Variable OSR growth is creating a autumn fungicide challenge for growers this back-end.

While early disease pressure in oilseed rape is much lower than last year due to dry conditions during August and early September, the variable nature of growth will complicate autumn spray decisions, according to experts from agronomy firm, Hutchinsons.

By the end of September, early-sown in many areas had reached the fourth true leaf stage, while the most backward crops were only just emerging. “The differences in this region are largely moisture related,” said Hutchinsons agronomist, Robert Barker, in Yorkshire.

“There are some crops on the Wolds that didn’t receive rain early in the season and then also got hit by flea beetle, so really struggled to get going.

"Equally, we’ve got other crops in areas that caught rain showers after drilling or were direct drilled to conserve moisture that established well and look much better. It’s a very variable picture, but at least disease pressure is generally low.”

There is a similar picture elsewhere in the country, according to Hutchinsons technical development director, Dr David Ellerton. “Last year we’d already seen plenty of phoma in crops by late-September and early October, whereas disease pressure so far in most places has been much lower.”

But, with later disease development, some growers may well be able get away with a single well-timed fungicide spray to protect crops from phoma and light leaf spot through the winter, he argued.

“Generally speaking, the optimum time for a single spray is late October or early November, however accurate timing is everything. If you spray too early, fungicide activity may run out of steam if phoma comes in late. In contrast, you can’t afford to delay too long and risk disease getting established in the crop.”

Forward crops at the 6-8 leaves by mid-October stage in wetter parts of the country may still be at higher disease risk and benefit from a two-spray approach – an earlier treatment if the threshold of 10-20% of plants infected is reached, followed by another spray three to four weeks later.

In smaller plants, phoma infection can potentially reach the stems more quickly, so he recommended a lower treatment threshold of 10% infection. Priority should also be given to susceptible varieties, such as Nikita, Mentor, Flamingo, Elevation, Kielder, Broadway, SY Harnas and V3240L, which are all more at risk from canker.

For Mr Barker, light leaf spot is the main driver for autumn fungicides, although it is generally well controlled with a single spray in early November, he said. “But we are seeing light leaf spot risk develop from mid-October onwards, so it’s best to monitor crops closely and use the industry risk forecasts to judge the optimum time to spray."

While the fungicide response of some oilseed rape varieties may not always be obvious, Dr Ellerton pointed out that there was an average yield benefit over untreated of 0.32 t/ha across all sites and varieties at Hutchinsons Regional Technology Centres in 2017/18 (0.75t/ha the previous year). There are big variations between varieties, with the highest response of more than 1.3t/ha, worth more than £400/ha at current prices.