3 March 2000

Cut strobilurin rates at your

own risk – scientist

GROWERS who slash strobilurin rates to save money are in danger of cutting their noses to spite their faces, says a leading independent scientist.

"It is a false economy," says ADASs Bill Clark. "That was true ten years ago with triazoles and it is even more true today with strobilurins. Fungicides are the last input to cut when times are hard."

Lower yield responses to strobilurins last season may add to the temptation to cut rates, or even leave them out. But for nearly all crops that would be a mistake, he maintains.

"The average response was still about 0.65t/ha over triazoles, and that was entirely a seasonal effect. The three year average is about 1t/ha."

Only where yield will be limited for some other reason, such as severe drought or take-all, should strobilurins be left out.

Total strobilurin dose on wheat should be 1.25-1.5 units, with the bias towards flag leaf, he continues. But the extra yield potential that brings is wasted if disease control with partner products is neglected.

"You have to take disease control seriously. If you get the T1 wrong it basically wrecks the platform for later applications."

A top-five triazole (see box) must be added to the strobilurin at a rate based on varietal disease resistance. Septoria susceptible varieties such as Consort merit at least a half-rate at T1, but on resistant types such as Claire or Hereward a quarter-rate is adequate.

"Some triazole is still needed as you can never be absolutely sure that there is no latent septoria there."

Older triazoles are another false economy. "Even a full-rate will not give the eradicant activity that a quarter rate of the new ones does."

Suggested strobilurin rate at T1 is a half dose, with more and preferably kresoxim-methyl on thinner crops. "Landmark has a bigger greening effect." In the north saving strobilurins for T2 and T3 makes sense, he adds.

Flag leaf applications should contain the bulk of the strobilurin dose, and a triazole dose again based on varietal resistance. Delaying flag leaf applications because no disease is visible on the upper leaves is a mistake.

"You will never see septoria on the top two leaves at flag leaf emergence – leaf 2 has not been out for long enough. You can delay if leaf 3 and 4 are still clean – then there is less risk. But heavy rain can still splash infection from the crop base."

Earwashes should top up the strobilurin rate if the target dose has not already been applied.

Cynics may argue that such "maintain rates" messages are manufacturer or distributor hype. But Mr Clark refutes those arguments.

"I have no axe to grind. The dose response curves tell the story and that is HGCA work."

[Panel]

GREEN STRAW GRIPES

Strobilurins may delay harvest, but that is where the extra yield is coming from, says Mr Clark. "People who complain that the straw is staying green longer have to realise that that is what is giving them the extra yield. During grain fill wheat adds about 0.2t/ha/day. The longer that goes on, the higher the yield."

[Box]

RATE ADVICE

* 1.25-1.5 units of strob total.

* Drought prone wheat exception.

* Triazole rate tuned to variety.

* Use one of: epoxiconazole, tebuconazole, fluquinconazole, metconazole, cyproconazole.

[Graphic]

Dose response curves for strobilurins

Green straw gripes

Strobilurins may delay harvest, but that is where the extra yield is coming from, says Mr Clark. "People who complain that the straw is staying green longer have to realise that that is what is giving them the extra yield. During grain fill wheat adds about 0.2t/ha/day. The longer that goes on, the higher the yield."

RATEADVICE

&#8226 1.25-1.5 units of strob total.

&#8226 Drought prone wheat exception.

&#8226 Triazole rate tuned to variety.

&#8226 Use one of: epoxiconazole, tebuconazole, fluquinconazole, metconazole, cyproconazole.