The fungicide resistance challenge

Monday, 18 February 2019

Robust fungicide programmes have helped support high yields in wheat and barley. Such approaches have particular value where varietal disease resistance is low or where disease pressures are hard to predict. But there are, of course, costs associated with intensive spray programmes – particularly the development of fungicide resistance.

When fungicides are applied, susceptible fungal strains are usually controlled very effectively. However, any resistant strains present (through mutation or natural variation) are more likely to survive and reproduce. This process of ‘selection’ makes each subsequent generation more difficult to control. In the absence of any fitness costs, resistant strains may come to dominate the population, causing disease control to fail.

In wheat, septoria tritici is currently of greatest concern, with significant shifts in sensitivity to strobilurins and azoles in UK populations. Isolates with mutations that confer reduced sensitivity to succinate dehydrogenase inhibitors (SDHIs) are also present across the UK, with increasing complexity and frequency each year.

For barley, ramularia is the greatest concern, with significant shifts in sensitivity to strobilurins, azoles and SDHIs in UK populations.

Though good disease control can be achieved in both crops by using robust fungicide programmes, it is essential to use comprehensive anti-resistance strategies to slow resistance development and preserve the efficacy of both existing and new chemistry.

How to manage fungicide resistance

A good resistance management strategy does not need to compromise disease control. In fact, if done well, such strategies should result in robust and sustainable control. Strategies should:

  • Exploit all practical, non-chemical control methods to reduce disease risk and slow epidemic development
  • Limit the time over which the pathogen population is exposed to the fungicide
  • Use effective mixtures and alternate fungicides with different modes of action
  • Use the minimum dose required to effectively control target pathogens

Non-chemical control

Varietal resistance to disease remains patchy, with some popular varieties having low or no resistance ratings. This is especially true for barley. Modern wheat varieties, however, have improved resistance to septoria and rust. Results from AHDB and industry partners show that such varieties can be managed with fewer fungicide inputs, reducing selection pressure and still giving equivalent outputs to more susceptible varieties. Even for barley, where markets permit and the wider agronomy package suits, varieties with resistance to the diseases prevalent should be selected. Good varietal resistance also provides the added bonus of greater flexibility in spray timing.

Appropriate husbandry techniques can also be used to reduce disease pressure. For example, avoiding very early sowing of winter wheat can help reduce early-season septoria pressure. The same is true in spring barley, as very early sowing (December to February) can increase rhynchosporium pressure. Controlling volunteers and reducing crop debris can also help reduce rhynchosporium, net blotch, brown rust, yellow rust and mildew pressure.

Fungicide futures

As part of the Fungicide Futures initiative, AHDB has published practical measures to combat fungicide resistance in pathogens of wheat and barley. The publications provide bestpractice information on treatment frequency, timing, dose, mixtures, alternation, multi-sites and programmes.

Fungicide Futures is a joint initiative between AHDB and the Fungicide Resistance Action Group UK (FRAG).

This article is taken from Grain Outlook, spring 2019