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Wet weather woes for OSR phoma infection

Agronomy Issues
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.

Recent days of repeated heavy rainfall have now exceeded the trigger point for new infective phoma spore release from stem cankers on last year’s oilseed rape stubble.

With many crops still at small growth stages after delayed emergence, as a result of dry conditions in August, there is a serious risk of rapid infection spread in susceptible plants this season, warns Syngenta Technical Manager, Georgina Wood.

“The characteristic signs of leaf lesions typically break out seven to 10 days after initial infection. Growers need to now 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 fungicide treatment such as Plover, 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. 

Syngenta iOSR growers have reported crops at widely varying growth stages this season. Those planted before rainfall in early August had largely grown away well, whilst those drilled later were generally slow to emerge in dry conditions with many also significantly impacted by cabbage stem flea beetle damage.  

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.

Post harvest rainfall Doncaster

Post harvest rainfall Cambridge

 Analysis of rainfall records on the Syngenta website, shows in Yorkshire, for example (above, left), there has been at least 35 days with rain events since OSR harvest in mid-July, with more than 26 since the start of August. Southern and eastern counties (above, right) have been drier, typically with around 27 days with rain since harvest and now approaching 20 since the start of August.

Post harvest rainfall Kelso

“The Borders of Scotland, however, have fared far worse (above), with 47 days rain since mid-July and 34 since the start of August. After all the headaches that has caused growers with harvest and field work, they now face prolonged serious 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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