Verticillium control reliant on resistant varieties

GROWERS have been urged to consider growing varieties resistant to the chemically uncontrollable disease, verticillium in oilseed rape to contain its spread.

Arable plant pathologist, Philip Walker, of ADAS Boxworth reminded growers that there is still no chemical treatment for verticillium stripe (also known as verticillium wilt) in oilseed rape, so growers totally rely on in-built resistance.

It is not a major problem in Scotland – yet – but is has been reported that this soil-borne disease can persist for 10 years in the soil, so adjusting rotations to manage can be impractical. Mr Walker advised a rotation of at least one in five for verticillium, as well as sclerotinia in rape. "When monitoring levels in the soil, there is usually a general trend for decline over time, but it rarely gets to zero, demonstrating its persistence,” he pointed out.

There was a high severity of verticillium in crops this year due to dry conditions in the spring. “This year, in ADAS trials on confirmed verticillium sites, symptom expression was seen at least two weeks earlier in late June (ie when desiccants are applied rather than when desiccation is complete)," he said.

"Symptoms included yellowing, or grey stripes, usually on one side of the stems of rape plants. Underneath these vertical stripes, when the outer stem layer is peeled back, you could see grey vascular tissue and, using a hand lens, tiny black dots called microsclerotia.

“These can survive in the soil for at least 10 years, germinating in the autumn in response to root exudates from newly-planted crops and invading the roots of oilseed rape seedlings. The crop shows no symptoms at this stage.

"At stem extension, the infection spreads from the roots into the vascular system, colonising the stem. This causes premature ripening and yield reduction," he explained. “The dry conditions this year will mean that more farmers will have seen verticillium symptoms in crops, particularly in hotspots such as the East and South-east.”

Cultural control strategies such as adjusting drilling dates have been investigated, but not been proven to be effective. “For those who have seen verticillium this harvest, the only control option available at the moment is to grow a resistant variety,” he argued.

There are differences in verticillium resistance between varieties, with many more modern varieties that have high vigour, good standing power and possibly pod shatter resistance able to compensate for or tolerate this disease, even where a high degree of infection is seen.

“In ADAS trials this year, there have been clear differences between susceptible and resistant varieties in the field. The variety Quartz, for example, is considered highly susceptible and had a severity index score of 97 out of a 100 for this disease in our trials.

"Many modern varieties are in the 50-65 range showing moderate resistance and are able to withstand this disease,” he said. "AHDB has been looking at ways of introducing a resistance rating to verticillium in the AHDB recommended list and this is expected to be introduced in 2023."

Neil Groom, general manager for breeder, Grainseed, said: “To manage verticillium, you need resistant varieties. There is no other real option. “Once you have identified this disease on your farm, you need to plant a variety with known proven resistance. Trials show distinct differences in susceptibilities between oilseed rape varieties.

"Tested on sites with known verticillium infection, varieties which are close to the reference variety, Flamingo, are classified as tolerant. Other ones, like Quartz and Harper, are classified as susceptible.

"Grainseed oilseed rape varieties such as Es Mambo and Keeper have proven resistance to verticillium, following several years’ independent trials and commercial experience. Mambo has been the most resistant variety in AICC trials.

"Working with plant breeder, Mike Pickford, over the last five years, we have been focussing on breeding clean varieties for the UK to enable growers to reduce inputs, reduce risk of weather delays in fungicide application and improve genetic resistance," he said.

"Keeper is the first of these clean varieties and growers will notice the greener stems at harvest. This helps the plants keep photosynthesising and building yield and results in very large seed at harvest."

In the UK, though, there is limited data on yield losses. Mr Walker said he had seen yield increases of 1 t/ha in resistant varieties and the converse for susceptible ones, but with the current price of oilseed rape seed in the region of £650/tonne, this could make a significant difference to the balance sheet.

“Yield loss is due to reduced seed size, as the infection restricts water and nutrient uptake. High risk conditions include warm, dry springs, summer drought, tight rotations, use of infected seed and stressed crops.” added Mr Groom.