16 April 1999
Flak for FRAC over strob dose theory

By Andrew Blake

FUNGICIDE makers promoting what they consider responsible use of strobilurins face a rough road.

The stumbling block is their argument over maintaining doses to reduce the risk of resistance.

There is broad agreement that it is advisable to restrict the number of strobilurin treatments to any one crop and use effective partner products to offset resistance risk. But the advice from the companies Fungicide Resistance Action Committee that low doses should be avoided is harder to accept, say critics.

“I think FRAC has got it wrong,” says Peter Taylor, chairman of the Association of Independent Crop Consultants. “Where is the evidence that higher doses give less resistance than lower doses? I believe the number of doses used is far more important.”

FRAC guidelines are sensible in advocating the use of mixtures wherever cereal disease is already present, says IACR-Long Ashton pathologist Derek Hollomon.

“It makes sense to apply strobilurins no more than twice to a crop. Anything that reduces use reduces selection pressure on diseases. But in terms of dose rate I am less convinced. Using a strong mixture partner is much more important as far as an anti-resistance strategy is concerned.”

With so-called disruptive resistance, as found with strobilurins, high fungicide doses soon select out resistant strains, says Chris Longhurst, global research and development director for Dow Agrosciences. “So at first glance, using high rates could be a bad move.”

“Maintaining the dose at recommended rates will help to stop resistance from developing,” argues James Brown, geneticist at John Innes Centre. “Killing as many susceptible spores as possible means that you are also killing those that are potentially resistant.”

Dick Neale for agchem distributor Hutchinson says it important to distinguish between full strobilurin label dose and that recommended for target diseases in the guidelines. “The recommended rate is sufficient to control all spores and prevent resistance build up. The use of multiple low doses is irresponsible.”

Nottinghamshire farmer Robert Sutton says there is an overall assumption that resistance is caused by repeated low-dose applications. “In all probability nothing could be further from the truth.

” Nick Lawrence for BASF acknowledges there is no direct field evidence for the FRAC guidelines. “Its very difficult to prove a negative.”

The basis for what is clearly a precautionary principle is that repeated low-dose applications were widely used in Germany, where mildew resistance to strobilurins was first found, he explains.