With Phoma stem canker still being the Number one disease of winter oilseed rape, growers may need to consider their variety selection very carefully as many of the newly listed varieties are much more susceptible and will need more investment in the crop to keep disease at bay.
Neil Groom, Technical Director for Grainseed, points out that generally the rating for resistance to Phoma stem canker in the current HGCA East/West listing appears to have plummeted in recent years. “Over 70% of HGCA varieties have a rating of 5 or below and will need at least 2 autumn fungicide sprays. The new variety Incentive has a Phoma disease rating of 4, as has Charger, Avatar, Sesame, Rhino and Troy. Even worse is the rating for another new hybrid Marathon at a 3. According to plant pathologist Dr Faye Ritchie of ADAS an HGCA rating of 7 or above significantly reduces the need for a fungicide, so growers should be looking for varieties with this high score.”
“Each rating also has a financial value. With each HGCA rating being worth at least £20 per ha to growers, it is well worth looking at varieties which stand out in terms of their stem canker ratings. For example, Es Astrid has a rating of 7.0, the new conventional variety Es Alegria also has a rating of 7.0 and Es Alienor is 7.4. If you calculate from the variety Marathon up to Alienor that could give £90 extra to the grower, equivalent to 0.36 t per ha with rape at £250 per t,” he calculates.
“In addition Es Alienor and Es Alegria are known to be exceptionally vigorous in the autumn. This means rapid early plant growth that extends the distance that the fungus has to grow through the plant to infect the stem base, reducing impact of the disease. This characteristic will be doubly important this autumn as vigorous varieties will also help establish strong enough plants to survive without the benefit of neonicotinoid seed dressings.”
“Growers can also be assured that the resistance to Phoma stem canker in these three rape varieties is very robust, being based on multi-gene resistance to all races of Phoma. This resistance will not break down and will remain stable,” says Neil.
Neil Groom concludes that Phoma is still the most important and most costly disease of oilseed rape, with 60% of rape being grown in high Phoma risk areas. “Phoma causes premature plant death and yield losses of up to 50%. Yield losses of 0.5 to 1 t per ha are common when control is lacking. NIABTAG advises growers to spray when 1 in 10 plants shows Phoma leaf spotting. But if you are growing a variety with a strong resistance you don’t need to spray until 2 plants in 10 have leaf spots. Growers should not take their eye off the ball and should look for the key agronomic characteristics that make up the best varietal package and that includes vigour and multigene Phoma resistance.”