Tuesday, December 18, 2018

Disease ratings in rape varieties undervalued

June 29, 2018 by  
Filed under Crops

Growers are being urged to take more notice of disease ratings in oilseed rape varieties with one breeder saying they should be used more – as in wheat or barley.

“Considering disease ratings in wheat and barley is second nature when choosing which varieties to grow, but in rape this appears to be an undervalued benefit, says Neil Groom, technical director for Grainseed.

“Choosing a cereal variety with a strong disease resistance package can save you money or at least give you more flexibility in terms of widening the spray window. It is the same in rape. I do not understand why farmers do not utilise this benefit to a much greater degree in oilseed rape.”

Oilseed rape suffers from a number of damaging diseases. Some 91% of rape crops across the UK suffered from phoma leaf spot in spring 2017 and 40% of plants within the crop had this disease,  according to Crop Monitor.

This makes phoma the most widespread disease. It was recorded in all regions at high levels of incidence, confirms Crop Monitor. Phoma could become even more prevalent in the future as climate changes, suggests Mr Groom.

There are some varieties with good resistance ratings to this disease, says Mr Groom. But he warns: “Phoma is probably the most significant and costly disease of rape, causing premature death and yield losses up to 0.5 to 1t/ha.”

The AHDB list for 2018/2019 details 20 rape varieties. But only two have an 8 rating for phoma resistance. Of the others, two have a 7 rating, three a 6 rating, five a 5 rating, six a 4 rating and two have a poor 3 rating.

Grainseed rape variety Mambo has a rating of 7.8, Alegria 7.4, Decibel CL 8 and Aquarel CL 8. “This resistance is also multigene and is resistant to all known strains of Phoma stem canker, so is unlikely to break down in the field,” says Mr Groom.

“Choosing a variety with a high resistance rating to Phoma can protect the crop and save you money. It has been calculated that each resistance rating point is worth at least £20/ha benefit.”

Highest incidence

Light Leaf spot is the next most widespread disease and affected 68% of crops and 15% of plants within the crop last spring, according to Crop Monitor. The north of England showed the highest incidence, with 88% of crops infected and 36% of plants.

Light Leaf spot can also be a costly disease, and has the ability to stunt plants and cause premature pod ripening which can lead to large yield losses, Mr Groom explains.

“A couple of years ago most varieties had a 5 or 6 rating but now many more have a 7. However 7 is the highest rating out of a 1-9 rating system. Mambo has a 6.4 rating for Light leaf Spot and Aquarel CL a 6, Alegria has a 5.4 and Decibel CL a 5.”

Varietal resistance is also very important when it comes to Verticillium wilt which is more widespread, probably due to it being a soil-borne disease and tighter crop rotations. The first option is growing a resistant variety and the second is widening crop rotations.

That is because are no approved chemical solutions to Verticillium, which can cause heavy yield losses due to lower seed weight, fewer pods and seeds per pod, premature ripening and plant death.  As a result, resistant varieties play a key part in combating the disease.

“CropMonitor doesn’t look at this disease nor do AHDB publish resistance ratings on their list. But the Association of Independent Crop Consultants and ADAS have been investigating and assessing varietal differences.

“Our varieties Mambo, Alegria and Alienor have all come out very well in terms of resistance to this disease in these assessments.” says Mr Groom.

Growers should choose a rape variety with robust disease resistance across the most important diseases – as they would do when selecting wheat.

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