Declininglevels of Septoria sensitivity to azoles and the acute vulnerability of SDHIsto resistance development means wheat growers must take particular care inmanaging high early pressures from the disease not to mention significant threatsfrom both yellow and brown rusts this season.
With new Septoria tritici variants highlyresistant to prothioconazole and epoxiconazole discovered over the past twoseasons and known to be spreading, Agrii regional technical adviser, Will Fosshas no doubt that well-planned mixtures of fungicide active ingredients will beessential from the word go.
Alongside stacked triazoles, he sees the most effective SDHIs as having anincreasingly important part to play together with both multi-site protectants andappropriate adjuvants. He also urges growers not to forget strobilurins especiallywhere rusts pose a significant risk.
The activity of prothioconazole and epoxiconazole is declining sharply atalmost exactly the same rate, he pointed out. And we know that differenttriazoles select for different Septoria strains. So using tebuconazole andmetconazole in programmes alongside them as well as fungicides with differentmodes of action will help reduce the selection pressure on resistant strains.
Typically, underhigh disease pressures our trial work shows the best epoxiconazole + metconazoleand prothioconazole + tebuconazole combinations deliver 20-30% better controlof Septoria on Leaf 1 than full rate epoxiconazole for yield advantages of upto 0.6 t/ha.
Analysis of five seasonsof fully replicated Agrii wheat fungicide trials across 14 varieties and sixsites has further underlined the particular value of SDHIs where Septoriapressures are high.
Across the complete dataset, the average response to fungicide treatment was1.5 t/ha, rising to more than 2.3 t/ha in higher disease years. Within this,programmes involving SHDIs delivered an average 0.5 t/ha or more extra yieldthan those without them. And in higher disease years this advantage rose to 0.9t/ha.
Interestingly too, while there was no difference in the extra SDHI responsebetween relatively susceptible and resistant varieties in higher disease years, in lower disease years susceptiblevarieties showed double the response to SDHIs of relatively resistant ones.
Our detailed studiesalso show some SDHIs and SDHI combinations work markedly harder than others,observed Will Foss. The most effective programmes weve seen in our work haveconsistently involved either bixafen + prothioconazole + spiroxamine or fluxapyroxadwith epoxiconazole + metconazole.
We mustnt forgetthe extra value of the multi-site protectant, folpet either. Across our trialslitre-for-litre it has delivered an average 0.15 t/ha yield advantage overchlorothalonil. But as a partner for bixafen, in particular, its advantage wasan impressive 0.4-0.5 t/ha in 2012.
Strobilurins have a particularly valuable role to play where rusts are also aconcern, he added. For instance, weve found pyraclostrobin is the best partner for SDHI/triazolemixtures, boosting yield responses even in last years low disease season by0.15-.25 t/ha.
Weve also seen the most appropriate adjuvants from T1 boost yields by morethan 0.4 t/ha, specific weights by 1.5 points and margin over input costs bynearly 40/ha. But responses are highlyproduct specific so well-informed selection is crucial to success.
So what Septoriatreatment approach does Will Foss advocate for the greatest value this season,bearing in mind current disease pressures and resistance management needs ?
Well, the firstessential, he is adamant, is keeping on top of the disease from the outset witha robust T0. Doing so, he stresses, willminimise the danger of pulling the T1 forward to counter developing problemsand so stretching the gap between T1 and T2 which Agrii studies show can penaliseyields very substantially.
In 10 years of trials, weve seen anaverage T0 yield response of just under 0.4 t/ha, reported Will Foss. Thelowest T0 response weve recorded in a season has been 0.16 t/ha, while in thelast high disease season (2012) it averaged almost 0.8 t/ha.
In most cases, this season Id recommendBrutus (metconazole + epoxiconazole) or Capalo(metrafenone+epoxiconazole+fenpropimorph) either alone or at a reduced ratewith Phoenix (folpet) as the best start.
Id then be thinking about either Boogie (bixafen + prothioconazole + spiroxamine) or acombination of Imtrex (fluxapyroxad) plus Brutus with or without Tucana (pyraclostrobin),depending on the diseasespectrum, at T1 and T2. And Id also be including the specialist adjuvant,Kantor in most cases.
The promptest and most effective Septoria treatmentwill be vital with two thirds of this seasons wheat area in varieties with aclear susceptibility to the disease, Will Foss insisted. At the same time,good spraying technique is likely to pay dividends too.
Especially beyond T1, more haste and less speed has to be the best approach withsusceptible varieties that already have disease in the lower canopy. We sawthis in 2007 where high forward speeds,low water volumes and use of air induction nozzles with their low muzzlevelocity sometimes resulted in poor brown rust eradication.
In an ideal world Id be sticking with a flat fan nozzle delivering200l/ha at 6-10 kph to ensure sufficiently good penetration, he suggested. Wherethis isnt practical and higher work rates are needed early eradication ofactive disease from T0 will be vital. Higher rates and greater eradicantactivity/protection will also be important where spraying is more focused ongetting across the acres than achieving the best coverage. Of course, more resistant varieties and lowerdisease pressures will allow faster work rates and lower water volumes.