Wednesday, February 26, 2020

Light leaf spot is latest challenge for oilseed rape after mild winter

January 30, 2020 by  
Filed under Crops

A mild winter without any prolonged cold spell has encouraged light leaf spot to develop in oilseed rape – prompting warnings for growers.

A conducive environment for light leaf spot to develop, coupled with more and more rainfall, has furthering the likelihood of new infections – particularly latently within the crop, suggests ADAS arable plant pathologist Philip Walker.

A high number of samples identified to contain light leaf spot confirms the suspicion that there was a lot of latent disease building in the crop. Looking ahead to the coming months, Mr Walker warns farmers to be particularly careful if weather conditions continue to stay mild.

New infections

“If there is a significant drop in temperatures, we are unlikely to see new infections, and any pre-existing infections will pause in development. However, if conditions remain mild then there will likely be more and more infection events in previously symptomless fields.

“A crop carrying high levels of light leaf spot infection into stem extension will be negatively impacted at this vital yield building stage of development. However, the challenge is finding an opportunity to travel, given the amount of rain we’ve had.”

Some 62% of samples assessed in December by Bayer’s SpotCheck initiative contained light leaf spot symptoms three days after incubation. The situation is being exacerbated by waterlogged fields which makes it difficult to combat the problem.

The difficulty completing fieldwork is an observation shared by Bayer commercial technical manager Grant Reid. While some rape crops were struggling last autumn, mild conditions towards the end of 2019 allowed the crop to continue growing away, he says.

Variable crops

“There is a lot of variation in oilseed rape crops – some crops are what I’d term normal for this time of year, others are struggling. Ground conditions have not been ideal for travelling, and I suspect there are a lot of crops out there which have not received a fungicide spray yet.”

Having said that, Mr Reid says it is important to remember that oilseed rape is a resilient crop, which branches quite a lot. “If you have a crop of around 15 plants/m2, don’t write it off and be patient,” he adds.

“It is a long time until spring, so continue assessing ground conditions and utilising SpotCheck to understand disease levels in the crop, and when you can, apply a fungicide such as Proline (prothioconazole).

Given difficulties with drilling winter crops, particularly wheat, Mr Walker says it may mean that oilseed rape is one of the only paying crops this season. This makes it important to utilise all tools in the armoury to put rape in the best position as it enters stem extension, he says.

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