CHINK IN STROB
CHINK IN STROB
ARMOURY
Mildew strains resistant to the new generation of strobilurin fungicides have been confirmed in Germany. David Millar finds out if UK growers should be concerned.
"THEY would say that, wouldnt they." Not Mandy Rice-Davies this time but many of the growers being asked by spray manufacturers to apply only full rates of the new strobilurin fungicides.
And, their reaction to pleas to take precautions against disease resistance could be just as disdainful as that of the naïve and headstrong Miss Rice-Davies.
However, the existing strobilurin manufacturers, Zeneca and BASF, together with three companies with strobs in the pipeline, have already formed a working group to look at strobilurin resistance and plan guidelines aimed at safeguarding their use if growers pay heed.
There is now no dispute that wheat mildew resistant to the strobilurins and upcoming chemistry with similar mode of action has been identified in northern Germany. Not just in a small area of Schleswig-Holstein but over a substantial chunk of Germanys main wheat growing regions. The problem was first noted in crops treated with the German equivalent of Landmark (kresoxim-methyl + epoxiconazole) in repeat low doses.
It is accepted that there is cross-resistance to all other strobilurins as well as upcoming chemistry such as famoxadone from DuPont. Monitoring carried out by Zeneca, the makers of azoxystrobin (Amistar), found wide variation but still up to 45% resistant spores in air samples taken over a 1,000km route in northern Germany.
Breakdown
Steve Heaney, a fungicide resistance specialist with Zeneca, points out that similar surveys in East Anglia, Ireland and France last summer found no resistant spores of wheat powdery mildew, barley powdery mildew, brown rust or net blotch. The German monitoring also showed no resistance to diseases other than wheat mildew.
While Mr Heaney admits the breakdown of strobilurin control of wheat mildew is unexpectedly fast, it was likely that the versatile wheat mildew fungus would be the first to adapt. The resistant strains would always have been present but their frequency was stepped up dramatically in Germany by the use of mildew-susceptible varieties and the use of low dose fungicide applications.
"We believe the main factor in driving this process so quickly has been the use of lower than recommended doses of strobilurins," says Mr Heaney. "I am surprised it happened so quickly but my understanding is that strobs have been used intensively in this area of Germany since 1996."
The resistance found in Germany is a black-and-white resistance giving virtually zero mildew control with full-rate strobilurin. However, the mode of action of the strobilurins and a few, new non-strobilurins is completely different to other fungicides so there is no cross-resistance with morpholines, quinoxyfen, DMI or other materials.
Mr Heaney points out that resistance management is about keeping the resistant spores at a frequency that doesnt damage overall control of the fungus population. Zeneca and the other manufacturers are continuing to monitor the disease populations but cant predict how soon a similar resistance might develop in parts of the UK.
"You have to keep the whole thing in perspective. The only place and the only pathogen in which we have detected resistance is in a region of northern Germany and in wheat powdery mildew.
Holding off the widespread onslaught of mildew resistance will depend on growers and their advisers tackling resistance management. The strobilurin working group will issue firm guidelines next month but the members are clear that full dose rates will form of a strategy to stave off resistance development.
Effectiveness
The temptation for growers is to spray curatively and cut dose rate to improve gross margin. Early use of a full rate strobilurin, with the addition of a specific mildewicide in high risk situations or where mildew is already establishing, will introduce differing modes of action and greatly increase the effectiveness of disease control.
Tony Grayburn, of BASF, which is also monitoring UK sites this year for resistance development, says strobilurins should be used as recommended at the T1 (GS30-31) timing. Growers who expect difficulty with mildew should use whatever products they normally apply or, if mildew develops, a morpholine to take it out.
BASF in the UK is not issuing specific advice for growers but Mr Grayburn accepts the best fit with kresoxim-methyl for protective control from the early timing would be quinoxyfen (Fortress) although there is also a case for an early morpholine. In Germany BASF is supporting the use of a tank mix with quinoxyfen at the T1 stage.
John Sellars of Dow AgroSciences, agrees quinoxyfen, which has no curative action, should be used at the early GS30-31 optimum timing although it is cleared through to GS49. "Growers who are always chasing mildew with morpholine chemistry can make a single application of quinoxyfen and the problem is sorted," he says.
Recommended label rate for quinoxyfen is 0.3 litre/ha but growers last year were bringing its cost down to £10/ha by spraying at a third of label rate. All the spray manufacturers agree that it is tempting to cut the rates of partner products but growers should remember it is because of low-rate use of strobilurins that they are being asked to adopt resistance strategies.
Appropriate dose use is advocated by David Clark, of Bayer: "Although mildew hasnt been a major disease over the last two years, experience with azoxystrobin shows adding a good mildew triazole such as tebuconazole makes a significant difference," he says.