Fungicide resistance management in cereals

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Fungicides are essential for the maintenance of healthy crops. However, for some diseases, their effectiveness has been seriously affected by the development of fungicide resistance in target pathogens.

Fungicide treatments are, and are likely to remain, essential for maintaining healthy crops and reliable, high-quality yields. They form a key component of integrated crop management, and their effectiveness must be maintained for as long as possible.

The majority of modern fungicides have single-site modes of action, acting on specific biochemical pathways in the target fungal pathogen. Once a fungicide is used on a pathogen population, individual isolates of the fungal population which have a reduced sensitivity to the fungicide will be selected by repeated use of fungicides with the same mode of action. Multi-site fungicides are less prone to the development of resistance in the target pathogen and these older fungicides still have a very important role in the resistance strategy for the more modern fungicides.

To determine the sensitivity of a fungal isolate, tests are carried out using multiple doses of the fungicide in order to determine an EC50 value which is the dose that provides 50% inhibition of the isolate compared to a non-fungicide-amended control. Some isolates can have increased EC50 values which are still within the normal range of sensitivities of the population so these are unlikely to affect field performance of the fungicide. Others may have EC50 values outside the normal range of sensitivities and these may affect field performance if they become frequent in the population. If these isolates remain at low levels they may have no impact on the field performance of the fungicide when used at commercial doses.

Resistance can arise rapidly and completely so that disease control is lost in a single step, such as the G143A mutation affecting the performance of strobilurin fungicides. More commonly, resistance develops gradually so that the pathogen population becomes progressively less sensitive, such as the development of resistance in Zymoseptoria tritici to the azole fungicide group. When resistance develops slowly, with multiple mutations affecting sensitivity individual isolates may show reduced sensitivity in laboratory tests but there may be no loss of field performance.

To have the greatest impact, anti-resistance strategies need to be implemented as soon as fungicides are introduced to the market, before any shifts in sensitivity are detected.

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