Struggling OSR needs Tailored Agronomy

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Unusually dry weather across much of southeast England resulting in some oilseed rape crops being written off has heightened the need for a much more flexible approach to agronomy this autumn.

Kent-based Hutchinsons agronomist David Shepard says just 10-15mm of rain fell across a large area of Kent during August and September, which has hit oilseed rape hard and is creating some real challenges for agronomy.

“Everything is quite backward due to the weather, with the best crops around the four leaf stage,” he says. “But some crops are only at the cotyledon stage and we’re still waiting to see how they will fare in the next couple weeks, before making a decision on them.”

The difficult conditions mean it is vital to monitor crops closely over coming weeks and tailor autumn treatments to crop requirements.
For Mr Shepard this already means a likely switch from a favoured two-spray approach to a single well-timed fungicide for phoma and light leaf spot protection.

“At the start of the season I’d have gone for two sprays, but given the backward crops and dry weather keeping visible disease levels low, it will probably be just a single spray now, with phoma being the priority.”

David Shepard H&S

A large area of more resistant varieties in the ground across his region will help to reduce phoma risk, but the underlying threat remains high so he still recommends applying a fungicide when the treatment threshold of 10-20% plants infected with phoma leaf spot is reached. The Rothamsted Research phoma risk forecast can help identify when this will be reached.

Boost Backward Oilseed Rape

Fungicides with growth regulatory activity should not be used on backward crops (i.e. those with around three leaves by early October) and instead it is worth considering products that can help increase rooting, such as those based on penthiopyrad + picoxystrobin.
Trials have shown excellent control of both phoma and light leaf spot from these actives, with an ability to considerably increase root mass, enabling better nutrient and water uptake without reducing crop size above ground, says Hutchinsons technical development director David Ellerton.
He also highlights recent AHDB research suggesting light leaf spot is becoming more widespread and spores are being produced earlier in the season than previously thought. He therefore recommends growers apply a fungicide that is effective against both diseases unless variety resistance is particularly strong.

“If light leaf spot is building earlier before symptoms are visible, it needs treating accordingly. Just because you can’t see it doesn’t mean the disease isn’t there.”

Products based on a combination of prothioconazole and tebuconazole are also an excellent option for controlling both phoma and light leaf spot, he says.

If no early spray has been applied for phoma, then Dr Ellerton says a routine protectant fungicide should be applied for light leaf spot in late October or early November.

Even where a phoma spray has been applied a second application may be required, especially if the first fungicide was applied very early, he notes.
Mr Shepard adds that foliar nutrition, such as phosphite or zinc-ammonium acetate complex could be worthwhile at the 1-2 leaf stage to give backward crops an extra boost, especially where dry soils are limiting nutrient uptake from the ground.

Take Control of Forward Crops

Any growers with forward crops (i.e. those with 6-8 leaves by early October) requiring growth manipulation and disease control should consider metconazole or tebuconazole-based products, Dr Ellerton adds. However these do not offer the level of disease control of earlier options if used alone.
Cereal growth regulator mepiquat chloride in combination with metconazole also has clearance for autumn application in forward oilseed rape, although should be applied at the 4-6 leaf stage for maximum benefit. Again this product will require an additional fungicide in many situations.

David Ellerton (3)

Beware of virus-carrying aphids

The other main disease to watch out for this autumn is Turnip Yellows Virus (TuYV), which is spread by the peach-potato aphid (Myzus persicae).
A recent survey by Hutchinsons and Bayer CropScience found oilseed rape crops in England on average had 63% leaf infection with TuYV, while Scottish crops showed 32% infection.

Aphid resistance to common insecticides such as pyrethroids and pirimicarb, together with the loss of neonicotinoid seed treatments means a change in control is needed this season, Dr Ellerton says. “There are no cheap and cheerful options for controlling aphids anymore and most are contact-acting, so aphids have to be present for them to work effectively.”

Both pymetrozine and thiacloprid have approval for use in the autumn to control aphids in oilseed rape and both actives will control resistant populations, he says.

“Hutchinsons trials have also shown the addition of an adjuvant based on orange oil considerably improved aphid control and reduced TuYV infection.”
Dr Ellerton recommends growers track aphid activity using monitoring services such as the AHDB/ Rothamsted email alerts and regularly inspect their own crops for aphids where risk is high.

Insecticide treatments can often be combined with the autumn fungicide, but only if the two timings coincide and neither application timing – nor resulting efficacy - is compromised.