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Wet summer triggers OSR phoma spore risk

Agronomy Issues
10.09.2020
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Phoma leaf lesions September 2019
New infections of phoma leaf lesions have already been spotted in southern counties, with crops in the north under increasingly high pressure with continued rainfall.

Repeated heavy rainfall over recent weeks has now exceeded the trigger point for infective phoma spore release from stem cankers on last year’s oilseed rape stubble.

Crops still at small growth stages after delayed emergence, as a result of dry conditions at the beginning of August, are at serious risk of rapid infection spread, particularly in the more susceptible varieties, warns Syngenta Technical Manager, Georgina Wood.

“The characteristic signs of leaf lesions typically break out seven to 10 days after initial infection. Growers now need to be alert for the first signs and assess thresholds for treatment,” she advised.

Georgina pointed out the key application timing is to stop development of leaf surface lesions and the invisible spread of infection down to petioles and stems, aiming to prevent the later formation of yield limiting stem cankers.

Phoma stem canker

“Plants currently with small leaves are more susceptible and a priority for early Plover fungicide treatment, since it takes less time for the infection to move to the stem, compared plants with larger leaves,” she advocated. Disease development is also faster in warmer conditions, which typically makes early season infections more damaging. 

The treatment threshold is normally considered around 10% of plants showing visible signs of leaf lesions. However, factoring in the size of plants, along with pressure of repeated rainfall events and windy conditions triggering spore spread this season, would indicate small crops may require earlier protection. 

It has been calculated that rain events on 20 days from the start of August is required for phoma cankers on last season’s trash to emit spores, which will infect this year’s crops.

Rain events in 2020 Kelso and Shrewsbury

 Analysis of rainfall records on the Syngenta website, shows in Kelso, for example (above, right), there has been at least 22 days with rain events since the start of August. Western counties (above, right) have been drier, but still approaching the 20 day trigger point since the start of August.

Rain events in 2020 Cambridge

“Even the eastern couties, which have typically ben far drier during late summer in recent years, have seen above average rainfall events since the start of August.

"After all the headaches that has caused growers with harvest and field work, they now face prolonged serious phoma disease pressure.”

Growers are advised that the early onset of phoma lesions which require initial treatment now, is likely to require a two spray-programme to cover the autumn infection period. The second application is typically applied where new infection and leaf lesions are identified four to six weeks after the first treatment, or when light leaf spot is first seen.

Georgina Wood OSR    

“Syngenta trials in recent seasons have shown initial phoma thresholds were reached an average 92 days before the first signs of light leaf spot, which highlights the crucial importance of the early sprays to specifically target phoma,” advised Georgina. 

‘With Plover having no growth regulatory impact on the crop, it’s a good fit for seasons such as this when phoma comes in early and crops are generally small.’

Lookout for more information on this season’s oilseed rape agronomy on the iOSR website pages.

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