Fungicide Resistance

Fungicide resistance occurs through evolution of a fungus when a stable mutation results in a reduction in sensitivity to a fungicide by a fungal population. Evolution is a slow process in animals that takes thousands of years but because fungi reproduce rapidly it can occur much more rapidly in their populations.

Fungicides with a single-site mode of action are at greater risk for resistance developing compared to those with multi-side modes of action.  Most fungicides being developed today have a single-site mode of action because they tend to have lower impacts on the environment and non-target organisms.

Key point: New fungicides e.g. SDHIs (and future fungicides) are likely to be more prone to resistance developing as they tend to have a single-site mode of action

Types of fungicide resistance
There are two types of fungicide resistance:
1. Those that develop from a change in a single major gene; resulting in total resistance to the fungicide (complete loss of disease control) e.g. Strobilurins for Septoria control in wheat.
2. Those that develop from changes in several interacting genes; resulting in reduced sensitivity to the fungicide (higher rates and/or more frequent applications needed to give the same results) e.g. Triazoles for Septoria control in wheat.

Typically cross resistance, where all fungicides with a similar mode of action are affected, occurs with both types of resistance. It could be argued that fungicide resistance is inevitable for all fungicides; it is simply a matter of time, the more often a fungicide is used the greater the risk of fungicide resistance developing.

Key message: As shown in figure 1, every time an active ingredient is used it alters the fungal population by killing the majority of the susceptible population and therefore allowing the resistant population increase as a proportion of the total fungal population.

Fungicide resistance management guidelines
Following resistance management guidelines slows down the process and minimises the risk of fungicide resistance developing.
• Use disease-resistant varieties
• Try to apply fungicides before disease symptoms appear i.e. preventative rather than curative applications
• Only use at risk fungicides when needed most e.g. do not use triazoles at T0 in winter wheat
• Tank-mix fungicides with a different mode of action and ideally include multi-site fungicides (e.g. Bravo/CT500 or Phoenix) because these have low resistance risk. 
• Follow best practise spraying guidelines to maximize spray coverage. Better coverage, especially on the underside of leaves, allows better performance of multi-site fungicides.
• Monitor disease control performance and report any suspected loss of efficacy.

Many of the popular winter wheat varieties grown are susceptible to Septoria which increases the risk of fungicide resistance developing

Current situation in Ireland
It is over 10 years since Strobilurins were lost for control of Septoria in wheat when total resistance developed almost overnight. In recent years, performance monitoring by Teagasc has shown that the performance of the top straight Triazole products Opus (Epoxiconazole) and to an even greater extent Proline (Prothioconazole) has slipped, particularly in terms of curativity.  It is important that resistance management guidelines, e.g. applying no triazoles at T0, are followed to prevent further slippage of their performance.

Thankfully performance monitoring shows no drop in the performance of the new SDHI containing products (e.g. Adexar, Aviator, Keystone, Treoris); with these products now providing a yield benefit of over a ton per hectare over straight Triazoles. However these products need to be protected by the inclusion of a multi-site fungicide i.e. Bravo/CT500 or Phoenix whenever they are used.

The situation is better in barley with low levels of Net Blotch resistance (partial) to Strobilurins found in Ireland and no Triazole resistance to date. However, we cannot afford to be complacent with much higher levels of Net Blotch resistance to Strobilurins found in the UK and reports of Rhynchosporium resistance to Proline (Prothioconazole) in Scotland. Proline (Prothioconazole) in particular is of critical importance to barley disease control programmes and it is therefore vital to follow best practise resistance management guidelines e.g. always mix products with different modes of action against the disease targets when spraying for disease.

Key message: Follow best practise resistance management guidelines to prolong the life of fungicides

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