Course: Fungicide management in cereals | Last Updates: 7th October 2015
Experiments designed to find the best timings to control Septoria tritici have determined the first main spray timing for disease control. Those trials found "timing 1", hence T1, should target final leaf three emergence.
Many people still regard T1 as growth stage 31-32, but there is a big difference between these two growth stages. T1 sprays should not be going on at GS31 because the target leaf three is not present, so the spray will not have the planned effect.
The emergence of final leaf three is not always easy to determine. It often coincides with growth stage 32, but how many people can accurately identify it? The following notes and images should help.
Accurately identifying growth stage 32 is key to T1 spray timing
Identifying the correct T1 timing
The best way to be sure is to dissect out the top three leaves (figure 1). This is not as difficult as many people imagine but needs practice, as the flag leaf is less than 1cm long at this stage and can easily be missed.
The next best guide is to determine the nodal growth stage (figure 2). Again, this needs practice and some care. Just feeling for nodes will not tell you what growth stage the crop is at – you must dissect out the top leaves or at least split the stem to see the positioning of the nodes.
The T1 fungicide should be applied when leaf three is almost fully emerged on the main stems. Tillers will be a little further behind in growth stage.
How can T0 sprays reduce pressure on T1?
The T0 spray timing is much less critical than the T1 timing but can give additional control of Septoria tritici, eyespot and the rusts. The spray timing normally coincides with GS30-31 and fits with other farm operations such as plant growth regulator or herbicide applications, and is usually low in cost.
The aim of the T0 is to decrease the inoculum levels present in the crop, and to increase the flexibility growers have in timing T1s.
The main targets for the T0 fungicide are leaf four and below (leaf three will not yet have emerged).
Chlorothalonil has traditionally been used at this stage as it gives excellent protectant activity against septoria and in wet weather it moves up onto the emerging leaf three, giving additional disease control. To get effects on other diseases such as rusts you need a triazole and maybe specific eyespot products if that disease has established early.
T0 sprays can't replace T1 sprays as very little fungicide is translocated (moved) to leaves that were not emerged at the time of spraying. Also, the gap between this spray and the flag leaf spray would be too long and exposing leaves two and three to almost certain disease infection. An intermediate spray applied at GS33 would be needed in these circumstances.
Why timing is important
The "spray window" for effective disease control on a particular leaf layer is relatively narrow. Spray too early and the leaf is not fully emerged, so it does not capture the spray, giving poor control. Spray too late and the disease may have already become established, again resulting in poor control. The optimum spray timing is when a leaf has just fully emerged.
This is typical for fungicides which have both protectant and eradicant activity, such as triazoles. The activity of protectant fungicides (such as chlorothalonil and mancozeb against Septoria tritici) falls off more rapidly if sprays are applied too late.
Correctly timed, the T1 timing can give a high level of disease control on leaves three and four, both of which can affect disease spread to the upper two leaves, and provide some protectant control on leaf two.
In practice, with large areas to treat, fungicide spraying usually starts before, and ends after, the optimum time. Crops at high risk from disease should be targeted for near optimal spray timing. Crops at lower disease risk have more flexibility in terms of fungicide timing.
Septoria should be the primary target for T1 sprays
Targets for T1 timing
The main target for T1 sprays is Septoria tritici, but increasingly, with broad spectrum products, other diseases are also being controlled. Both brown and yellow rust as well as eyespot can all be controlled with a single spray at this timing.
Table 1 provides a summary of the main targets and product choices at the T1 spray timing.
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