As featured in Arable Farming Magazine
Spotlight on light leaf spot
by Arable Farming September 2020
Three years of SpotCheck data reveal how the light leaf spot threat to oilseed rape has spread across the UK.
Bayer launched the SpotCheck initiative in 2017, to help growers identify light leaf spot in oilseed rape crops.
Since then, more than 1,800 samples of leaves collected from potentially infected OSR crops have been assessed in conjunction with ADAS, and the results show what many have suspected – light leaf spot is now endemic across the UK.
ADAS plant pathologist Philip Walker explains that light leaf spot is difficult to spot in the early stages.
He says: “In its early phase, light leaf spot expresses itself as small white dots scattered across the top or underside of the leaf, where the fungus is erupting from the leaf surface.
“They can look like tiny grains of sugar, but without a hand lens or microscope they are difficult to spot.
“Additionally, they may not appear on every leaf, or on every plant, so it is very easy to miss.
“However, spotting the early signs of light leaf spot is crucial to protect yield, since fungicides provide mainly protectant activity.”
SpotCheck assessed a total of 646 samples between October 2019 and April 2020. The assessment involved visually inspecting 30 leaves collected from the crop by growers and agronomists, after the leaves had been incubated at room temperature for 48-72 hours.
In 2019-20 the number of SpotCheck samples testing positive for light leaf spot increased by 19% on the previous season, and they started to arrive in October, four weeks earlier than in 2018.
The warm, wet autumn, which favoured disease development, was the likely cause for earlier disease onset in winter 2019.
Mr Walker says that cumulative data from three years of SpotCheck corroborate what many growers have suspected.
He says: “Light leaf spot is now an endemic disease in the UK and no longer just an issue for growers in the north of England and Scotland.
“We received positive light leaf spot samples from 47 counties in 2019-20, covering most of the oilseed rape growing areas of England, Scotland and Wales.
“Just a few counties remained free of light leaf spot, however only a small number of samples were received from these areas.”
Compared to the smaller number of positive samples reported from October to December, disease incidence rapidly increased from January onwards, peaking in March.
The number and spread of counties recording positive samples also increased until the geographic distribution ranged from the Scottish Highlands to the Isle of Wight north to south, and Norfolk to Monmouthshire east to west.
While samples from multiple counties showed high disease incidence of between 81-100% of leaves infected, only around 55% of samples exhibited positive signs of light leaf spot following visual inspection in the field.
This, says Mr Walker, is a consequence of the difficulty in identifying the disease in the field.
“It is possible that growers are missing the signs of disease in the field, or there is confusion around the symptoms of light leaf spot.
“It also shows the benefit of incubating samples to enhance the expression of the disease.
“This doesn’t require a laboratory and is something growers can do themselves to monitor the progress of the disease throughout the season.”
Ella Crawford, commercial technical manager at Bayer, suggests that identifying the early signs of disease can improve decisionmaking on when to spray.
She says: “Bayer’s trials last season showed that an autumn fungicide delayed the onset of disease, but that a spring spray was vital to provide protection for as long as possible.
“Plots that were untreated in spring saw disease much earlier than treated plots.
“The chemistry available is largely protective so needs to be applied at the early signs of disease. October/ November timing is still important and while growers in the South may traditionally focus on phoma leaf spot at that time, ensuring you treat for light leaf spot too will protect the crop over the winter period.”
Craig Simpson, Bayer’s commercial technical manager for southern Scotland, says that changing weather patterns are also increasing the risk of phoma further north.
“This past winter we didn’t have many days below 4degC, which contributed to high light leaf spot incidence from February onwards, but we also saw more phoma identified in SpotCheck samples than before.”
Both Ms Crawford and Mr Simpson suggest an application of Proline (prothioconazole), which targets both light leaf spot and phoma, in early October, followed
by a second application in early to mid-November, where crop investment allows, to provide robust protection through winter.
“An early spring application is essential,” says Ms Crawford, who advises growers plan to spray around February or March, depending on disease incidence.
However, crop monitoring may indicate an earlier January application is required, especially in high risk, early-drilled crops, or if only one autumn spray was achieved.
Varietal choice has also had a clear impact on disease incidence and severity, with samples showing those varieties with a Recommended List light leaf spot rating of 6 or higher, had reduced disease incidence.
Tips for managing light leaf spot
- Utilise genetics, choosing varieties with a strong light leaf spot rating, ideally 6 or above
- Reduce exposure to spores released at harvest by drilling later, if possible. Or, avoid drilling near fields previously growing osr
- Monitor crops weekly for disease, incubating leaves to help identification, or send to SpotCheck for testing
- Apply a protectant fungicide as soon as symptoms are seen in samples or in the field. Where phoma is also a risk, prothioconazole targets both diseases
- For best protection apply a fungicide for light leaf spot twice, once in autumn and again in spring
- Source: Bayer