Mildew on the offensive

11 December 1999

Mildew on the offensive

Oh no – strobs are under threat. Gilly Johnson asks how we can best prolong the life of this valuable new chemistry.

MILDEW resistant to strobilurins has arrived in the UK. Suspect strains are now confirmed in the south east, East Anglia, and the north of England.

Its not a panic situation; these traces have been picked up at low levels only, and there are no reports of practical problems with disease control so far.

But it has set alarm bells ringing throughout the industry. "Im surprised that resistant mildew is here so fast, given that strobs have only been around commercially for two years, and that mildew has been at low levels in that time," says Bob Simons of Bayer.

Problems with resistant mildew in northern Germany have been widely reported. Unlike some morpholine resistance which involves a gradual decrease in sensitivity, this mechanism involves one target site and is either switched on or off, which makes it potentially more serious. It appears resistant strains spread fast within the space of one season in Germany; could the UK follow suit?

So far, only wheat mildew is implicated; barley mildew is a separate species. Mildew is a tricky pathogen in that it completes many lifecycles during the season, so resistance can evolve more quickly.

Trying to assess likely risk of mildew in the UK this coming spring is difficult. Conditions have to be just right – warm and humid – for mildew to develop, even though early infection could look bad. Heavy rain can wash spores away, which might explain why crops escaped attack last spring.

The potential risk factors are:

&#8226 early drilling will promote lush winter crop growth

&#8226 popular new wheat Claire is susceptible

&#8226 mild winters and springs are the trend

&#8226 high levels of early mildew are already being seen.

If mildew does return with a vengeance, the scene is set for a difficult situation. In the resistant hot spots in Germany, growers are now applying strobs in conjunction with mildewicides such as quinoxyfen or Corbel. Strobs plus mildewicides have been sold as twin packs in Germany. What can UK growers do to protect their crops and preserve the efficacy of strob treatments?

There are voluntary resistance management guidelines already in place. These were devised by the Fungicide Resistance Action Committee (FRAC), a specialist body with members drawn from different agchem companies. The working group Strobilurin Type Action and Resistance (STAR) has a brief to focus specifically on prolonging the efficacy of strobs.

In essence, the recommendations put a limit of two strob sprays per cereal crop per season, keeping rates up (preferably full rate) and using mildewicides as mixtures with different modes of action. Mixing strobs is not an answer: resistant mildew is resistant to all the strobs: Amistar, Landmark types and the new trifloxystrobin product in the registration pipeline.

But these guidelines have not been universally welcomed. Some agronomists are ignoring the FRAC advice and recommending the little-and-often, repeat low-dose approach. This breaches the guidelines on two counts: low rates, and more than two strob sprays per crop.

In defence, their argument is that resistance risk is minimal with the repeat low-dose programme, because mildew protection is applied well in advance of the disease taking hold.

Their suspicion is that the full rate recommendation is there primarily to protect agchem company profit margins. This accusation is refuted by both Zeneca (manufacturers of Amistar) and BASF (Landmark). But it is recognised that in practice, low – perhaps 0.6 litre/ha – if not very low rates are the norm. Tony Grayburn at BASF thinks that the time has come to devise more workable, flexible guidelines, which take UK growers reluctance to apply full rates of any product into account.

The dose issue apart, consensus among all agchem manufacturers is that growers need more specific advice on mixture strategies and appropriate rates.

There is a wide range of products with different modes of action against mildew (see chart); these could be brought into a resistance management programme in tandem with strobs at realistic rates. Growers cant rely on commercial strob mixtures alone – they havent the clout on mildew, particularly at the lower rates favoured.

Potential mix partners include quinoxyfen (Fortress), a protectant mildewicide which offers completely different mode of action and long-lasting protectant activity, according to manufacturer Dow AgroSciences. For high risk Claire, the advice is for half-rate Fortress (0.15 litres/ha) in the T1 mix.

Then theres Bayers spiroxamine (Torch), which is a new morpholine with eradicant activity; a half-rate in high risk T1 situations would be advisable, suggests Bayer. The older morpholines such as Corbel are also effective. New triazole tetraconazole (Juggler and Emminent), cyproconazole (Alto) and cyprodinil (Unix) also have useful mildew activity.

Dr David Ellerton of distributor group ProCam is confident that resistant mildew need not present a problem. "The dose rate issue isnt as relevant, and you can argue the case either way. More important is to put in a mix partner, and theres ample choice. Dont rely on strobs, or strobs plus certain triazoles alone. Tailor the dose rate of the mix partner according to risk."

Each grouping shows products with similar mode of action

The benzimidazoles carbendazim, mbc, carbamate etc

Demthylation inhibitors Certain triazoles such as cyproconazole, epoxiconazole, flusilazole, tebuconazole, triademenol, tetraconazole, etc

Quinolines quinoxyfen

Morpholines fenpropimorph, fenpropidin, tridemorph, spiroxamine

Anilinopyrimidine cyprodinil

Strobilurin type azoxystrobin, famoxadone, kresoxim-methyl, trifloxystrobin

Source: FRAC

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