Fungicide and bactericide use strategies to avoid resistance development in plant pathogens in New Zealand
There are several possible reasons why a fungicide applied to a crop might fail to control disease, including spray operator error or poor sprayer calibration, excessive wind or washoff by rain, old or ineffective fungicide product, or resistance of a plant pathogen to the fungicide.
Fungicide resistance is usually not the reason for disease control failure but does sometimes occur. Fungicides at risk from resistance are mainly the synthetic ones developed since the 1970s that are specific in the way they affect their target fungi. Many older fungicides, such as captan, thiram, mancozeb, metiram, copper and sulphur, which have a non-specific mode of action and are active against a broad spectrum of diseases, are not considered to be at risk from resistance development in fungi.
Instances of resistance in New Zealand have been recorded to the following fungicide groups: benzimidazoles (also known as MBCs), dicarboximides, demethylation inhibitors (DMIs) and phenylamides (acylalanines). Fungicide groups considered to be at risk from resistance development, but to which resistance has not yet been recorded in New Zealand, include, anilinopyrimidines, dodine, quinone outside Inhibitors (QoIs), also known as strobilurins or STAR compounds, and morpholines.
Although there are instances around the world of pathogens developing resistance to particular fungicides, there are no groups of fungicides that have completely gone out of use because of resistance. Resistance can sometimes be managed because the frequency of resistance in pathogen populations may decrease if use of the at-risk fungicide is reduced or stopped.
How fungicide resistance develops and how the risk can be minimised
Resistance arises when repeated use of an at-risk fungicide leads to the selection of pathogen strains that are less sensitive (more resistant) to the fungicide. The overall population then appears more resistant than it was before the at-risk fungicide was available. Resistance can develop in two ways, directional selection and disruptive selection.
Pathogens have natural variation in their sensitivity to fungicides and a small proportion of strains requires a higher concentration of fungicide to kill them than most of the population. These less sensitive strains arise from natural gene mutations. Resistance develops if the less sensitive strains encounter a sub-lethal dose of fungicide and continue to survive. When this happens the sensitivity of the whole population gradually shifts towards resistance. This type of resistance is selected most rapidly by the repeated use of the at-risk fungicide, especially at reduced application rates, and by poor spray coverage.
This occurs when a single gene in the fungus mutates, allowing a new biochemical pathway that is not affected by the fungicide to permit the fungus to survive in the presence of the fungicide. This type of resistance can give rise to high levels of resistance very rapidly.
Development of both types of resistance is delayed by limiting numbers of applications of the at-risk fungicide group, mixing the at-risk fungicide with a fungicide from a different cross-resistance group to ensure that no part of the pathogen population survives when the at-risk fungicide is used, or alternating the at-risk fungicide with fungicides from a different cross-resistance group.
The use of non-chemical disease management practices, including crop hygiene to minimise inoculum carry-over and crop canopy management to make the crop environment less suitable for disease development, can also delay the development of resistance. Effective spray coverage through the use of suitable spraying equipment and its correct calibration is critical for preventing development of fungicide resistance.
Fungicide and bactericide groups
Fungicides are grouped according to the way in which fungal pathogens develop resistance to them (cross-resistance groups), rather than by their chemical structures. The fungicide groups discussed in these resistance management strategies are those recognised by the European Fungicide Resistance Action Committee (FRAC) and the equivalent North American committee (NAFRAC).
The nine fungicide groups that are included in these strategies either have instances where resistance has developed in fungal plant pathogens or are considered to be at risk from resistance development. There is also one strategy for the use of the bactericide streptomycin sulphate against certain plant pathogenic bacteria.
- DMIs (demethylation inhibitors)
- Phenylamides (acylalanines)
- Qo inhibitors (QoIs) (strobilurins)