Protecting our future - using fungicides wisely

Fungicides are vital for the control fungal diseases. Without effective disease control we would not be able to grow high yields of quality cereal crops in a profitable way for the farmer and at an economic price for the purchaser.

Discovering new fungicides is a very long, and increasingly difficult and costly task. Yet, if not used wisely, fungicides can be rendered ineffective very quickly. Diseases are very adept at developing resistance to fungicides we rely on.

What is fungicide resistance?

In practical terms a fungus can be considered to show reduced sensitivity to or be resistant to a fungicide when the normal use pattern of the fungicide results in reduced or no disease control.

Around the world many cases of disease resistance have occurred but we are fortunate in New Zealand that apart from some isolated cases (Septoria leaf blotch and powdery mildew becoming resistant to strobilurin fungicides), fungicides are still highly effective in cereals.

How does resistance develop?

There are two main types of fungicide: mult-isite fungicides and single-site fungicides.

Multi-site fungicides are active against the fungus in multiple ways and are generally not prone to resistance development. While they form a valuable component of fungicide programmes, options for New Zealand cereal farmers are limited.

Single-site fungicides are active against a specific biological process in the plant. Due to this the possibility that a fungus can overcome the activity of a fungicide is greater and resistance to this type of fungicide occurs more often.

Resistance to single-site fungicides can occur in two ways:

  • Monogenic resistance occurs when the mutation of only one gene is sufficient to significantly reduce the activity of the fungicide. 

    Often the mutant fungi with the modified gene (these are naturally occurring mutants found in any population) is just as healthy and fit as the normal fungi and so when they survive the fungicide they rapidly reproduce and soon become the dominant population. At this point the fungus becomes ineffective. Because the population is healthy if applications of the fungicide are stopped it has no impact on the fungal population and this means the fungicide can no longer be used.

    Strobilurin fungicides, and to a lesser extent SDHI fungicides, show this type of resistance development. In New Zealand Septoria leaf blotch has used this mechanism to become totally resistance to strobilurin fungicides but there is currently no evidence that SDHI fungicides such as Aviator Xpro have been affected.
  • Polygenic resistance describes a situation where several gene mutations have to accumulate to lower the sensitivity of the fungus to the fungicide. This takes far longer than the development of monogenic resistance and very rarely results in fungicides that are ineffective. DMI fungicides can exhibit this type of reduced sensitivity and in Europe to control Septoria leaf blotch, much higher doses of DMI fungicides are needed. Testing by Bayer in New Zealand has shown that this isn’t the case here; Septoria leaf blotch is still very sensitive to DMI fungicides such as Proline or Prosaro.

How to reduce the chance of resistance developing?

There are two main mechanisms to reduce the development of disease resistance. Both are equally important and both should be practiced.

Mixing modes of action. Applying two or more types of fungicide, ideally in mixture, but possibly in programme, that have different modes of action is an effective way of delaying resistance development. It means that if the fungus is able to resist one fungicide it is likely to be controlled by the other fungicide. It is important to ensure that both fungicides are active against the disease and that their duration of activity is similar.

Dose rate. Applying a robust dose rate is very important to delay the onset of resistance as this ensures that disease populations are effectively controlled. Failure to control a disease population results in lost yield and reduced quality; worse still because a component of the fungicide mixture being applied will be a DMI, using low doses can accelerate the development of insensitivity.

For more information about fungicide resistance download the booklet “a guide to Fungicide Resistance”, below.

A guide to fungicide resistance