Monday, July 15, 2019

Tolerant varieties offer ‘best solution’ to Verticillium

July 2, 2019 by  
Filed under Crops

Growers are advised to monitor rape crops for Verticillium early this month – and choose a tolerant variety in future if it is found.

Unlike other yield reducing diseases of rape, soil-borne or seed-borne Verticillium stem stripe has no chemical solution, says plant pathologist Faye Ritchie of ADAS Boxworth. It can remain present in soils for years – so the main way to live with it is to grow a variety with good resistance.

“You have little other choice,” says Dr Ritchie. “Extending rotations is likely to be impractical and there are no approved chemicals. For those who have seen or see it this harvest, they need to make a proactive decision to grow a resistant variety this autumn.”

AHDB has been looking at ways of introducing a resistance rating for Verticillium in the AHDB recommended list. Symptoms are seen when the crops starts to ripen usually towards the end of June or early July, says Dr Ritchie.

“Look out for premature ripening of branches and grey stripes down stems, often on one side only, but it can be the whole cross-section of a stem. Underneath these vertical stripes, when the outer stem layer is peeled off, you will see grey vascular tissue.

“Using a hand lens, you can see tiny black dots or microsclerotia. The microsclerotia are assumed to survive in the soil for over 10 years to infect subsequent crops, germinating in response to root exudates.”

Last year, the Defra oilseed rape disease survey reported the levels of Verticillium were low, at just 5% of crops affected nationally. “The highest levels regionally were in the Midlands, with over 20% of crops affected. In 2017 the percentage of crops affected nationally was 20%.”

Neil Groom, technical manager for Grainseed says resistant varieties can help growers control Verticillium sustainably. “Once you have identified it, you need to start thinking seriously about how to manage it,” he says.

“Unlike other diseases of rape, there is no approved fungicide to control it and so growers must rely on cultural control measures – growers must choose varieties with known resistance and avoid using seed from crops known to have the disease.”

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