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A new disease rating on the latest recommended list for oilseed rape aims to help growers choose varieties resistant to verticillium. This year  is... Why resistance is ‘only way’ to combat verticillium

A new disease rating on the latest recommended list for oilseed rape aims to help growers choose varieties resistant to verticillium.

This year  is the first time verticillium resistance ratings have been included in the Agriculture and Horticulture Development Board list. Top conventional oilseed rape variety Pinnacle is rated as having intermediate resistance to the disease.

“Varieties with higher ratings have been assessed for a longer period of years, hence more data has been available,” says Pinnacle breeder Mike Pickford.

“I would fully expect Pinnacle to rise up to the moderately resistant rating in the future.

Verticillium tends to be sporadic but has hotspots such as Cambridgeshire and East Anglia, says Mr Pickford. But it has been reported in the  rape growing areas of Lincolnshire and Herefordshire too.

Disease symptoms

Verticillium was first found in England in 2007 when it was called verticillium wilt. Last year, the disease was renamed verticillium stripe to fit more accurately with symptoms when they are seen in the crop.

Initial symptoms include premature ripening which gets more obvious as the crop matures. Verticillium affects the whole of the plant architecture, showing as yellow or brown discolouration on the stem surface.

Peeling back the epidermis of infected stems will reveal grey vertical stripes through the stem tissue. This is the infection found in the xylem or water conducting parts of the stem.

As the crop senesces, these grey vertical stripes will form tiny black dots called microsclerotia. They can be seen using a hand lens, and will return verticillium infection back into the soil.”

In extreme conditions verticillium stripe can cause up to 34% yield reduction, says Neil Groom, general manager for Grainseeds. Being a soil-borne disease, it will be building up in the soil each year.”

No treatment

There is no chemical treatment available, says Mr Groom. “Plant pathologists say that the only way to minimise this disease is to extend your rotations or grow varieties with known resistance.”

Because the microsclerotia can survive in the soil for 10 years, the rotation needs to be ten years, This strategy is unworkable and impractical, so growers who have experienced the problem only have resistant varieties to fall back on.

“There is no other option of minimising Verticillium other than growing a resistant variety Grainseed oilseed rape varieties such as Pinnacle have proven resistance to verticillium, following several years’ independent trials and commercial experience.

Growing a variety with strong all-round disease resistance can save money, but variety selection is made prior to drilling. “Once the crop is in the ground, there is nothing else you can do to control verticillium,” says Neil.

“Working with plant breeder Mike Pickford, we have been focusing 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.”

ADAS has been running trials for ten years on sites with a history of verticillium. And differences in varietal resistance have been observed between rape varieties for a number of years in the field by breeders.

“In one Grainseed trial on a known disease site in Cambridgeshire, susceptible varieties were scoring a high 70 Index which means 70% of the plants were dead from verticillium. On more tolerant varieties, we were seeing much lower scores of 20.”

AHDB trials

AHDB trials have been running for four years. Rather than the usual 1-9 rating, insufficient data differentiation means verticillium resistance is denoted in three categories – moderate resistance, intermediate and susceptible.

Pinnacle is categorised in the intermediate category. In trials, it looks clean and growers will notice its greener stems at harvest, says Mr Pickford. “This helps the plants keep photosynthesising and building yield and results in very large seed at harvest.

“More Importantly, data on verticillium resistance shows Pinnacle to be as good as the variety Catana which is recognised as the resistant control variety.”