Wheat growers should resist the urge to cut back dramatically on their key flag leaf fungicides later this month despite dry weather which has led to generally low disease levels.
Many growers have already or are looking to cut costs on fungicide timings if they have late drilled disease-resistant varieties, but the T2 flag leaf spray is too critical to force big changes.
Jonathan Blake, fungicide expert at crop consultant Adas, says some growers may look to cut rates, but the three-pronged fungicide strategy of SDHI-azole-multisite is needed for most crops at the T2 stage.
“When the full canopy is out then this is the optimum time for disease control, so it is too risky to cut back dramatically and then see three weeks of rain,” he tells Farmers Weekly.
Wet spring in the west
Wheat’s most yield-damaging and wet weather-loving disease, septoria, is being seen at low levels in the very dry east of England, but at typical seasonal levels after a wet spring in the west.
Disease levels, of course, depend on drilling date, varietal resistance and geography, so fungicide rates could be trimmed in the drier east but are likely to be kept higher in the wetter west.
“Growers are still likely to need a half to three-quarter rate of an SDHI-azole product at T2 as we consistently get a good yield response at that stage,” adds Mr Blake, who is based at Adas’s Rosemaund site in Herefordshire.
The T2 timing is when the flag leaf, or last leaf to appear, is fully emerged and generally occurs in the last week of May in many areas of Britain.
Mr Blake points out that a dry spring in 2012 was followed by a wet summer, so cutting back on T2 fungicides too severely could be a risky strategy for 2019, as severe septoria can cut yields by up to 5t/ha or about half the expected final harvest yield.
Many growers in the drier east have trimmed back on earlier T0 and T1 sprays due to the low disease levels, but growers should focus on the threat of “losing control” of disease as the efficacy of both SDHIs and azoles against septoria is declining.
Mr Blake’s colleague Phil Bounds points out the importance of variety, as Extase, with the best resistance to septoria available, may need only a half rate of SDHI/azole compared with the disease-susceptible Santiago, which could well need nearly a full rate.
With Extase, he would advise cutting rates rather than omitting one of the three components of the T2 spray, and even suggests add a second multisite into the mix to give better protective action for all varieties.
However, drilling date and climate as well as variety will play a big part in deciding the right cocktail of the T2 fungicide spray.
“Late-drilled Extase in East Anglia will be a lot different from a September drilling in Herefordshire,” he says.
Examining an untreated fungicide trial site in north Herefordshire, Mr Bounds found septoria clearly well established on leaf 5 of susceptible Santiago while leaf 5 of Extase was clean of the disease, although it was present on leaf 6.
Herefordshire, with its annual rainfall of 600-750mm, way above East Anglia, has seen the increased popularity of good septoria resistance varieties such as Graham, Siskin, Sundance and Dunston in recent seasons to help fight the disease.
While advising caution over cutting rates, experts also advise against “chasing” fungicide resistant septoria by increasing rates of SDHIs and azoles above current farm practice.
Analysis by Adas of AHDB fungicide performance data shows that although septoria is becoming harder to control with SDHIs and azoles, growers should not be tempted to increase rates because this would not be economical and would encourage resistance.
Multisite fungicides – such as chlorothalonil, folpet and mancozeb – are not affected by resistance, but the first of these, which is the most commonly used, is set to be banned by the European Union after failing toxicity tests.
Chlorothalonil will be available for this season and a use-up period is likely to cover most of spring 2020, but experts are now looking at the use of alternative protective multisites.
In addition, Adas’ work has shown the advantage of using more than one multisite as all three mentioned work in different ways.
The loss of chlorothalonil will be partly compensated by the likely launch in 2020 of a two new fungicides – one with a different mode of action to the two systemics currently used, SDHIs and azoles, and also a new azole with very good activity against septoria.
The new mode of action product Inatreq (fenpicoxamid) from Corteva should be a welcome addition, as should the new azole Revysol from BASF, which is claimed to give septoria control as good as other azoles when they were first introduced.
Main SDHI/azole products which have been covered by the AHDB fungicide performance data
Ascra – SDHIs bixafen and fluopyram + azole prothioconazole
Elatus Era – SDHI Solatenol (benzovindiflupyr) + azole prothioconazole
Librax – SDHI fluxapyroxad + azole metconazole
Disease rating for selected winter wheat varieties from the current AHDB Recommended List
|Variety||Septoria||Yellow rust||Brown rust|
Disease resistance scores are in a 1-9 scale where 9 is good resistance and 1 is very susceptible.
*Santiago has been removed from the current AHDB Recommended List
Average annual rainfall levels
- Herefordshire – 650-800mm
- Essex – about 500mm
The driest area in Britain is on the Essex coast where rainfall levels can dip below 500mm.
“Septoria is becoming harder to control with azoles and SDHIs. However, farmers should not be tempted to increase rates, because this is not economic and would drive resistance faster,” says Paul Gosling, who manages fungicide performance research at the AHDB.