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Crop Advice & Expertise

Light leaf spot is a national problem: Findings from 600 samples over the last year

A new sampling and disease identification service has highlighted how easy it is to underestimate light leaf spot in oilseed rape crops.

Key takeaways

  • Light leaf spot is an increasing national problem
  • Disease is often undiagnosed until too late
  • SpotCheck allows earlier diagnosis
  • Prompt identification improves spray timing and efficacy – an autumn spray is worthwhile
  • Proline offers good protection against light leaf spot and Phoma

The disease, which is notoriously difficult to identify, has been recognised as the number one disease threat to oilseed rape by the DEFRA winter oilseed rape pest and disease survey.

To address this growing problem, Bayer’s SpotCheck initiative was launched last year to give growers a conclusive diagnosis of light leaf spot and other diseases’ prevalence in crops.

The project, with support from ADAS and the Association of Independent Crop Consultants (AICC), assessed more than 600 samples between October 2017 and March 2018. Growers were asked to send in 30 randomly selected leaves, which were checked by ADAS for symptoms. If none were found, the leaves were incubated for 48 hours before being reassessed.

 

A national problem

Claire Matthewman, OSR Fungicide Campaign Manager for Bayer, believes the difficulty identifying light leaf spot and its changing geography has created an extremely challenging scenario for growers.

“The first SpotCheck samples containing light leaf spot were from the east of England and Lincoln, as well as the north,” she says.

“In November, after incubation, samples were found to contain the disease as far south as Hampshire. Assessing the whole season, SpotCheck found it in every oilseed rape-growing region in the UK.

“That shows just how much of a national disease it has become, and how many growers have light leaf spot in their crops who may not have seen it before.”

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Underestimating disease

When submitting samples to the initiative, growers were asked to state whether they had seen light leaf spot in the crop they were sampling.

Results showed growers and ADAS both identified light leaf spot in 23% of all samples, but where light leaf spot was not observed by the grower, 33% of those samples were later confirmed to have the disease. This shows how easily disease presence in the crop can be underestimated.

Philip Walker, ADAS Arable Plant Pathologist, says light leaf spot is one of the most difficult diseases to identify in the field, especially for growers unfamiliar with it.

“Incubating leaves at room temperature in a sealed bag for two to three days to allow disease to express, and for the latent inoculum in the leaves to become visible, is much more reliable.”

Early in the season, growers should look for white sugar-type spores on the top or underside of the leaf and may find it useful to magnify the leaf surface with a hand lens, he says.

“Later in the season, look for lesions on the leaf, which can be confused with symptoms of powdery mildew or fertiliser scorch. This season, light leaf spot was slower to develop in the field, so we were still seeing early symptoms in February, which was when the main epidemic occurred.”

Sean Sparling, Chairman of the AICC, says it was particularly difficult to control the disease in 2017/18:

“Light leaf spot set off early this spring, and, as it was a difficult season for spraying, the disease proliferated. Prior to SpotCheck, the best diagnostic readily available was leaf incubation. During the season, I was sending in leaves to SpotCheck that were seemingly healthy, but showed not only light leaf spot present, but also surprisingly high levels of downy mildew and Phoma.

“The initiative demonstrated that we need to recognise disease levels in the crop might be worse than we think and just because we can’t see it doesn’t mean it isn’t there.”

 

Hide and seek

Mr Sparling’s observation was echoed in the SpotCheck analysis. Weekly assessments were carried out on untreated plots of a susceptible variety in Hereford, Cambridge and North Yorkshire. Plots were assessed for visible light leaf spot symptoms and then samples were incubated for 48 hours and reassessed.

“In Herefordshire, symptoms were identified after incubation as early as the first week of November, although field symptoms were not seen until 11 weeks later,” explains Mr Walker.

He says the fungus can develop while the disease is latent in the crop and, due to light leaf spot’s polycyclic nature, it can increase at an alarming rate: “It only takes a small population for an epidemic to take off in the crop.

“Light leaf spot is difficult to control and fungicides are only able to knock down the disease. If you do this as soon as disease is present within the crop it will markedly decrease the epidemic threat.”

Research in 2016/17 with the AHDB found that a November fungicide spray gave a yield response of 0.37 t/ha, and a November and March fungicide spray gave a yield response of 0.41 t/ha.

“This demonstrates the relative importance of the autumn spray, versus the spring spray, in knocking down the level of inoculum going into future cycles,” Mr Walker says.

If growers are considering spraying for light leaf spot in autumn, workload is an important factor, adds Ms Matthewman. She explains why products like Proline (prothiconozole) can be helpful.

“Most growers will be spraying in the autumn for Phomaanyway. So selecting a product with activity against Phoma and light leaf spot, such as Proline, is a good way of protecting the crop from disease, without the extra workload and fuel costs of an additional pass.”

Mr Sparling adds: “Fungicides give you three weeks’ protection against light leaf spot at best, so you need to understand their place within the life cycle of the disease and how disease and fungicide will be impacted by upcoming weather. Based on these facts, decide when is the right time to spray.”

 

Insights from our regional technical managers

 

East

Jack Hill, Bayer’s Commercial Technical Manager for Norfolk, says:

“If you see classic light leaf spot symptoms then the horse has already bolted, but knowing when to apply a protectant spray is exceptionally challenging. “SpotCheck is a brilliant tool to identify risk before physical symptoms appear – some fields in the Norfolk area had a 70% incidence in the spring, which was very valuable information for growers who then needed to act fast.”

 

North

Adam Tidswell, Bayer’s Commercial Technical Manager for Yorkshire, says:

“Generally, we saw less light leaf spot in the north in autumn 2017 than we did in 2016. This was because there were more spray days last season, so most crops had an autumn fungicide application.

“Having said that, we were still seeing SpotCheck results with up to 100% infection after incubation, which shows the importance of SpotCheck in understanding disease levels within the crop, beyond what the eye can see.”

 

West

Jon Helliwell, Bayer’s Commercial Technical Manager for Oxfordshire, Gloucestershire and Wiltshire, says:

“For me, the most interesting finding from SpotCheck was the difference in symptoms observed in the field, versus those found by ADAS after incubation. It shows the earliness and ease with which light leaf spot can manifest itself within the crop unnoticed, and how important tools like SpotCheck are in supporting in-field assessments.”

 

Looking ahead

After a successful first season, SpotCheck will continue in 2018/19.

“SpotCheck forms an important contribution to the tools available to growers to identify light leaf spot,” says Ms Matthewman. “The speed at which samples are processed means growers can diagnose and react as soon as inoculum is found in the crop.”

Mr Sparling concludes: “Two of the most costly things in this industry are complacency and assumption. If you are complacent and assume light leaf spot isn’t in your crop and let disease get a foothold, you are in serious trouble.

“It is vital we have as many tools available as possible and SpotCheck is a very useful way of assessing whether potentially yield-robbing diseases lie hidden in what might be seemingly healthy crops.”

Find out how you can participate in the SpotCheck initiative here.

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