Variable OSR growth is creating a autumn fungicide challenge for growers this back-end (see also above).
While early disease pressure in oilseed rape is much lower than last year due to dry conditions during August and early September, but variable growth will complicate autumn spray decisions, according to experts from agronomy firm Hutchinsons.
By the end of September, early-sown forward crops 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,” says 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 pretty 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 incidence in crops by late-September and early October, whereas disease pressure so far in most places has been much lower.”
However, given recent heavy rain and very variable growth stages, disease risk must be closely monitored and fungicide programmes carefully tailored to in-field risk, variety susceptibility and growth stage, he says.
But, with later disease development than last year, some growers may well be able to 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 6-8 leaves by mid-October in wetter parts of the country may still be at higher disease risk and benefit from an earlier spray if the treatment 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 recommends 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 says.
“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 varieties may not always be obvious, Dr Ellerton said there was an average yield benefit over untreated 0.32t/ha across the board at Hutchinsons Regional Technology Centres in 2017/18 (0.75t/ha the previous year).
There are big variations but the highest response of more than 1.3t/ha, worth more than £400/ha at current prices.