The treatment of fungal plant pathogens through the use of fungicide applications is and will remain essential for maintaining healthy crops and reliable high-quality yields. Their use in modern cropping systems form a key component of integrated crop management, and their effectiveness must be sustained as long as possible. Understanding what is meant by the term ‘fungicide resistance’, where this is being observed and how to minimise the risk of it developing, will be the focus of a mini-series on fungicide resistance.
For the first part, we will be looking at how fungicides work, what is fungicide resistance and how is this different to fungicide sensitivity in practical terms.
Types of resistance
In terms of resistance to fungicides there are two types that concern agriculture.
Multi-step (or continuous or quantitative) resistance: when the pathogen becomes less sensitive to the fungicide and higher rates are required to maintain adequate control.
Disruptive (or discrete or qualitative) resistance: when the pathogen can suddenly tolerate quite high levels of fungicide and rates required for adequate control are often impracticable.
Multi-step resistance has been coined reduced sensitivity by researchers as it is a gradual, step by step process where the fungus gradually becomes less sensitive to fungicides. This is the case with Septoria in wheat in south-western Victoria. Researchers have recently detected this reduced sensitivity in the fungus to azole fungicides. This means that growers will need to apply higher rates of fungicide to achieve adequate control. These rates will continue to increase until the fungus is so insensitive to the fungicide that the rates needed are no longer commercially viable.
Disruptive resistance on the other hand is more like an on-off switch. A fungus which was susceptible to the fungicide suddenly is resistant to commercially viable rates making the fungicide ineffective even at higher rates.
For a specific fungus to be labeled as fungicide resistant, reports from resistance being observed in the field must be confirmed by further studies identifying if, at the organismal level, there is a reduction in sensitivity of the fungal isolate(s) to the specific fungicide.
How Fungicides work
Mode of action
Fungicides are effective at controlling fungal growth through interfering with critical cellular processes. How a specific cellular process is inhibited by a particular fungicide is called the mode of action (MOA). The Fungicide Resistance Action Committee (FRAC) lists 11 modes of actions, A through I, P and M, in its 2018 mode of action poster.
The term fungicide resistance, as used by FRAC, refers to an acquired, heritable reduction in sensitivity of a fungus to a specific anti-fungal agent (or fungicide) and there are four main mechanisms by which fungi can become resistant to the fungicides traditionally used to control them.
Alteration of the target site so that sensitivity to the fungicide is reduced: The site at which the fungicide would usually affix to the fungi is changed so that it can’t fit well or at all, and is unable to block the desired cellular process.
Detoxification of the fungicide: Fungal cells modify the fungicides so that it is non toxic to the fungi when it is metabolised.
Overexpression of the target: More target sites are created and the cellular process is able to continue as not enough of the target sites are now shut down/blocked.
And Exclusion or expulsion from the target site: The fungicide is expelled through natural biological pumps that exist to expel foreign substances.