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Two-Pronged Approach to Tackling Light Leaf Spot

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A two-spray strategy is key to tackling light leaf spot in many crops of oilseed rape this autumn, according to leading agronomy firm Hutchinsons.
With high disease risk forecast and recent AHDB research suggesting spore infection may be occurring earlier than previously thought, the traditional approach of one autumn fungicide may prove ineffective in many situations, the firm’s northern regional technical manager Cam Murray says.
“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.”

Cam Murray Hutchinsons

Mr Murray’s 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 suggests.

Forward crops (i.e. those with 6-8 leaves by early October) may require growth regulation and in such cases consider metconazole or tebuconazole-based products, adds Hutchinsons technical development director David Ellerton. However these may not offer the level of disease control required if used alone.

Cereal growth regulator mepiquat chloride in combination with metconazole also has clearance for autumn application in forward oilseed rape, although he says this should be applied at the 4-6 leaf stage for maximum benefit. Again, this product will require an additional fungicide in many situations.

Mr Murray reinforces the importance of varietal resistance in tackling light leaf spot and suggests that where growers have sown a particularly disease resistant variety (e.g. rated 7 for LLS) there may be scope for a single autumn fungicide.

“However, if you do go down this route, make sure it goes on earlier rather than later,” he notes.

LLS with spores

All growers are reminded that although light leaf spot has traditionally affected crops mainly in Scotland and northern England, it is frequently being found across southern England. Likewise, phoma incidence appears to be spreading further north as climatic conditions change, highlighting the need for constant vigilance and robust agronomy.

Hutchinsons are involved with the University of Hertford and a number of breeders in projects to increase the durability of varietal disease resistance and progress will be communicated to growers via newsletters and our trial site open days next summer.

Beware of virus-carrying aphids

One issue growers should be aware of in future 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 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, says 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 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.