Healthy potential for remaining OSR
The key question for iOSR growers now, is just what impact the high levels of CSFB larvae are going to have on yield?
The threshold for yield effect has been set as low as just five larvae per plant. Yet most crops are reported with infestations well in excess of this number – with iOSR growers across the country seeing populations of 20 to 40 or more larvae per plant.
iOSR meeting host, Ian Lutey, manager for R H Topham at St Ives, hopes that keeping crops as healthy and green for as long as possible could help to mitigate against the effects of CSFB larvae.
He applied Amistar and trace element manganese at mid-flowering, as much for the physiological stress relief and greening as the Sclerotinia protection.
“The effect of larvae damage is evident in almost every plant,” he said. “In some it is restricted to the side branches, so we may lose some of those but retain the rest of the plant; in others the main stem is damaged.
“We don’t know what impact it’s likely to have on final yield, but everything we can do to keep the plants healthy to the end has got to help.”
Northamptonshire grower, Jeff Bradshaw, had also used Amistar to extend the green leaf retention of his remaining crops that were looking healthy, but also showing extensive sign of CSFB. Furthermore, this year he’d planned a late N application at the end of flowering to push yields.
Other iOSR growers highlighted experience that the late application had been effective in increasing seed yield, but cautioned that it came at the expense of reduced oil content that resulted in limited, if any, increase in gross output. However, the combination of prolonged green leaf to capture sunlight and the extra N may prove more successful.
In Norfolk, Chris Eglington calculates to establish big, strongly branched plants, at plant populations of 15 to 20 ppm2, with precision drilling at low seed rates. With his multi-branched plants Chris believes there is potential to still achieve good yields even with some losses, but he acknowledges that this season the CSFB larvae pressure is unprecedented.
Syngenta Technical Manager, Georgina Wood, explained an ingenious and simple technique to assess CSFB larvae numbers, which would help growers calculate the potential impact on yield from different levels of infestation.
“Lay a sample of plants on chicken wire suspended above a plastic box with 50 mm of water in the bottom, ideally in a greenhouse or somewhere light and dry,” she suggested.
“The larvae think the plant is going into natural senescence and drop out, as if falling to the ground to pupate; only they fall into the water and drown. A squirt of washing up liquid in the water ensures they sink.”
The water can be drained through a sieve and the larvae numbers easily counted. The technique has proved especially useful in assessing trials with high larvae numbers.
“It has also been evident, from the significant variation in larvae sizes collected, just how prolonged the infestation pressure went on for this season,” she added.
Read more from the latest iOSR group: