Choose seed treatment carefully, rape growers told

RAPE growers are being advised to balance time and cost against local pest variations when making decisions on seed treatments.

While seed treatments can delay the first spray – relieving pressure on a busy workload – they come at a price, said Ben Freer, agronomist with NIAB TAG. So it’s important to evaluate the pest risk first.

“Seed treatments are usually justifiable. Although slightly more expensive than foliar sprays, the benefits accrue through getting the crop established more quickly – always the number one priority.”

Benefits include defence against the flea beetle, which can destroy the crop before it fully emerges. “There is no spray effective against a pest at this stage,” said Mr Freer.

But growers could be lulled into a false sense of security as few seedlings have suffered shot-holing in the last couple of years. Yet beetles will have enjoyed recent soil conditions and loose, dry, cobbly seed beds favour their movement.

“One of the easiest ways to assess the likely extent of the problem, and to guide your seed treatment decision, is to check inside trailers of harvested rape – the cabbage stem flea beetle is black, shiny and jumps as well as it flies.”

Mr Freer says the relatively new neonicotinoids, found in Modesto and Cruiser, have proved valuable in reducing the occurrence of Turnip Yellow Virus (TuYV), which according to some authorities can reduce yields by up to 25%.

“With no strong visual signs, this virus has the potential to restrain yields significantly. It is spread by the peach potato aphid, which will have thrived in the recent warm weather,” said Mr Freer.

While seed dressings will not stop individual plants becoming infected, once the aphid has drawn sap from the plants it will die, preventing further spread. But dressed seed much be properly drilled.

As a condition of use, treated seed needs to be buried to keep it away from birds and other animals, so crops established by autocasting or broadcasting would fall foul of this,” said Mr Freer.

NIAB TAG trials data does not easily separate the two leading products. Modesto has more active ingredients which may prolong the insecticidal activity, while Cruiser contains the fungicide metalaxyl?M which combats downy mildew.

“Each is superior to traditional Chinook (beta?cyfluthrin + imadacloprid) but the choice is really dependent on individual circumstances,” said Mr Freer.