14 April 2000

Tune strob choice as runners line up for T1

Sprayers are lining up, ready

for the off at T1 on wheats

across the nation. As

growers place their orders

with suppliers, ADASs Bill

Clark has some 11th hour

tips on what to back.

Andrew Swallow reports

IT is neck and neck in the race to be the best strobilurin, according to a leading independent scientist.

Working out exactly which will prove to be the best bet means tuning strategy to individual circumstances, says ADASs Bill Clark.

"The choice of which strob to go for will depend on the disease control you need, local prices, and how technical you want to be – particularly with your choice and dose of triazole."

The simple answer for septoria-prone crops such as Consort or Riband is straight Landmark, (epoxiconazole + kresoxim-methyl) he says.

"It does not really matter which strobilurin, so long as you use at least a half rate. But the triazole has to be epoxiconazole to get the best Septoria control."

Current pricing makes Landmark the cheapest strobilurin plus epoxiconazole option, so not only is it the simplest, is also the most profitable option where Septoria is the main aim, he says (see table).

But on more Septoria resistant varieties, such as Equinox or Madrigal, applying a half-rate of strobilurin as Landmark can be unnecessarily costly. A straight strobilurin plus alternative triazole, or a lower rate of epoxiconazole, will be more economic, he says.

"If you have got a mildew or rust susceptible variety you could be wasting money on the Opus in Landmark," he warns. Switching to the stand alone strobilurins also allows growers to tune that part of the equation to the variety profile.

"Twist is stronger on Septoria, and Amistar better on rusts. This year Twist offers mildew control too, but with mildew resistance around how long that will last depends on how the industry uses the product."

Where any mildew is seen in the crop, Mr Clark advocates adding a specific mildewicide such as Patrol (fenpropidin) or Torch (spiroxamine). While Twist (trifloxystrobin) plus Opus, or straight Landmark may control mildew, it is no defence against resistance, he warns.

"Using straight Landmark was what got the Germans into trouble."

But Twist is weak on rust, so where rust is a concern, the strobilurin should be Amistar (azoxystrobin) or Landmark. "Yellow rust is not even on the label for Twist," he says.

"And the triazole does not need to be Opus, which eases the decision away from Landmark. It is relatively easy to control rust with low doses of triazole, a quarter to half-rate depending on the level of rust in there. Keep the strob at half-rate.

"The truly systemic nature of Amistar will help protect new leaves against rust," he adds.

No matter how clean the crop, no strobilurin should be used alone because of resistance risks. Very low level mildew resistance would quickly escalate and net blotch resistance is also a real risk. "We have to take the risk seriously."

"In practice most of the yield response comes from T1 and T2 timings anyway, except for Scotland, where strobilurin earsprays are more valuable." &#42

Strobs on Septoria


Margin over cost (£/ha)

Three doses of:

0.6 Az + 0.6 Opus 600

1.2 Tf + 0.6 Opus 610

0.6 Landmark 650

1999 ADAS results @ £65/t.

Septoria susceptible varieties.

Spring 2000 chemical prices.

OCT/NOV DRILLED EQUINOX

&#8226 At GS30-31, leaf 3 tip visible.

&#8226 Yellow rust a significant risk.

&#8226 Septoria protection secondary aim.

&#8226 Two options:

Spray now: 1/2 rate Amistar plus 1/4-1/2 rate of tebuconazole, metconazole, cyproconazole or epoxiconazole. Systemic nature of Amistar protects emerging leaf 3.

Wait for leaf 3 fully out: 1/2 rate Landmark.

&#8226 At flag: Three-quarter rate strob. Amistar plus 1/4 rate triazole cheapest? May need half-rate triazole if yellow rust present – Landmark back in equation.

&#8226 Similar strategy for: Madrigal, Reaper, Rialto.

EARLY DRILLED CLAIRE

&#8226 At GS31-32.

&#8226 Septoria pressure negligible.

&#8226 No rush unless mildew rampant.

&#8226 Spray at GS32-33, Leaf 3 fully out.

&#8226 Use half-rate Twist plus low rate triazole with mildew activity eg, cyproconazole. If mildew established, add specific mildewicide, or mix Landmark and Ensign (fenpropimorph + kresoxim-methyl).

&#8226 Possibly delay T2 to ear emergence but must monitor disease. Decision as T1 but three-quarter rate strob.

&#8226 Similar strategy for: Hereward.

OCT/NOV DRILLED CONSORT

&#8226 At GS30-31, leaf 3 tip visible.

&#8226 Less Septoria visible but inoculum still there.

&#8226 Two options:

Spray now: 1/2 rate Opus + Twist: vapour action protects emerging leaf 3.

Wait for Leaf 3 fully out: 1/2 rate Landmark, but may need extra Opus to eradicate established Septoria.

&#8226 Flag leaf: three-quarter rate strobilurin + minimum half-rate Opus. Do not delay beyond GS39.

&#8226 Similar strategy for: Savannah, Charger.

EARLY DRILLED CONSORT

&#8226 Already at GS31-32.

&#8226 Leaf 3 near full emergence.

&#8226 Spray ASAP, but with what?

&#8226 Opus at minimum half-rate: Septoria established on lower leaves so must assume all leaves infected.

&#8226 Half-rate of any strobilurin, but Landmark likely to be most economic.

&#8226 NB persistence not an issue – flag leaf out in 4-5 weeks.

&#8226 At flag leaf: Opus again + three-quarter rate strob. Twist an option for GS37 timing as vapour action should help protect emerging flag leaf.

&#8226 Similar strategy for: Riband & Savannah.

Strob decisions. Septoria levels are likely to steer strobilurin fungicide choices in many

cases.

Septoria is clearly visible on leaf 5 of this Sept-drilled Consort. The adjacent leaf 3 (top)is sure to be carrying infection after last weeks heavy rain, warns ADASs

Bill Clark.