Two weeks ago, I wondered if it might be possible to consider a T1 fungicide application on winter wheat without an SDHI fungicide in the mix. Temperatures were very low, growth was very slow, but development was ongoing and there had been very little rain. Or at least there were very few periods of prolonged leaf wetness because the showers were generally accompanied by drying winds.

But two weeks ago was too early for the T1 because the third-last leaf was only peeping in many crops. It seems that this year is late for the T1, but in many recent years, we sprayed before the third-last leaf was fully emerged. This is normally early May in north Leinster and seven to 10 days earlier in the south.

It is important to remember that having a crop sown very early or very late may not change this timing much, as such crops can often produce one extra or one less leaf, leaving development close to normal because it is partially triggered by day length.

So we are not far off the mark for the correct T1 timing. If you work a T1 date of 5 May into a programme, this gives the T2 around 26 May and the T3 around 16 June.

So there is no major challenge in trying to get the intervals to fit and if the forecast is bad, these can be tightened.

The same will apply for crops further south, but the dates will be a week or so earlier, as harvest will be earlier also.

The challenge in wheat is to keep the upper three leaves as clean as possible. We know from the past that crops can still do 10t/ha with quite a bit of septoria present. Disease control is more about preventing the development of an epidemic, rather than keeping every single leaf clean.

In wheat, the duration of grain fill is very significant, as the level of grain fill can contribute to higher yield. If you are short of grain sites, the fewer grains can still be filled better to deliver yield.

Getting prolonged grain fill is not so much about the T3 during heading or flowering as it is about the T2 post-flag leaf emergence.

Septoria is always building in a crop and its natural progression is influenced by weather and wet in particular. But septoria will still take hold of a crop even in a dry year, as it still manages to move about the canopy in dew.

Of all the fungicide treatments, the T1 tends to have the most variable timing. This is mainly due to growth and development, but it is also to do with weather and opportunity to spray. In the past, many have targeted the T1 when the second node can be seen, but this is not a true indicator of when third-last leaf has emerged.

Correct timing

The only way to know the correct timing is to physically open up the unfolded leaves on some of the bigger stems in the crop. In doing this, there are a few things you need to be aware of.

Opening the leaves also means opening and unrolling the leaf sheaths. This is a relatively delicate process, particularly when you get to the younger leaves that are not yet strong. A sharp blade is essential to help with this.

A second important point is that the leaf sheath ends at the node. Indeed, the node is technically the junction box where all of the plumbing in the leaf is connected with the plumbing in the stem to move nutrients up and down the plant.

Dissecting the leaves

1 Select a sample of stronger stems to dissect. With each stem, begin by pulling off the other tillers and cut off the root mass at the base of the plant. Also, remove the leaf sheaths of the fully emerged leaves. Work on a clean surface where you can leave things down in order.

2 Take a look at the plant before you begin dissecting. With your blade, open the fold of the outer leaf sheath. Every second leaf sheath is rolled in the opposite direction. You need to get the sheath unfolded at its base where it joins the node. Leave this last unfolded leaf safely to one side – this is likely to be the fourth-last leaf. Then begin the process with the next leaf.

3 This leaf is likely to be only partially emerged, so the leaf and sheath will be quite soft and easy to damage. Repeat the process of unfolding the leaf sheath down to the base to fully disconnect this leaf. Leave this beside the previous leaf.

4 If the previous leaf was the third-last leaf, then there are two more to go. These will be much smaller and more delicate, so even more care is needed. If you try to force them off, you can end up with one leaf within the other. So be gentle with this one, bearing in mind that if there is one more, it may be little more than 1cm to 2cm in length.

5 If the third-last leaf was only emerging on your plant, the flag leaf will still be small. However, if the third-last leaf was fully emerged, you would not get a third leaf inside.

If you had all of the leaves laid out as you progressed, you can now see the exact leaf stage of that plant.

All similarly sized stems are likely to be at the same stage, so you should not need to repeat this process. Crops and varieties may differ. Realistically, you need the third-last leaf fully unfolded rather than just emerged to catch the fungicide.

Given the amount of recent rain it seems inevitable that there is latent septoria in new leaves at this point. This means that T1 fungicides will need to include a high dose of triazole, a contact and probably an SDHI.