Oilseed rape protection is a priority

Article taken from the Farmers Guide

Phoma spores blowing in from OSR stubbles are infecting occasional plants and the disease will increase rapidly with further rainfall, warns Norfolk-based Agrovista agronomist Craig Green.

Left untreated, the phoma fungus infects the leaf and grows down through the leaf petiole into the stem, where it can no longer be controlled.  Once in the stem it sits as a ball of cells, developing in the spring to form cankers, which can cut off and kill entire systems.  In bad cases yield losses of 0.5t/ha are not unusual, and can rise to 0.8t/ha.

Initial leaf infection can occur quickly – it takes just 120 day-degrees for symptoms to appear – 10 days at 12°C, says Craig.  Growers should look for pale lesions, which become dotted with black fruiting bodies.  Spraying threshold is 20 per cent plants infected or 10 per cent on smaller plants, he advises.

“Smaller plants are at the most risk as the fungus has less of a distance to travel inside the leaf to the stem – and with many oilseed rape crops being planted in late August/early September with limited rainfall, there are quite a few smaller crops this year.”

Capitan 25 (flusilazole) and Proline (prothioconazole) provide control with no PGR effects, at 0.4 and 0.32 litres/ha respectively.  However, the former is being revoked and will no longer be available to buy after 12th October, though the product can be used for a further 12 months, says Craig.  Where PGR activity is also needed early for the most forward crops, Sunorg Pro (metconazole) will do a good job, he notes.

Light leaf spot is likely to reappear.  “Last year was terrible – it is certainly no longer a northern disease.  It usually comes in during November through to February.  A second phoma spray if needed will also help control it as this time.”

Some OSR stubbles remain green with volunteers, a deliberate tactic to decoy pigeons off new crops.  However, he advises spraying these stubbles off as they not only harbour phoma but also increase populations of peach potato aphids that transmit turnip yellows virus to new crops and also barley yellow dwarf virus (BYDV) to emerging cereals.

To protect newly-sown OSR against these aphids a new tool is available in the form of Plenum (pymetrozine).  “This is very welcome because peach potato aphid carries a high level of resistance to pyrethroids and pirimicarb,” says Craig.  “A treatment will still be beneficial in heavy infestations on crops treated with neonicotinoid seed dressings.”

Cereal volunteers in OSR can compete severely with the crop and also harbour septoria tritici and net blotch which can which can infect newly emerging neighbouring cereal crops.  Craig recommends Shogun (propaquizafop) at 0.3-0.5 litres/ha.

Black-grass also continues to appear in OSR.  “It is important not to let the weed get too big between now and the application of Kerb (propyzamide) or Crawler (Carbetamide). “Aramo (tepraloxydim) will take out easy-to kill black-grass, while Centurion Max (clethodim) works well on more resistant populations.  “Where we have a real problem we’ll apply Crawler at the end of September/beginning of October at 2.5kg/ha and follow with full-rate Kerb in November.  It’s expensive, but works very well.”

Cereals will get a robust pre-em herbicide programme wherever black-grass is present, regardless of resistance status.  “It’s tempting to rely on Atlantis (mesosulfuron + iodosulfuron) to take light infestations out.  All too often it doesn’t due to poor timing or weather and that’s how problems begin,” says Craig.

Herold (DFF + flufenacet) at 0.3 litres/ha plus Trooper (flufenacet + pendimethalin) at 2 litres/ha plus Absolute (DFF + flupysulfuronmethyl) at 120g/ha will be followed by Avadex (tri-allate) seven days later.

“All our trials show Avadex increases black-grass control by an average of 18 per cent,” he notes.

Where annual meadow grass is the target he recommends Hekla (chlorotoluron + DFF) or Churchill (DFF + pendimethalin), or either Herold or Trooper at the above rates.

Barley yellow dwarf virus could be a problem this year in all cereals due to a shortage of Deter (clothianidin) seed dressing.  “If seed is not treated start spraying an approved pyrethroid from the one-leaf stage onwards if aphids are present and keep monitoring.  If seed has been treated, spray 6-10 weeks after drilling if aphids appear, depending on seed rate and time of drilling,” says Craig.

*Craig Green is an agronomist with Agrovista and is based at the company’s Great Ellingham depot, Norfolk (craig.green@agrovsita.co.uk)


03 October 2013

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