Why leaf three should be T1 fungicide target

5 April 2002

Why leaf three should be T1 fungicide target

By Andrew Swallow

CROP sprayers moving into apparently forward winter cereals could be going too early to get the best results from T1 fungicides, warns ADAS.

The key is to get T1 fungicide onto the newly emerging leaf three, stresses ADAS disease expert Bill Clark. Fail to do that and disease control could suffer, yield falter and costs rise.

The aim is to protect that leaf from infection, allowing greater flexibility with fungicide choice and timing at the vital T2 timing when the flag leaf emerges.

But the combination of early drilling and a mild winter means some crops could show up to three nodes before leaf three has emerged. "Going by nodal growth stages for T1 timing is not good enough, Im afraid," says Mr Clark. "You could be way out."

Only by peeling back the leaves to reveal the developing leaves in the stem can growers and agronomists be confident of identifying which leaf is emerging and hence target sprays accurately, he stresses (see panel).

Earlier this week at Boxworth, Cambs, winter wheat ranged from GS30 to GS32, in some cases within the same field. But not one plant had leaf three emerging. "You have got to dissect to check."

If backward patches of crop are yet to reach leaf three emerging then Twist (trifloxystrobin) or Acanto (pyraclostrobin) could be considered. "They will redistribute themselves in the crop and protect leaf three as it emerges," he says.

"If part of the field has gone beyond full leaf three emergence that is when products such as F500 or Opus come into their own – you need very good eradicant activity."

With septoria widespread and sufficient moisture in crops to spread it to upper leaves a top five triazole – epoxiconazole, cyproconazole, tebuconazole, fluquinconazole or metconazole – must be mixed with the strobilurin, he adds.

"Rate will depend on the eradicant activity required, which depends on the length of time leaf three has been out unprotected."

A well-timed T1 spray with leaf three at two-thirds emergence needs only 0.25-0.3 litre/ha rate of triazole. An older strobilurin such as kresoxim-methyl (as in Landmark) or azoxystrobin (as in Amistar) should be adequate.

However, where leaf three has been out for a fortnight it is time to call in the fire-brigade with a high rate of triazole, preferably epoxiconazole, and consider switching to pyraclostrobin (as in Opera) for the strobilurin.

Getting the timing right will save growers about £4/ha (£2/acre) on the triazole alone, he estimates, with the strobilurin switch and greater yield potential of tight timing likely to improve margins further. &#42


1 Select main tiller and take emerging leaf between finger and thumb.

2 Split open stem and peel back outer leaves to leave just emerging leaf and contents.

3 Open out emerging leaf and count developing leaves inside, taking care not to miss the 1-2cm flag leaf.

Three developing leaves – you are holding leaf 4, be ready to check again and go in a week or so.

Two leaves – you are holding leaf 3, fill up the sprayer.

One leaf – youve missed it. Leaf three is already out and could be riddled with septoria. Strengthen mix accordingly.

Agronomist commentary

Most wheat in East Anglia at GS 30-31, T1 expected mid-April. Mildew main visible disease, but latent septoria likely to show soon. Concern for 20% of crops that are too thick and "Wellie" height already. Dick Neale, Hutchinsons

A few very early sown wheats past GS31 in NW Norfolk, most only just GS30. Not a lot of disease, eyespot drying up.

Peter Riley, Morley Agricultural Consultants

Early-drilled wheat at GS32 in Lincs, most still GS30. Little disease present. Bridget Carroll, Aubourn

Most wheat due T1 second-half April having had "T0" already. Septoria quite high, but not exceptional. Does not believe in strobs at T1. Bill Butler, independent in Devon/Cornwall

Most wheat at GS31, some forward at GS32 but had "T0" spray. Application order more important than exact L3 emergence timing. Malcolm Harrison, AICC, Leics

Comments taken Tue, Apr 2.

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