Of all the fungicide applications to wheat, the T2 is the most important to get right, as ADAS senior research scientist Jonathan Blake explains.
Applied at flag leaf-emerged, the T2 spray is aimed at preventing disease on the flag leaf and eradicating infections which have already occurred on the leaf below (leaf two). These leaves are crucial for yield. Understanding how wheat yield is formed helps to explain why.
Building yield
The grain-filling period, from flowering to crop senescence, usually takes 6-7 weeks and is the key yield-forming period in wheat. Photosynthesis during this period accounts for about 75% of the final yield, with water-soluble carbohydrates - formed earlier in the season - making up the remaining 25%. The importance of this period was demonstrated last year when dull conditions throughout grain filling limited wheat yields across the UK.
Clearly, until we can manipulate the weather, we can't affect the amount of sunlight that will reach our fields. But what we can do is ensure that crops are well placed to make the best use of what they do get.
To maximise yield, the crop needs to have a canopy size sufficient to capture about 95% of the sunlight, usually achieved with a green area index of 6 (GAI = the ratio of total green area - one side only - to the ground area occupied). Larger canopies tend to be less efficient. This green area needs to be healthy to maximise photosynthesis through the grain-filling period.
The top two leaves generally intercept about 65% of sunlight during grain filling. Ears account for about 20%, while leaf three and four together account for 10-15% of light intercepted. This means that in a 10t/ha crop, leaf one and two will contribute 4.5-5t/ha to yield. Ears contribute 1.5t/ha, and leaf three and four add a further 1t/ha (stem carbohydrates contribute the remaining 2.5-3t/ha).
The relative importance of T1, T2 and ear sprays should, to some extent, reflect the impact of each of these components on yield.
Septoria always requires control at T2
Timing of growth stage 39
Last year most growth stages were earlier than normal. The largest differences were at the early GS 30 and 32, which occurred 10 days or two weeks earlier. GS 39 also occurred about 5-7 days earlier.
In the spring, temperature governs the speed of plant development the unusually warm spring last season explains the more rapid early development. By GS 39, day length has an increased influence. So whereas we often see quite big differences in calendar dates of early growth stages, the emergence of the flag is more predictable, usually between 15 and 30 May, depending on latitude and variety.
Spray timing for a given field should be based on field observations. Ideally, the spray should be applied as soon as most flag leaves on main tillers have emerged. If weather delays spraying, septoria-prone varieties (rated 5 or less on the HGCA Recommended List), or crops where rust is developing, should be targeted for treatment first, as delaying flag leaf sprays is likely to be most costly on these. It does not pay to delay spraying until part of the ear is emerged in the hope of protecting the ear at the same time. Such tactics can result in loss of leaf two to disease and poor control on the flag.
Disease threats
Septoria tritici and rusts are the main targets at T2, assuming mildew has been effectively controlled at T1. Septoria always requires control at T2 irrespective of location. In the north and east yellow rust is an extra concern at this time; in southern and eastern England brown rust can be more problematic.
T2 applications should be both curative to remove any latent infection, and protectant to help maintain green leaf area and maximise the duration of grain filling.
Triazoles have been the key products in the control of septoria for many years. But recent shifts in efficacy mean that since 2004 just two, epoxiconazole and prothioconazole, have given consistently good control of septoria. HGCA fungicide performance studies in 2007 indicated that there was little difference between these two in either their curative or protectant ability on septoria. The inclusion of a strobilurin at T2 improves the control spectrum to rusts substantially, and can directly increase yields through greening effects. Yield and disease control benefits from their use often occur, even in pure septoria control situations.
Fungicide mixtures for T2
Base strategies at T2 should include either prothioconazole or epoxiconazole. Doses between half- and full-label recommended rate are usually appropriate. Higher triazole doses are needed on septoria-susceptible varieties under high disease pressure. Lower doses are appropriate on disease-resistant varieties under low disease pressure.
Strobilurins at T2 can boost yields; applications of about half label rates should be sufficient in most circumstances.
Delaying T2 flag leaf sprays can prove costly, especially on septoria and rust-prone varieties.
Boosting septoria control
Adding chlorothalonil or boscalid to a triazole generally improves septoria control. Although more widely used at T1, these products add persistence to control strategies at T2, and will be useful on susceptible varieties such as Ambrosia and Consort.
Boosting rust control
While it appears 2008 is less likely to see a repeat of the widespread brown rust epidemic of 2007, brown rust-susceptible cultivars such as Zebedee, Alchemy, Cordiale and Solstice still make up a third of the wheat area this season. Anywhere south of the Humber, strategies at T2 should look to control this.
Yellow rust is more restricted to particular varieties, with Robigus and Glasgow being most susceptible. These varieties still account for about 10% of the wheat crop, after encouraging yields last season. Extra rust control can be achieved by ensuring the triazole and strobilurin combination chosen for septoria control has good rust activity as well.
Adding spiroxamine or a morpholine will add further rust activity and provide some late mildew control. But if mildew has really got out of hand, one of the newer mildewicides (such as metrafenone or proquinazid) might be justified.