Farmers in mainstream oilseed rape-growing areas of the Westcountry who are seeking to cut autumn workloads will benefit from selecting varieties with better disease resistance ratings.
That is the view of product development specialist, John Miles, based on trials comparing phoma control programmes at ADAS last year.
"They show that the right variety selection will give you flexibility in terms of autumn spraying needs," he said. "In the ADAS work, even after two sprays, the susceptible control variety, rated 4 for the disease, still had phoma index scores of between 10 and 20. In comparison Cash, rated 7 for the disease, had an index score of 5 or lower, significantly cutting the risks of the disease leading to stem canker and significant yield loss in the summer." Mr Miles pointed out that strong variety resistance such as that provided by Cash did not negate the need for a sensible spray programme.
"With many crops needing at least two, if not three, sprays to keep the disease at bay through the autumn and winter, it simply buys you time to get round the whole crop," he said. "As a result, growers should be looking to put a proportion of their crop down to varieties that have the best resistance ratings possible."
The work which compared the use of Proline applied in mid-October, late-November and then again in late February showed that treatment which included the first of these spray timings gave the best results. There were also positive yield increases from a two or three-spray programme building on this mid-October treatment, with larger responses on the 4-rated variety for phoma.
But where sprays were delayed for whatever reason, the less resistant variety had yields that were between 0.5 to 0.6 tonnes per hectar lower than that of the 7-rated variety for phoma resistance.
"This suggests that in a year when early spraying conditions are more difficult, a variety with a better disease resistance rating will be invaluable," he added. "It has to be stressed that these results were from 2010 and 2011, which was not a bad season for phoma, so as a result, responses are likely to be even higher in a year when the disease is more prevalent. My advice to those growing rape in the South West would be to ensure that at least a third or half of your area is in more resistant types."