2 March 2001

First disease control decisions on horizon

Whether cereals were sown in good time and are lush and

showing signs of disease, or went in later and are immature

but still clean, first disease control decisions are looming

THERE is more septoria showing than usual for this time of year because of the wet conditions, says Neil Paveley of ADAS. "So many growers will base their first decision on that."

There have been no significant reports of yellow rust and mildew is not a particular problem. "Weve had some severe frosts, especially in the north, which are effective at killing out yellow rust inoculum.

"But mildew needs watching because the three new varieties, Deben, Biscay and Option, are reasonably susceptible to the disease."

A pre-T1 spray will not be required on most crops this year, advises Dr Paveley. "Its used to prevent yellow rust becoming too much of a problem early on and to control mildew in susceptible varieties. So its not appropriate this season and growers can save on chemical and application costs."

T1 timing

The T1 spray, applied when leaf 3 has emerged, should be based on a triazole for both broad-spectrum eradicant and protectant disease control. The aim is to protect leaf 3 and control eyespot.

Top choices are epoxiconazole, tebuconazole, metconazole and cyproconazole, Mr Paveley believes. "Its a case of seeing what you can get for the best price."

Achieving effective control of disease is the priority when deciding what to add to the selected triazole, says Mr Paveley. "The situation has changed from last year. Resistance to the strobilurins means that this chemistry shouldnt be used for mildew control. So dont rely on kresoxim-methyl at this stage."

Choice of product for mildew is now between quinoxyfen and the morpholines.

"Use quinoxyfen if mildew is a problem early on, as it has good persistence. But where spraying gets delayed, the morpholines are a better bet because they have disease knock-down."

Where crops are at high risk from eyespot, cyprodinil should be included at the T1 stage. Chlorothalonil can be used as a cheap protectant on septoria-susceptible varieties such as Riband and Consort.

"The other thing to bear in mind at T1 is that the different crop types will reach key stages at different times," says Mr Paveley. "Backward crops are likely to have leaf 3 out by GS31, whereas in forward crops it will be out at GS32.

"So look at the leaves to make your decision on spray timing, not at the nodal growth stage."

Product dose

Growers should value the benefits of varietal resistance when deciding how much fungicide to apply, says Mr Paveley.

"Look at the disease ratings of individual varieties and study the difference between their treated and untreated yields. This shows how much support a variety needs."

For example Riband gives a potential 3t/ha response to fungicide but Hereward only half that. "So the amount of fungicide needed is different."

There may even be a case for omitting the T1 spray on backward crops of disease-resistant varieties this year, he suggests. "Judge it on disease pressure and the resistance on the variety. The appropriate dose may be zero."

T2 timing

The T2 spray, made when the flag leaf has emerged, protects the crucial yield-contributing leaves.

A strobilurin will be a key component at this timing, for its green leaf area enhancement and yield benefits. Dr Paveley believes that strobilurin use is best at T1 and T2 for growers in the midlands and south, while T2 and T3 timings are best for the north and Scotland.

"There is a prolonged ripening period in these latter areas, so the protectant activity of a strobilurin is put to good use later in the season," he says.

Again, the spray should be based on one of the better triazoles. Yield responses to T2 sprays are always economic, so typical T2 treatments are in the range of half to full rates.

Dr Paveley advises against delaying this spray until the ear has emerged, even if the upper leaves appear clean. "If control on the leaves is lost, the penalty is very severe," he warns. "So stick to the spray timing."

T3 timing

The T3 or ear spray tops up foliar disease control and protects the ear. Yield and quality responses are variable, so high doses are not justified.

"If its cold and wet at flowering, the T3 spray is economic," he advises. "But if its hot and dry, you are unlikely to get a response."

Grain discolouration from sooty moulds can persuade milling wheat growers to spray with either azoxystrobin or tebuconazole, to prevent shrivelling. "But low solar radiation can also cause shrivelled grains, so its difficult to assess the effectiveness of this action."

Azoxystrobin is a good choice where grain filling is expected to be prolonged. Other preferred products at this stage are tebuconazole and metconazole for their activity against fusarium ear blight.

Spray costs

Avoid cheap and cheerful fungicides which are less effective against disease. Instead, pay attention to varietal resistance and use it as an opportunity to save on costs of the spray programme, advises Dr Paveley.

Rate cutting, where applicable, should be done in a strict order. "The most scope is with the T3 spray, then T1 and finally T2.

"The flag leaf makes the greatest contribution to yield, so higher rates are more appropriate at T2 than at T1. But if you cut the T1 dose to the point where disease is not adequately controlled, control will be difficult later in the season." &#42

FUNGICIDEDECISIONS

&#8226 T1 Protect leaf 3 and control eyespot.

&#8226 T2 Maintain yield-building flag leaf.

&#8226 T3 Guard against ear diseases.

&#8226 Use varietal resistance to guide doses.