March 18, 2014
Resistance Concerns Raise Septoria Control Bar
Declining levels of Septoria sensitivity to azoles and the acute vulnerability of SDHIs to resistance development means wheat growers must take particular care in managing high early pressures from the disease – not to mention significant threats from both yellow and brown rusts – this season.
With new Septoria tritici variants highly resistant to prothioconazole and epoxiconazole discovered over the past two seasons and known to be spreading, Agrii regional technical adviser, Will Foss has no doubt that well-planned mixtures of fungicide active ingredients will be essential from the word go.
Alongside stacked triazoles, he sees the most effective SDHIs as having an increasingly important part to play together with both multi-site protectants and appropriate adjuvants. He also urges growers not to forget strobilurins – especially where rusts pose a significant risk.
“The activity of prothioconazole and epoxiconazole is declining sharply at almost exactly the same rate,” he pointed out. “And we know that different triazoles select for different Septoria strains. So using tebuconazole and metconazole in programmes alongside them as well as fungicides with different modes of action will help reduce the selection pressure on resistant strains.
“Typically, under high disease pressures our trial work shows the best epoxiconazole + metconazole and prothioconazole + tebuconazole combinations deliver 20-30% better control of Septoria on Leaf 1 than full rate epoxiconazole for yield advantages of up to
Analysis of five seasons of fully replicated Agrii wheat fungicide trials across 14 varieties and six sites has further underlined the particular value of SDHIs where Septoria pressures are high.
Across the complete dataset, the average response to fungicide treatment was 1.5 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 yield than those without them. And in higher disease years this advantage rose to 0.9 t/ha.
Interestingly too, while there was no difference in the extra SDHI response between relatively susceptible and resistant varieties in higher disease years, in lower disease years susceptible varieties showed double the response to SDHIs of relatively resistant ones.
“Our detailed studies also show some SDHIs and SDHI combinations work markedly harder than others,” observed Will Foss. “The most effective programmes we’ve seen in our work have consistently involved either bixafen + prothioconazole + spiroxamine or fluxapyroxad with epoxiconazole + metconazole.
“We mustn’t forget the extra value of the multi-site protectant, folpet either. Across our trials litre-for-litre it has delivered an average 0.15 t/ha yield advantage over chlorothalonil. But as a partner for bixafen, in particular, its advantage was an impressive 0.4-0.5 t/ha in 2012.
“Strobilurins have a particularly valuable role to play where rusts are also a concern,” he added. “For instance, we’ve found pyraclostrobin is the best partner for SDHI/triazole mixtures, boosting yield responses even in last year’s low disease season by 0.15-.25 t/ha.
“We’ve also seen the most appropriate adjuvants from T1 boost yields by more than 0.4 t/ha, specific weights by 1.5 points and margin over input costs by nearly £40/ha. But responses are highly product specific so well-informed selection is crucial to success.”
So what Septoria treatment approach does Will Foss advocate for the greatest value this season, bearing in mind current disease pressures and resistance management needs?
Well, the first essential, he is adamant, is keeping on top of the disease from the outset with a robust T0. Doing so, he stresses, will minimise the danger of pulling the T1 forward to counter developing problems and so stretching the gap between T1 and T2 which Agrii studies show can penalise yields very substantially.
“In 10 years of trials, we’ve seen an average T0 yield response of just under 0.4 t/ha,” reported Will Foss. “The lowest T0 response we’ve recorded in a season has been 0.16 t/ha, while in the last high disease season (2012) it averaged almost 0.8 t/ha.
“In most cases, this season I’d recommend Brutus (metconazole + epoxiconazole) or Capalo (metrafenone+epoxiconazole+fenpropimorph) either alone or at a reduced rate with Phoenix (folpet) as the best start.
“I’d then be thinking about either Boogie (bixafen + prothioconazole + spiroxamine) or a combination of Imtrex (fluxapyroxad) plus Brutus with or without Tucana (pyraclostrobin), depending on the disease spectrum, at T1 and T2. And I’d also be including the specialist adjuvant, Kantor in most cases.
“The promptest and most effective Septoria treatment will be vital with two thirds of this season’s wheat area in varieties with a clear 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 with susceptible varieties that already have disease in the lower canopy. We saw this in 2007 where high forward speeds, low water volumes and use of air induction nozzles with their low ‘muzzle velocity’ sometimes resulted in poor brown rust eradication.
“In an ideal world I’d be sticking with a flat fan nozzle delivering 200l/ha at 6-10 kph to ensure sufficiently good penetration,” he suggested. “Where this isn’t practical and higher work rates are needed early eradication of active disease from T0 will be vital. Higher rates and greater eradicant activity/protection will also be important where spraying is more focused on getting across the acres than achieving the best coverage. Of course, more resistant varieties and lower disease pressures will allow faster work rates and lower water volumes.”