USING A two-spray strategy is key to tackling light leaf spot in many crops of oilseed rape this autumn, according to agronomy firm, Hutchinsons.

Disease risk is high and recent AHDB research has suggested spore infection may be occurring earlier than previously thought.

This means that the traditional approach of one autumn fungicide may prove ineffective in many situations, the firm’s northern regional technical manager, Cam Murray, said.

“Light leaf spot is a real issue in this area and early control is certainly better; think of it like the protectant approach to septoria control in wheat.

“It is vital to get control of the disease from the start. If we don’t there’s little chance of stopping infection spreading from the leaves to the stem in the spring, especially as none of the main fungicide options offer very strong curative ability.

"Prothioconazole is generally the strongest active on light leaf spot, but that still only offers relatively limited curative control.”

His preferred approach is to apply the first fungicide spray in early autumn at the 5-6 leaf stage of the crop and follow this up with a second spray towards the end of October or early November before worsening field conditions prevent travel.

Prochloraz and propiconazole is a relatively cost-effective mix at the earlier timing, with prothioconazole and tebuconazole preferred for the second spray, he suggested.

Forward crops – those with 6-8 leaves by early October – may require growth regulation and in such cases consider metconazole or tebuconazole-based products, added Hutchinsons' technical development director, David Ellerton.

Although light leaf spot has traditionally affected crops mainly in Scotland and northern England, phoma incidence also appears to be spreading north as climatic conditions change.

Beware of aphids bearing 'gifts'

Growers should also be aware of turnip yellows virus (TuYV), which is spread by the peach-potato aphid (myzus persicae).

A survey by Hutchinsons and Bayer CropScience found oilseed rape crops in Scotland on average had 32% leaf infection with TuYV, while English crops showed 63% 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, said Dr Ellerton.

“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 said.

“Hutchinsons trials have also shown the addition of an adjuvant based on orange oil considerably improved aphid control and reduced TuYV infection.”