Understanding how fungicides work – their mode of action – not only helps in the design of spray programmes when new diseases or resistant strains of pathogen appear, it helps you minimise the risk of fungicide resistance evolving in the first place. Have a look at my earlier blogs on insecticide resistance, as the principles behind resistance management are similar for insecticides and fungicides. As the number of active substances authorised for horticultural use contracts, it’s vital to use treatments in a way that prolongs their effectiveness but delays or avoids the occurrence of resistance.
As with insecticide resistance in pests, a pathogen that develops resistance to a fungicide is often cross-resistant to others that share similar chemistry – but resistances that affect all the active substances targeting one area of an enzyme or one step in a biological process don’t necessarily confer cross-resistance to those targeting another part. And in some cases resistance to one group of actives can make the pathogen more sensitive to another.
Some fungicides are formulated with two or more different actives, each with a different mode of action (more commonly so than in the case of insecticides). These can be particularly useful in your resistance management strategy. Syngenta’s Switch, for example, contains cyprodinil, which interferes with the production of digestive enzymes by fungal cells; and fludioxonil, which inhibits spore germination.
A product’s chemistry is just part of the story when it comes to judging whether pathogens are likely to evolve resistance. Some species of fungi or oomycetes are particularly prone to random mutations, any of which might make them less sensitive to a fungicide – and if they are particularly fast to reproduce, the resistance is more likely to spread. Resistance risks are also affected by factors such as the persistence of the fungicide on the crop or how frequently it’s applied.
An international expert panel – the Fungicide Resistance Action Committee (FRAC) – takes all these factors into account to group and code-number fungicides according to their shared chemistry, the pathways in the pathogen’s biochemistry that they target, and the likelihood of cross-resistances evolving against them. They include biological as well as chemical products.
By selecting products with different FRAC codes you can draw up spray programmes and management strategies that minimise the resistance risk. And you could consider applying products with a mode of action targeted at a specific stage in the pathogen’s life-cycle – sporulation or spore germination, for example.
You won’t always find the FRAC code specifically listed on a product’s label but it will have been accounted for in the label’s resistance management instructions.
As we develop our website for Ornamental Controls we’ll be adding more guidance on our products’ modes of action – something we believe will be increasingly important when you’re planning crop protection programmes to integrate cultural, biological and chemical approaches that do the job while minimising the risks of resistance in the pests, pathogens and weeds you want to manage.