Growers often ask when will a new fungicide  start losing effectiveness due to resistance. This question guides both on-farm disease management and the stewardship strategy for sustainable fungicide use.

To answer this question, it is important to first recognise that the ingredient for resistance already exists in the pathogen population, as pathogen strains carrying mutations that confer fungicide resistance emerge naturally in the field. Secondly, it is necessary to make a distinction between the time required until fungicide resistance is selected and field failure.

This is because fungicide resistance might already be present in the environment, even before the first use of a fungicide, or selection can occur very rapidly, often within just a growing season, from its first application. On the other hand, field failure can be a fast or slow process depending on factors such as the fungicide mode of action (MoA), the agronomic practices and the pathogen’s life cycle.

Once resistance to a particular fungicide has been selected, pathogen populations resistant to that same fungicide will begin to increase in abundance. However, fungicide performance will not be affected in the field until pathogen populations are dominated by the resistant type.

This means that fungicide lifespan will be shorter or longer depending on the strength and speed of the selection pressure. In other words, when the use of fungicides from the same group is high, selection pressure increases, resulting in a faster accumulation of resistant individuals. Every fungicide is different and, for some of them, such as multi-site fungicides, field failure has never been observed.

Fungicide resistance drivers

Growers and agronomists need to consider three main factors when they are determining the risk of developing fungicide resistance: the type of fungicide and use pattern, characteristics of the target pathogen and the specific agronomic practices being used. These factors can then be assigned to the matrix in Table 1 to determine the combined risk of fungicide resistance developing.

Fungicides:

- The repeated use of the same single-site mode of action fungicide allows for a faster selection of resistance mutations in the field, since pathogens carrying these mutations have a competitive advantage over sensitive ones. Under this scenario, resistant pathogens will rapidly increase in frequency, becoming the dominant type. For medium-high and high-risk fungicides, such as SDHI and QoI (Groups 7 and 11), only one target site mutation is required for resistance to develop. For medium-risk fungicides, such as DMI (Group 3), several mutations need to occur for resistance to develop and this often takes a longer period (Table 1).

- Minimal or no rotation of fungicides combined with high-risk agronomic practices, increases the likelihood of an even-faster selection and spread of resistance. This has been the case for SDHI fungicides and net blotch diseases of barley, where the lack of chemical and crop rotation and the use of susceptible varieties have led to the fast selection of resistance in medium and high-rainfall zones.

Pathogens:

- Short life cycle pathogens – the risk of fungicide resistance evolution increases in pathogens able to produce many offspring rapidly, which often translates into multiple disease cycles in each season, such as wheat powdery mildew. This is exacerbated in paddocks where susceptible varieties are being grown.

- Sexually reproducing pathogens – pathogens with a sexual stage in their life cycle have an increased risk of fungicide resistance evolution due to the ability to generate higher variability, such as Septoria tritici in wheat, blackleg in canola and the net blotches in barley.

- Long-distance dispersing pathogens – windborne and seedborne pathogens will have a faster regional impact in terms of resistance spread than those that are, for example, stubble-borne. However, even stubble-borne pathogens can disseminate over long distances because of plant material movement (transport of hay) and, more locally, the use of contaminated farm machinery across paddocks.

Agronomic practices:

- Short or no crop rotations – continuous planting of a particular crop allows for large pathogen populations to build up, especially if the pathogen is stubble-borne. This practice often requires more-frequent fungicide applications and higher label rates earlier in the season due to higher disease incidence.

- Susceptible varieties – growing susceptible varieties will require more-frequent use of fungicides and higher label rates to control bigger disease epidemics. Allowing pathogens to produce large populations increases the risk of faster resistance selection due to mutations arising during pathogen reproduction.

- Stubble retention – no-till farming increases the risk of early disease outbreaks due to the accumulation of diseased plant residues from previous seasons that infect the new crop as soon as it emerges. The lack of an adequate stubble management plan is especially relevant for no or short rotation strategies where susceptible varieties are grown (see also above).

By way of example, to determine the combined risk of fungicide resistance developing using Table 1: if a fungicide from Group 3, which has a medium risk, is applied to manage a high-risk pathogen powdery mildew in a situation deemed a high agronomic risk, the overall risk for fungicide resistance when considering these combined factors is high.

Table 1: Matrix to evaluate the risk of developing fungicide resistance within Australian cropping systems. Plot the pathogen risk against the fungicide risk and agronomic risk to determine the combined risk of developing fungicide resistance

Source: Modified from  www.frac.info. Not all diseases listed as examples may have fungicides registered for their treatment.

The only factors that a grower cannot control when it comes to managing fungicide resistance are environmental conditions. While one can choose where to grow a crop, rainfall each season and across Australia can be quite variable, ranging from drought to waterlogging.

Wetter regions are more conducive to disease development, requiring more-frequent fungicide applications. These regions also allow for longer seasons and under these conditions additional fungicide applications are often required. The same applies to wetter seasons. Farm trafficability is a major consideration in very wet seasons as paddock access can be severely affected, which results in delays to fungicide sprays.

Ultimately, the combination of the above factors in time and space drives the speed at which fungicide resistance develops and field failure occurs and, critically, determines whether specific fungicides will be available to the industry in the long run.

Fungicide resistance mitigation

It is well-accepted that integrated disease management practices can slow the selection and increase of fungicide resistance in pathogen populations. As such, any strategy aimed at reducing the frequency or rate of fungicide application required, or that slows the development of pathogen epidemics during the period when fungicide is present, will have a dramatic impact on the speed at which resistance emerges and spreads in the landscape. Careful monitoring of movement of hay and seeds is also useful to limit regional spread of resistance.

Following best disease management and anti-resistance strategies will slow the selection of resistance. These strategies are more effective when implemented early, when the proportion of the pathogen population that is resistant to the fungicide is still small. For this reason, good stewardship of fungicides should consider all resistance risks ahead of the release of a new product and provide management guidelines adapted to the fungicide, and the crop and disease targets.

Developed with GRDC support, the AFREN Fungicide Resistance Management in Australian Grain Crops guide provides comprehensive advice on best disease and fungicide resistance management practices.

More information: Associate Professor Fran Lopez-Ruiz, fran.lopezruiz@curtin.edu.au