Are you reaping all the benefit of strobilurins?

6 March 1998

Are you reaping all the benefit of strobilurins?

Strobilurin fungicides will be widely available for the first

time this year. But cereal growers face a bewildering array

of advice on how best to use them.

Robert Harris finds some simple answers

NEW products they may be, but strobilurins still need more familiar fungicide partners to provide robust protection and achieve top yields.

Although they are powerful and very persistent protectants, strobilurins are poor eradicants, says David Jones, cereal pathologist at ADAS Rosemaund. Apply them when disease is present, even before it can be seen, and that infection will continue to develop, he warns.

Adding other chemistry widens the spectrum of disease control. "Strobilurins in general have broad activity. But there are quite important differences in their strengths and weaknesses."

Azoxystrobin (Amistar), from Zeneca, gives good Septoria tritici control. "But it is not as good as Opus. Septoria nodorum control is good, and it has good rust activity. But it is not very active on mildew."

Rival BASFs molecule, kresoxim-methyl, is strong on mildew, but lags slightly behind Amistar on septoria control, says Dr Jones. However, that is academic as the product is sold only in mixtures.

Three versions are available. Landmark contains kresoxim-methyl + epoxiconazole (as in Opus); Mantra contains both those and fenpropimorph (as in Corbel); Ensign is a kresoxim-methyl + fenpropimorph formulation.

Amistar users should take note, he says. "Growers would find it very difficult to get the best out of it on its own."

Improvement in visible disease control may not be noticeable. But both products delay green leaf senescence, lifting yield as a result. "With Landmark especially, we have seen some striking effects in keeping lower leaves green."

Yield benefits

Chlorophyll content was higher, especially in lower leaves which absorbed more light, he notes. The variety Pastiche has good all-round disease resistance, yet Landmark-treated plots outyielded untreated ones by 1t/ha (0.4t/acre), and Opus-treated plots by 0.7t/ha (0.28t/acre). "That is more than would be expected by disease control alone."

Strobilurin-induced yield benefits were seen on other trials last year although disease control was similar to Opus. At ADAS Boxworth, untreated plots of Brigadier suffered from severe yellow rust infection, and yielded 5.6t/ha (2.2t/acre). Opus treatment raised that to 7.7t/ha (3.1t/acre), but spraying with Landmark pushed output a further 0.5t/ha (0.2t/acre).

At Rosemaund, untreated Consort suffered badly from Septoria tritici, yielding just 3.75t/ha (1.5t/acre). Plots sprayed with Opus jumped to 8.8t/ha (3.6t/acre), but Landmark coaxed another 0.9t/ha (0.35t/acre) of grain from the crop.

"Amistar alone gave slightly better yields than Opus on Brigadier and Pastoral, but on Consort, where septoria was the main disease, it produced 0.4t/ha less. It was unable to eradicate the disease.

"In most seasons, leaves are infected as they emerge – this demonstrates the danger of using Amistar alone. But in trials on Riband, Amistar mixed with a triazole was broadly comparable to Landmark, giving growers approximately £30/ha more profit than when using triazoles alone."

To achieve extra yield, growers should not reduce rates too much, particularly on more responsive varieties like Riband, Consort and Brigadier, Dr Jones advises. "You can soon get to the point where you are losing money."

Although the trials used full rates of product, Dr Jones expects few growers will follow. "Rates will depend on price. I suspect Amistar will cost about £30-35/ha at full rate, and Landmark £38-40/ha."

Timings remain at or close to current practice. On responsive varieties, he advises starting with half to two-thirds rate of Amistar/triazole mix or Landmark at first to second node detectable (GS31-32).

Where septoria or yellow rust has established, triazole content could be increased to three-quarter rate, he advises. Where mildew has gained a hold, Mantra could replace Landmark.

"Given strobilurins persistence, it may pay to go at GS31 rather than GS32. That allows tank-mixing with growth regulators, and there is less disease to eradicate," he suggests. "Although the triazole adds flexibility, dont risk going so late that some crops are at GS33 before spraying."

The main exception to that early season advice is eyespot, he advises. "Where that is the main target, you would be better using prochloraz. If severe Septoria tritici or yellow rust is a concern, add a triazole or use Sportak Delta."

Second application

The second strobilurin application should be applied at flag leaf emerged (GS39), says Dr Jones. "The top two leaves are the most important for yield, so growers should reserve the biggest spend for this timing."

Between three-quarter and full rates of the same products should be used on responsive varieties. On more resistant types, half rates should suffice for the first timing, but use three-quarter rates for the second spray, he says.

"Although disease control remains very effective at lower rates, it is less clear what happens to yield responses. I would be reluctant to pull back further."

Strobilurins can be used as an ear treatment. "Amistar particularly keeps grain clean," says Dr Jones. "But pull rates well down – the effect seems largely cosmetic, and the economic benefits are not yet clear."

Barley growers could find Amistar especially useful for net blotch control, which current chemistry struggles to control. "It is also good on barley rusts. But it is not strong enough to cope with a serious rhynchosporium epidemic."

If mildew also threatens, growers should add morpholine. Where rhyncho is expected, Landmark or Mantra or Amistar plus triazole are favoured. "But there is less evidence of leaf greening in barley, so I am a bit more cautious about yield benefits until I get more information." &#42

Yield potential: Strobilurin fungicides offer more than pure disease control.

The latest compounds may need extra help, especially where septoria (left) is severe, says David Jones.

&#8226 Two molecules – kresoxim-methyl from BASF, azoxystrobin from Zeneca.

&#8226 Powerful protectants, but weak eradicants – both need partners for best results.

&#8226 KM mix ready formulated, AZ sold as straight.

&#8226 On wheat, use half rate strob/triazole at GS31-2, three-quarter rates if disease present.

&#8226 Apply three quarter rate at GS39.

&#8226 Reduce rates at both timings on more resistant varieties.