The success of fungicide resistance management strategies was debated at an Association of Applied Biologists conference last week. Mike Abram heard the arguments

Genetically modified crops with higher levels of in-built disease resistance might be the only way fungicide resistance can be overcome, delegates at the Association of Applied Biologists conference on fungicide resistance heard last week.

The conference concluded with little doubt that current resistance strategies needed improving – virtually all current fungicide chemistry had some problems with resistance.

But there was little consensus about how, or indeed whether, they could be improved.

Combating fungicide resistance was a battle, ADAS plant pathologist Bill Clark acknowledged. “I started to count the number of occurrences, but gave up – it is enormous, so it is clear pathogens are well developed to overcoming chemicals.”

Current strategies to prevent breakdowns rely on four basic premises:

  • Limiting exposure, primarily by reducing the number of applications in a season;
  • Avoiding eradicant use;
  • Mixing or alternating with different modes of action
  • Manipulating doses, generally described as avoiding the use of multiple low doses and promoting using high doses.

But there is limited evidence that all of these work, Mr Clark said. “The one that clearly works is limiting exposure. Any exposure of the pathogen to a product will inolve a degree of selection.”

But avoiding eradicant use was mostly impractical, he suggested. “Otherwise you will have prophylactic use, which is imprecise and not acceptable to consumers.”

While Mr Clark noted dose rates had always been a contentious issue, SAC researcher Fiona Burnett presented research from recent strobilurin barley mildew resistance trials that clearly showed high dose rates increased resistance.

Those trials also indicated mixtures helped, at least delay resistance build-up. Mixing a strobilurin with fenpropimorph kept the frequency of strobilurin-resistant isolates to a similar level to where strobilurins hadn’t been applied, Dr Burnett said.

But the University of Reading’s Michael Shaw said there was no mathematical reason why mixtures should have any effect on slowing resistance build-up.

A mixture partner acting independently would kill both non-resistant and resistant isolates indifferently, so in theory it shouldn’t affect the proportion of resistance in the population, he suggested.

“It might reduce the overall size of the population, ie give more control, but won’t alter the resistance level.”

That wasn’t to say mixtures couldn’t work, he admitted. “Some may work because of the chemical interactions between the product are synergistic, but you won’t know that until you do the experiments.

“My quarrel is firms marketing mixtures as if they will save fungicides per se – they won’t.”

But mixtures could help reduce, or delay, resistance if they allowed doses to be reduced, Dr Shaw acknowledged, which was exactly what TAG trials had shown to be possible.

The trials showed the total dose of epoxiconazole required to give effective control of septoria was reduced by nearly two-thirds when mixed with chlorothalonil compared with epoxiconazole alone (see table), the organisation’s Stuart Knight said.

Mixture partners

“It shows adding mixture partners at the right time can substantially decrease the amount of product required, which will reduce selection pressure.”

Timing was everything for the strategy, he pointed out. “If applications are delayed increasing the triazole dose is much more beneficial than adding chlorothalonil.”

Using mixtures had been readily adopted by growers, but it was usually driven by disease control rather than resistance management, Mr Clark said. “It is likely to slow resistance development rather than prevent it.”

Understanding of the components of existing strategies needed to be improved, he suggested. “We also need to improve assessment of resistance risk of new products. It is very difficult but would help both the registration authorities and manufacturers.”

But the key to winning the war against fungicide resistance was to improve host plant resistance to fungal diseases, possibly through the use of genetic modification, he said.

“It is almost impossible for pesticide manufacturers to develop anything other than products with single-site modes of action because of the tight environmental and operator controls now in place.

“We haven’t had any new broad-spectrum fungicides [such as chlorothalonil] for forty years because of all the hoops they have to jump through.”

It meant resistance to any new active ingredient was “inevitable,” he said. “With development costs continuing to rise it is also inevitable growers will have to manage disease with fewer actives than they currently have.

“The only way to do that is to have better host crop resistance.”

So far, breeders hadn’t really been able to deliver on the challenge to provide durable plant resistance in arable crops, he said. “For 30 years they have let me down!”

But in the longer term he believed GM technology could help breeders reach that goal. “I’ve come to the conclusion it maybe the only way we can win the war.”

  • Fungal diseases adept at overcoming chemistry
  • Anti-resistance strategies only partially effective
  • Reducing exposure important
  • Mixtures may be effective in helping reduce rates
  • Genetic modification possible route