The Syngenta oilseed rape growers group’s latest meeting focused on the importance of phoma timing following last year’s Phoma Alert.
Results from last year’s Phoma Alert highlighted the importance of phoma timing, which was long before any signs of light leaf spot (LLS), says Syngenta technical manager James Southgate.
Even with the low pressure season in dry conditions, ADAS crop monitoring for the Syngenta Phoma Alert website highlighted phoma treatment thresholds were typically reached twice before LLS symptoms kicked in, particularly on the western and northern sites.
“Although the initial phoma infection was relatively late, it was notable reinfection after initial treatments was quite quick when pressure increased,” he told the iOSR farmer group. “In fact, when infected crops reached the second treatment threshold in December, there was no LLS reported."
“The implication for agronomic decision-making was to treat purely for phoma with Plover at the first application, but given the late timing of the second treatment to bolster the LLS prevention with a Plover and tebuconazole tank mix,” he advised.
In the driest eastern counties, Mr Southgate highlighted Phoma Alert indicated some crops could get away with just one treatment last autumn, using the tank-mix option.
Although the iOSR growers had identified disease resistance as a key attribute of their variety selection, more recently the emphasis has been on LLS ratings and less on phoma which they know can be readily controlled in autumn.
“Some 80% of varieties have a phoma stem canker rating of six or below,” says Mr Southgate. “But when it comes to leaf spotting in autumn, Phoma Alert revealed little or no difference between crops with stem canker ratings of four or six. The advice in the field is to treat them all the same when thresholds are reached.”
He advocated growers should assess the risk to crops, based on seedling size, drilling date, weather conditions and the appearance of phoma spots on the leaf.
“Warmer conditions at the time of infection will lead to quicker movement of infection down the leaf petiole and into the stem; the smaller the leaf at the time of initial spotting, the greater the risk of spread. Growers should be monitoring crops and treating with Plover as soon as the phoma leaf spot threshold is reached,” he advised.
“Early infection in autumn typically leads to bigger and more damaging stem cankers in spring, with greater yield losses,” he pointed out.
THE increase in higher yielding Hyvido winter barley area, giving the chance to outcompete black-grass in the rotation, has also created a valuable early entry opportunity for OSR establishment. However, earlier OSR planting limits the availability for stale seedbed cleaning.
iOSR growers were urged to target any volunteer cereals early and fast to limit competition for light, moisture and nutrients – which will ensure OSR seedlings establish quickly and consistently. Volunteers pose a greater threat in min-till situations, especially if dry conditions continue and weed germination is slow this season.
Norfolk iOSR grower Chris Eglington reported his precision drilled crop had remained remarkably clean and even all season, to the point this season, for the first time, he decided not to desiccate any of the acreage. However, to mitigate any risk of a delayed harvest, he has applied a podstick treatment across the whole area.
In Northamptonshire, Ian Matts opted to glyphosate most, but not all, areas – focusing on green tramlines or under trees, again with podstick on shatter-susceptible varieties. Unlike Mr Eglington – who only grows conventional varieties – most of Mr Matts’ crop next year will be hybrids, with the exception of some Elgar too. For him, disease resistance has a high influence on variety selection, along with yield and gross output.
Further south, in Berkshire, Joe Dilibero, plans to grow Exalte across most of his oilseed rape area next year, having had good results again this season – producing an consistent crop with just one autumn and one flowering fungicide spray.
Hot, dry weather over spring and early summer has been highly favourable for aphid pest populations, with numbers of Myzus persicae aphids being caught in most monitoring traps running higher than this time last year. Myzus is considered the key vector of TuYV in oilseed rape.
Furthermore, AHDB testing results of Myzus populations has already revealed two-thirds of early season populations were already carrying TuYV and capable of transmitting infection.
Syngenta insecticide specialist Max Newbert warned early autumn transmission of infection into oilseed rape plants is likely to result in greater susceptibility to stress and more severe yield loss. Early infection also increases the chance of secondary spread through the season.
“With more farmers delaying drilling and encouraging green stubbles to control black-grass, there is a higher risk of infection being spread to oilseed rape; mayweed, chickweed and groundsel are all wellproven hosts for TuYV,” he added.
The threat potential has been heightened by widespread resistance to pyrethroid sprays among Myzus populations. “It’s reached such a high level pyrethroid applications will not offer any substantial control. Growers should now opt for Plenum as a means for control,” he advocated. “Independent research has demonstrated that Plenum was the most effective in preventing the spread of TuYV in autumn oilseed rape crops.”
THE legacy of last autumn’s phoma can be clearly evident in oilseed rape crops at harvest. The effect seen is typically stunted plants with thin main stems and far fewer branches which have died off early – with all the implications for a reduced yield, warned Syngenta technical field trialist Tom Clark.
Look closer on stems or trash of harvested stubbles and there is often evidence of cankers developed on the stem base of crops, typically displaying a key characteristic of the black speckled dots around the canker. Breaking open the stem will reveal a decaying mush, which can be easily differentiated from sclerotinia if there are no black sclerotia evident within the stem.
“Last season’s infection will inevitably have an impact on yield this harvest. But it also poses a serious risk for the new crop in autumn,” warned Mr Clark. “Old crop cankers, on the surface or mixed in with the stubble trash, will be the source of infective spores. Rain events over autumn trigger a continuous further spore release and infection events.”
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