Oilseed rape crop main stem growth is shooting ahead this season, but the secondary side shoots trying to catch up are more vulnerable to pest and disease attack. Pollen beetle control and green leaf management will be essential for crops to reach their yield potential.
Oilseed rape crops racing towards flowering, and then checked by cold weather, will need careful management to avoid excessive loses to pests and diseases. Rapid main stem extension could lead to an extended flowering period with increased risk of Sclerotinia, whilst backward secondary shoots are more vulnerable to extended pollen beetle attack.
Greater plant variability could mean crops benefit more than ever from longer green leaf retention at the end of the season, to enable crops to even up and increase pod fill.
ADAS Oilseed Rape Specialist, Dr Peter Gladders, reports early crops running up to three weeks ahead of last year, with the main raceme on some plants extending towards flowering. Bigger plants are seeking to dominate and shade out competition, but it does give greater opportunity for these stronger plants to send out more side shoots and generate yield potential.
"There is still time to get on with a PGR to limit the tall raceme and create a more efficient branching network," he advises. With the strong winter growth, plants have generally developed deep root networks and many growers have taken heed of advice to delay nitrogen, adds Dr Gladders.
PGR and fungicide timing
Timing of PGR and flowering fungicide applications is going to be extremely important this season, according to Rod Burke of Syngenta. He highlights that with side branches taking time to catch up with the main stem, green leaf retention is going to be crucial if the later pods are to successfully fill with seed.
"Independent trials and farmers’ own experiences have shown an Amistar application at early flowering maintains a healthy green leaf for longer," reports Mr Burke. "Sunlight interception in the post flowering period generates 90% of oilseed rape yield, with seed oil levels increasing by 1% a day at the end of the season. Green leaf captures the energy to drive yield."
He advocates growers with advanced crops which have already reached the yellow bud stage and where Sclerotinia is not a major risk, should tank mix Amistar with a PGR triazole, such as metconazole, to encourage the healthy leaf canopy and side shoots to develop.
For later crops, which are generally more even, Mr Burke believes a PGR now, followed by an Amistar application when the bulk of the crop reaches yellow bud, will reap the greatest benefit. Where the risk of Sclerotinia is high, however, then Amistar is best applied at early flowering in all crops, he adds.
Sclerotinia risk
Dr Peter Gladders warns that with Sclerotinia activity already started in the soil, crops are going to be at risk of infection right through flowering, and if that is protracted by the later flowering side shoots the risk will be inherently higher. "Assess the risk when deciding on application timing; the main risk is usually at mid-flowering, but fungicides must be applied preventatively," he advises.
Some growers in Scotland were caught out last year when Sclerotinia infection occurred late in the flowering period, with some crops suffering significant losses. "But the cost and wheeling damage of a second late spray towards the end of flowering are probably not justified unless you are expecting 15-20% infection," adds Dr Gladders.
Increased pollen beetle threat
Oilseed rape flowers at the green bud stage on side shoots, when the main stem is already in flower, are at greatest risk from pollen beetle attack, warns Rod Burke of Syngenta. Migrating beetles are attracted even by isolated flowers or early flowering volunteers, but it is the feeding damage on the green buds that causes major loses, he points out.
"This season the secondary side stems could be effectively in the same risk category as backward crops, with the threshold for Hallmark Zeon treatment reduced accordingly," he advises. Conventionally treatment is advocated for winter OSR crops if inspection reveals more than 15 beetles per plant at the green-yellow bud stage, but growers may need to consider revising the threshold down to around five beetles per plant.
Hallmark Zeon is recognized by the British Bee Keepers Association (BBKA) as presenting a low risk to bees when used as recommended. Hallmark can be tank-mixed with Amistar for application during oilseed rape flowering to provide pest plus disease control from one application. The ideal timing is either early or late in the day when bees are less active. However, growers are advised to avoid a pyrethroid insecticide tank mix with triazole fungicides when bees are foraging.
