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New Angle on autumn OSR disease control

Agronomy Issues
05.10.2020
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Autumn diseases in OSR leaf

Phoma outbreaks have hit oilseed rape crops early this season, making a two-spray autumn strategy core to most agronomists’ programmes to target early light leaf spot too.

With early sown OSR escaping the worst ravages of cabbage stem flea beetle and establishing well, investing in autumn disease control will pay dividends, advises Syngenta Technical Manager, Georgina Wood.

Georgina Wood OSR

“After struggling to get oilseed rape crops established over successive seasons, some growers have reverted to a ‘wait and see’ approach before spending on the crop.

"With the stronger and more consistent crops this year, previous trials and experience emphasises the value of phoma and light leaf spot control in the autumn,” she advocated.

Repeated rainfall this season, which was so valuable for crop establishment, has resulted in release of phoma spore flushes from stem cankers in the previous crop trash. The high risk of infection moving quickly in warm conditions, from leaf lesions down to the stem, has already seen crops receive an early Plover phoma fungicide.

“As autumn progresses, follow up fungicide applications will look to cover further phoma outbreaks, as well as initial infections of light leaf spot .” 

Light leaf spot on autumn OSR leaf

Miss Wood highlights trials have shown a two spray strategy with Plover, followed by Angle, has given the most effective activity on both phoma and light leaf spot (above). Results showed just 3% LLS infection with Plover fb Angle treatment, compared to over 35% in untreated plots and 20% infection from a single Plover application.

“The results reinforced the value of the two-spray strategy and the importance of starting the protectant LLS programme in the autumn," said Miss Wood.

"At recommended rates, Angle contains the full rate of difenoconazole to continue excellent phoma control, along with trials showing the activity on light leaf spot.”

Independent AHDB OSR fungicide assessment on four sites across the UK has shown Angle provided the same level of LLS activity as boscalid or prothioconazole, with dose rate response work indicating Angle having excellent intrinsic activity.

Miss Wood added that with the potential for LLS to continue to be active right through the winter, particularly in cooler conditions, growers should be alert to the possible need for follow up treatment in January or February.

OSR trials site showing leaf spotting

“The polycyclic nature of LLS means that once infection is in the crop, asexual spores can spread the pathogen by rain-splash onto surrounding plants. Syngenta spore monitoring in the eastern counties has also revealed wind borne spores, produced on decaying leaves, can be present throughout the winter and early spring.”

She recommends growers continue to monitor plants for early signs of LLS infection. Leaves of plants with suspected infection can be sealed in a clear plastic bag on a windowsill, to see if they develop small white spores.

“There’s no treatment threshold for autumn or winter LLS, with infections best targeted as soon as symptoms are seen.   The reoccurrence of phoma levels reaching threshold should also be monitored and follow up sprays applied accordingly.”

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