27 March 1999

EARLY WARNING SIGNS

Heads are expected to roll in any revolution. David Millar reports on moves to prevent the strobilurins going to the guillotine ahead of their time.

HARDLY has the bandwagon for the new generation strobilurin chemistry picked up speed than resistance now threatens to thrust itself into the wheel spokes.

All could still be well for these revolutionary fungicides – in which every major agrochemical manufacturer is furiously investing – provided growers and their advisers listen to the recommendations of the Fungicide Resistance Action Committee (FRAC).

Thats the official story – one which gamely suggests sticking to recommended dose rates, a maximum number of sprays, and bringing additional products into mixtures designed to keep resistance at bay. All this at a time when advisers are under pressure from their customers to seek out cost-effective disease control.

This choice between short-term cost savings and medium- to long-term safeguarding against resistance is a dilemma recognised by FRACs strobilurin working group, made up of the manufacturers Zeneca, BASF and Novartis. It is made particularly difficult by no evidence whatsoever of any resistance or insensitivity to fungal diseases showing up in the UKs main cereal areas.

Why then is there sufficient concern for the working group to have come out with firm recommendations asking, among other things, for no more than two sprays of a strobilurin, or a related fungicide, per crop per season? The answer lies across the North Sea in Schleswig-Holstein where the activities of German growers are accused of causing a major collapse in the strobilurin control of wheat powdery mildew.

Three years usage of repeat low-dose sprays of BASFs kresoxim-methyl + epoxiconazole product (Landmark) are blamed. Steve Heaney, of Zeneca, who chairs the FRAC strobilurin working group, says it is accepted by all strobilurin manufacturers that the resistance applies to all strobs and not just kresoxim-methyl.

Worse than that, it also extends to several upcoming non-strobilurins with a similar mode of action. The first members of this new cross-resistance grouping include famoxadone from DuPont and fenamidone from Rhône-Poulenc. Novartiss planned strobilurin, trifloxystrobin, is likewise affected.

"Resistant strains of powdery mildew are not controlled by field rates of strobs or related compounds," says Mr Heaney, whose company undertook a 1,100km mobile catch of airborne spores in Germany last year.

This uncovered patches of high level resistance across a wide swathe of Germanys main wheat growing areas. Resistance was also found in Denmark but similar spore checking in France, Belgium and East Anglia found no evidence there of any loss of strobilurin activity. Nor was there any sign in any country of any fall-off in strobilurin activity against other fungal diseases such as Septoria tritici, brown rust or net blotch.

There are several main reasons for the failure to control powdery mildew in Germany, according to Mr Heaney. Strobilurins have been in use in northern Germany since 1996 and from the start growers were using three sprays per year. Rates of under half the recommended dose are commonplace in recognised high risk mildew areas growing wheat varieties known to be susceptible to the disease. On top of these, growers have been relying on combinations of a strobilurin with a partner product ineffective against powdery mildew.

This scenario is what FRAC hopes to avoid with its recommended strategy for using strobilurins and related compounds on cereals in the UK. It argues the strategy is an effective one not just for dealing with a potential difficulty with powdery mildew but for safeguarding strobilurin effectiveness against all diseases. The strategy is based on full rates, appropriate limitation of use, use for prevention of disease, and use of strobilurins in mixtures.

&#8226 FULL RATE – This means sticking to the manufacturers recommended rate at a particular growth stage to control a particular target or spectrum of targets. Lowering doses may increase the risk of losing control of large populations.

&#8226 TWO SPRAY MAXIMUM – Only two applications of a strobilurin and related compound (STAR) should be applied to each cereal crop.

&#8226 HIGH RISK AREAS – Where strong powdery mildew attacks are common, STARs should be applied in mixture with an effective dose of a partner fungicide from a different cross-resistance group.

&#8226 LOWER RISK AREAS – If strong powdery mildew attacks are not common, STARs should always be applied, whether on their own or in mixtures, before powdery mildew is established and at the recommended rates to maximise disease control.

&#8226 ESTABLISHED MILDEW – Where powdery mildew, or any other cereal disease, is already established at the time of application, STARs should always be mixed with an effective dose of an effective curative partner.

THE THREE-STEP APPROACH

ZENECA has developed three anti-resistance programmes for winter wheats and one for winter barley using its strobilurin Amistar:

1.Feed wheat

0.6 litre/ha of Amistar plus reduced rates of an eradicant fungicide such as Opus, Flamenco, Alto or Folicur at GS31-32. At flag leaf emergence (GS39), the company recommends 0.8-1.0 litre/ha Amistar alone or, if necessary, with an eradicant, while for an ear spray (GS55-59) a reduced rate triazole with an appropriate rate of MBC is proposed, depending on weather conditions.

2. Milling wheat varieties, such as Rialto, Hereward, Malacca and possibly biscuit-making types

Zeneca suggests chlorothalonil (Bravo 500) with a triazole for the T1 spray, bringing 1.0 litre/ha Amistar in at flag leaf either on its own or with an eradicant, finishing off with a 0.3-0.5 litre/ha of Amistar as an ear spray either alone in dry conditions or with 0.4 litre/ha Folicur if it is wet.

3. Combination programme for feed or milling types

The company also proposes keeping Amistar for the later sprays, but opening with a mixture of chlorothalonil with a specific mildew protectant, such as quinoxyfen (Fortress), at GS26-30.

On winter barley, a triazole or Unix (cyprodinil) gets the vote as an Amistar partner both at stem extension (GS31) or if needed at flag leaf emergence (GS39-49).