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Crop Advice & Expertise

How to spot crop disease early with latent disease testing

It’s not uncommon for wheat leaves to be infected with disease, but without any visible signs. Latent disease testing is one way of countering this problem, whilst optimising and justifying disease control programmes.

Picture this - it’s approaching mid-May, and the flag leaf is just starting to emerge on your wheat crops. You know the planting date, the variety and its likely disease susceptibilities, and the weather, particularly rainfall, both over winter and so far during spring. You’ve walked the crop and have made an assessment of any visible disease infection.

For years, that has been the information on which decisions have been made about disease control at the flag leaf spray, which usually gives the biggest return on investment for fungicides. It’s a good base to make informed decisions, but alongside a definitive understanding of the next month’s weather, it doesn’t consider if there’s any latent infection within the flag leaf.

 

How long does it take for latent leaf infection to develop?

In the case of Septoria, it starts to develop after a fungal spore lands on the leaf, and hyphae start to grow inside the leaf. They continue to grow usually for another two to three weeks before you see the classic yellowy-brown lesions and black pycnidia symptoms on the leaf surface.

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A: Spores land on either side of the leaf and enter through the stomata

B: Fungus then penetrates the leaf

C: Fungus spreads and grows inside the leaf

D: Visible symptoms appear on the leaf

No disease symptoms are visible between A-C. The latent period is from B-C, which can be up to 28 days for Septoria or as short as 4-5 days for mildew and brown rust. Fungicides are effective from A-B; beyond this point, the fungal growth will be beyond chemical control.

 

The challenges of controlling latent leaf infection

The challenging parts of controlling this disease are that:

  • Fungicides are most effective in the early stages of the disease cycle
  • Different products and rates are appropriate depending on how far you are through this infection cycle.


Without knowing definitively whether the upper leaves are already infected, it becomes more challenging to know whether the fungicide programme can rely on protectant products or requires, usually more expensive, curative products that can still control Septoria later in the infection cycle, as well as what dose will be required.

“For example, if we knew the real levels of disease inside the crop’s upper two leaves at T2, we would be able to use this information to choose the most appropriate rate of Ascra, and decide whether to use a multi-site,” explains Rosalind Martin, Bayer’s Combinable Crops Fungicide Campaign Manager.

“This would not only help make your fungicide choice more appropriate, but also ensure better value for money.” 

 

Latent disease testing with Bayer

That’s why Bayer has been working on a rapid disease detection technology that potentially helps you understand whether leaves have any latent infection lurking within them unseen.

Leaves are passed through a quantitative polymerase chain reaction (qPCR) laboratory analysis to determine the amount of a particular disease’s DNA present in the leaf.

Initially the technology, which Bayer started testing seven years ago, was used as a research tool to help understand whether treatments had been applied in a protectant or curative situation in trials, or for more detailed analysis of the differences in latent disease present in varieties with different susceptibilities to Septoria. While the technology was helpful for those purposes, the turnaround time was too slow to be of any use for real-time decision making.

In 2019, Bayer proved it was possible to speed up this process to a matter of days (including postage). They expanded their programme to optimise the test and data interpretation to develop a tool to inform fungicide decisions.

Nearly 12,000 leaves were analysed in a 2020 season from samples sent in by growers and from various trials. Including the weekend, the average turnaround time was just three days from arriving at the laboratory, with nearly 50% analysed within a single day. The aim is to turn around all samples within a day of arrival.

 

Case studies from latent disease testing triallers

James Nott, Essex

Trialled latent disease testing to see the effect of drilling date and variety on Septoria

Result: 100-times more Septoria DNA in the Kerrin sample (drilled 5 December) than the Extase sample (drilled 5 October) on the same day

“What was amazing was how little Septoria there was in the Extase. To me, it suggested variety choice was more important than I thought compared with drilling date… [Latent disease testing is] very good at telling you the scale of what you are trying to manage and extremely useful in a low disease-pressure year… You can’t do every field, but you can use it as a tool to help understand how much fungicide to use.”

Peter Hewson, Gloucestershire

Trialled latent disease testing to compare Claire drilled in late October and Skyfall drilled in May

Result: The Skyfall returned a non-detectable result for Septoria, while Claire had 0.256 pico-grams of Septoria DNA – a very low result based on previous season’s data

“Usually, our major focus is Septoria plus whatever other diseases occur, but with last year being so dry, Septoria was not visibly present in the crop and didn’t look like being an issue because of the weather… My conclusion is that the result could make you readjust programmes. It might have warranted the inclusion of some Septoria protection, even though the main disease we were chasing was yellow rust. At T2, we may have gained something had we kept something a little stronger in against Septoria.”

 

Improving latent disease testing in the future

One possible improvement to the latent disease testing process could be for leaves to be bulked together. This would provide a greater representation of leaves from the field in one test and make it more efficient to process.

It’s also important to consider what constitutes a detectable level of Septoria DNA. Based on research papers from the likes of Rothamsted Research, Bayer have set a realistic detectable and non-detectable level for the test. Just knowing whether your plant is infected or not is massive in terms of fungicide choice - it’s important to understand whether you need a low, medium or high input programme.

The other area of focus is on-farm comparisons, which will help a grower understand the differences on his own farm between varieties, fields, or drilling dates, for example. In 2020’s farmer-led grower trials, comparing different varieties was the most popular comparison chosen, followed by drilling date. You could also compare irrigated and non-irrigated and coastal versus inland locations.

Ultimately, growers shouldn’t just look at the result and base decisions on that alone. It should always be tied into the other factors you would always use when making a decision – latent disease testing is just another layer of information.

However, with increasing pressure on chemical use within agriculture from consumers and from growers looking to maintain profitability and look after the environment, continually improving the level of information available for growers to make decisions about when and how to use pesticides is critical.

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This blog post is based on an article from the spring 2021 issue of Crop Focus magazine. The magazine is packed full of insight, advice and research from the world of arable farming to help you grow the most profitable crops possible. Over 90% of readers find Crop Focus an interesting read – it’s free to subscribe so sign-up today!