Why leaf layer emergence is key to accurate fungicide use

Growers will require “laser focus” to accurately monitor leaf layer emergence and hit critical fungicide timings in what has been an extremely variable cropping season so far.

What’s more, cereal varieties have a large range in the time taken for individual leaf layers to emerge which can make accurate fungicide timings a real challenge.

“Because of the poor winter conditions and variable wheat crop establishment this season, we’re likely going to be dealing with even more inconsistent leaf layer emergence this spring,” explains Dr Aoife O’Driscoll, senior crop protection specialist at independent crop research specalist Niab.

“How we manage this and hit those critical spray timings is the big question of 2024 – leaf layer inspections and dissections are some of the ways of closely monitoring crops,” she says.

Aim for two-thirds of leaf layer emerged

The optimal time to apply your T1 fungicide sprays is when two-thirds (66.67%) of the target leaf has emerged from the main tiller, continues Aoife.

Leaf layer emergence dictates the time that leaves are exposed to infection. However, this varies considerably according to varieties, seasons, soils, rainfall and temperature.

Many crops are sitting shorter than usual this season, which makes the traditional method of monitoring the height between internodes less effective at indicating what leaf layer is emerging.

Leaf dissections can be a more effective tool to accurately monitor leaf layer emergence.

Dr Aoife O’Driscoll © Niab

Highly variable emergence

Latest research from Niab reveals leaf layer emergence can be highly variable for some varieties, even within the same field.

Furthermore, some varieties with a higher level of disease resistance tend to have less variability in leaf layer emergence, which can make achieving optimal fungicide application timings easier.

For example, Extase with a septoria resistance score of 7.4 showed more uniformity in leaf layer emergence, with all leaf two’s emerging in three to four days.

Meanwhile Saki, with a lower septoria resistance score of 5.4 (no longer on the Niab Recommended List from 2023-24) was more variable, with the time taken for leaf two’s to fully emerge across the field being roughly 7-10 days.

“This raises some questions,” continues Aoife. “Is the resistance due to true genetic resistance, or is it do with disease escape as a result of being able to better hit fungicide spray timings in less variable cropping situations?”

“What we need is a laser focus on timings, get back to basics – these days, it is less about fungicide or product choice and more to do with accurate spray timings,” she says.

Disease levels and growth stages are very variable depending on sowing date, variety and site conditions. Together with disease risk, this will determine spray timings.

“The biggest drivers in how uniformly leaves emerge are variety choice and thermal time (the phyllochron). Late sowing decreases both the phyllochron and the total number of leaves emerged.

If we are to get cooler conditions in March and April, this could lead to further varied emergence of the same leaf layer within the crop.”

The trials

Data from Niab trials investigating leaf layer emergence revealed a stark difference in the time taken for two-thirds (66.7%) of the target leaf layer to emerge, with knock-on impacts for fungicide timings and efficacy.

The trials took place across four sites in 2022 and 2023, with four varieties monitored in 2023 and 120 leaves sampled for each variety.

Average percentage of leaf emergence (%)


Spray timing – leaf two

Spray timing – flag leaf emergence


4 May

11 May

15 May

19 May

25 May

























Source: Niab
* – Target leaf near 66.7% emerged
** – Target leaf fully emerged

Leaf two

With a hypothetical 4 May spray date targeting leaf two, varieties Gleam and Graham hit the average two-third leaf emerged “sweet spot”.

Saki was too early with just over 50% leaf emerged, while Theodore had gone too far with an average of 90% emergence across the leaves assessed.

By 11 May, 100% of leaf two had emerged on Gleam, Graham and Theodore, while Saki lagged behind, with all varieties surpassing the optimal two-thirds of the leaf emerged timing, potentially reducing fungicide efficacy.

© Niab

Flag leaf

When investigating optimum spray timing for the flag leaf, Graham was the only variety to be at the “sweet spot” on 15 May.

Theodore had already surpassed this with 85% of the leaf emerged, while Saki and Gleam were too early.

By 19 May, Gleam had surpassed the two-thirds optimum and by 25 May, all four varieties surpassed the optimum with Gleam, Graham and Theodore out in full flag leaf emergence.

Extreme cases may need separate spray passes

Variable wheat emergence last autumn, coupled with continuous rain showers could make early fungicide applications difficult to time, explains BASF’s business development manager Jared Bonner.

“In extreme cases there could be two crops or more in one field which may require separate spray passes.

“This will of course mean extra workload and cost, but in order to get the most from the crop and hit fungicide timings, it could well be worth the extra spend,” he says.

Leaf layer emergence is highly variable and its rate is governed mainly by temperature and variety. As the weather changes and fluctuates, it will be harder to achieve spray timings, continues Jared.

“Last year, we saw a lot of T1.5 sprays as growers could not get on the ground and this year may be similar.

“T1.5 sprays have more of a curative effect as crops play catch up, which will put everything under pressure.

“The most effective fungicides will require both protectant and curative properties,” concludes Jared.

Take home messages

  • Disease levels and growth stages are very variable depending on sowing date, variety and site conditions
  • Hitting fungicide timings will be even more challenging this season due to variable crop establishment
  • Leaf inspections and dissections will be an effective tool to monitor crops and identify what leaves are emerging
  • Growers need a laser focus on crop development and accurate spray timings
  • There could be two or more crops in one field which require separate spray passes.

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