leading to significantly higher levels of infection in the crop.

“The results are clear, try not to have a very tight turnaround between spray off and drilling where BYDV is a problem. With a short gap, aphids can survive on the dying volunteers and weeds to directly migrate to the newly emerged crop.”

Farmers should take care to apply glyphosate at the correct rate, adds Mr Siggs. Rates might need to be higher than usual this autumn because growing conditions were good before, during and after harvest.

“We are seeing some fairly vigorous and large weeds and volunteers which will require more robust rates than may be assumed, so get agronomic advice.”

Mr Siggs emphasises that an integrated pest management approach is essential for BYDV management, utilising tools such as T-sum and selecting resistant varieties. Delayed drilling is also effective and fits in well with weed control programmes.

“Because of the longer gap between application and drilling, there could be concern about additional weed germination,” he says.

“Monitor carefully before drilling and consider Roundup in the pre-emergence mix at 540gms/ha if there is further weed emergence. But keep strictly to the true pre-emergence timing – and of course get advice about it.”