Assess leaf 3 layer to fine-tune wheat T2 decisions
11 May 2018
Bill Clark, technical director, NIABTAG
The T2 spray decision is all about keeping disease off the top two leaves, particularly Septoria tritici. What happened earlier in the season will have a bearing on which approach to take.
Earlier sprays at T0 and T1 are about keeping septoria off leaf 3. This is because leaf 3, if infected, poses a real threat to leaf 2 and the emerging flag leaf, both of which are alongside leaf 3 as they emerge.
This year many crops missed their T0 spray. That put extra pressure on T1 sprays, and meant these applications needed ‘extra’ eradicant activity because lower leaves (4 and 5) were already showing symptoms as leaf 3 was emerging.
If T1 sprays are delayed, or people wait until most leaf 3s are emerged, there is a danger that the tips of leaf 3 (which have been emerged the longest) will have been infected for so long that septoria cannot be controlled.
This represents the ‘high eradicant’ situation. The top 2 leaves are at high risk and may need a relatively early GS39 spray to ensure eradicant activity prevents further disease development on leaf 2.
Any delay with the flag leaf spray in this situation is dangerous and needs a high dose SDHI (minimum two-thirds, probably three-quarter dose) plus azole, for example Adexar (Xemium + epoxiconazole ) or Librax (Xemium + metconazole).
The pictures illustrate the point. The crop had no T1 spray, equivalent to a poorly applied or delayed T1 in a commercial situation. Only a GS39 spray (perfectly timed) was applied, so testing the eradicant activity of the products.
Full-dose epoxiconazole alone showed very little eradicant activity and looks like the untreated crop, although there was about 30% control.
The Adexar-treated crop looks very green. The chemistry gave very good eradicant activity, especially on leaf 2, which was clearly already infected when the spray was applied. That shows the eradicant power of this SDHI.
If leaf 3 is clean (ie no visible symptoms) at GS39 and it has been relatively dry leading up to flag leaf emergence, then there are other options.
The flag leaf spray could be delayed to allow more of those leaves to emerge fully, although this is still quite dangerous if it turns wet. Or lower doses of SDHI/azole could be used, with chlorothalonil.
This is all very septoria-focused, but rust also needs to be taken into consideration, as any delays in flag leaf spray could be costly. Any rust active in the crop near to GS39 should be taken as a warning not to delay the spray.