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Today’s fungicides are available to the farmer either as ready-mixtures of
different active substances, or as solo products. Without knowledge of the strengths and weaknesses of individual active substances, it can be difficult to choose the right product: inappropriate use can encourage the development of resistance.
Before applying a fungicide treatment,farmers have to consider which active substance will bring the desired action against the most prevalent diseases. Successful,reliable control of pathogens depends on the user’s skill in selecting the right combination of active substances, mixed in the correct proportions, and applied at the appropriate rates. The choice of product or combination of active substances depends on which pathogens currently present the greatest threat, and, just as importantly, which are expected to become a threat later in the season. At first glance, the spectra of activity of different products may seem very similar, but the active substances they contain generally have their own particular strengths and weaknesses.
Fungicides either inhibit or eradicate fungal diseases that might otherwise cause severe losses in yield and quality. They differ in terms of chemical properties, activity against pathogens and specific mode-of action.
Fungicides disrupt the metabolism of fungal pathogens, inhibiting their development or killing them off. However, the various active substances act at different points (targets) in the metabolic pathways of the fungi. They most commonly inhibit or inactivate an enzyme involved in the pathogen’s metabolic processes, for example in the production of energy during respiration.
Some substances act very specifically, in so far as they block the activity of individual enzymes or groups of enzymes.
Other active substances inhibit several metabolic steps, meaning that they are less selective.
Uptake and distribution
Besides the necessary active substances,fungicidal products also contain formulates and additives (e.g. emulsifying agents) that serve a number of purposes: to ensure even distribution of the active substance in the spray solution; to strengthen the retention of the active substance on the plant surface;and to improve uptake into the plant’s tissues.
These properties make a significant contribution to the efficacy of the fungicide: however, they have no influence on the transport of the active substance once it is within the plant.
The path and extent of distribution on the plant surface and uptake into plant tissues are characteristics that allow for a differentiation among fungicidal active substances.
Fungicides can be roughly categorized as being non-systemic or systemic. The category a substance belongs to also indicates whether the fungicide can be used to provide protective action alone, or whether it can also be relied upon to provide curative action. Non-systemic active substances do not penetrate into plant tissues,and are therefore unable to reach fungal structures that have already developed within the plant. So these active substances can only be applied to obtain protective activity. In contrast, fungicides with systemic properties can also be applied after the pathogen has succeeded in penetrating into the plant’s tissues, as the internal transport of the active substance allows it to reach the fungal structures in order to kill them off. However, curative activity is only possible up to a certain point: by the time the plant’s tissues have already been damaged by the pathogen, it is usually too late. Moreover, as the infection progresses,the fungi tend to move towards the later developmental stages of their life-cycles,in which they tend to form structures that are less sensitive to the action of fungicides.
What are the characteristics?
Contact fungicides lack systemic activity,but they usually possess a very broad spectrum-of-action. They are unable to penetrate through the plant cuticle, so they cannot act within the plant’s tissues. They therefore offer a purely protective activity that is limited to the inhibition of spore germination.These fungicides either disrupt a number of metabolic processes, or they inhibit a single, essential function in numerous pathogens (e.g. disruption of respiration processes). The converse of their broad spectrum of action is limited selectivity. The user must therefore take a number of drawbacks into account:Only the initial developmental stages of the pathogen are affected. As soon as the fungus succeeds in penetrating into the plant, it can continue to develop uninhibited.
• The fungicidal spray deposit must form an even cover over the plant surface in order to guarantee adequate protection.
• Plant tissues that develop after the treatment has been applied remain unprotected.
Moreover, rain can eventually wash off the existing spray deposit. It may therefore be necessary to apply these active substances several times in succession, depending on the prevailing weather conditions and the stage of growth of the crop.
Systemic fungicides are active substances that are taken up extensively at the plant surface. They tend to form a depot of active substance from which a continuous transfer takes place, either into the plant,or across its surface. The result is a much extended duration of activity.
Systemic active substances also succeed in penetrating through the plant’s cuticle,so they can express their activity within its tissues. They are usually selective, and can usually be used both protective and curative. Substances with fully systemic activity are usually distributed within the plant’s tissues in the direction of the growing shoots: this does not apply to locally-systemic substances. In order to avoid damaging the target plant itself, systemic active substances are selected for their ability to inhibit metabolic processes that are specific to fungi at concentrations low enough to be harmless to plant cells.
The spectrum of action of these fungicides is commonly restricted to a group of related fungi, although the degree of selectivity differs among the various active substances.
The selectivity contributes to the environmental safety of the substances: it also allows a more targeted use of products against pathogens that are already present in a crop.
Few groups of active substances
Only a few groups of active substances are available for controlling the most important fungal diseases of cereal crops.
Products other than those from the triazole and strobilurin groups generally have a very limited spectrum of action that is usually restricted to the control of mildew and eyespot. The exceptions include the use of chlorthalonil as a contact fungicide against Septoria, and the broader activity of boscalid against Septoria and eyespot.
Triazoles are well-known as truly curative fungicides, whose strength means that they can be relied upon for initial treatments,or in situations where the disease potential is uncertain. The farmer cannot rely on curative sprays with the fungicide right through to harvest, because there is a risk involved in depending entirely on a single group of fungicides, even one as strong as the triazoles.
Resistance management
The repeated use of fungicides from the same class of active substance encourages the development of resistance, which can quickly become a fixed genetic feature,spreading rapidly within the fungal population.
Eventually, the resistant variant of the pathogen predominates, and all fungicides with the same mode-of-action are equally useless against it.
Here, the pathogen undergoes changes to several genes, so its sensitivity to the active substance decreases in stages. It is rare for this to lead to a product losing its activity all together: more commonly, the result is a decrease in the efficacy of control and/or a shortening of the original duration of action. Here,a single genetic change is sufficient to reduce sensitivity to the active substance to such an extent that only a minimal level of activity remains, or no activity at all, even if the fungicide is applied at the recommended rate. In other words, isolates of the pathogen are either sensitive or resistant.
Under conditions of high selection pressure,the resistant isolates can reproduce explosively within a single season, and the active substance becomes useless.
In order to prevent the development of resistance to fungicides, spraying programmes should include active substances with different modes of action. Unnecessary applications and treatments with reduced application rates should also be avoided – these not only increase the risk of resistance development, they also represent a false economy if they fail to check the progress of the disease.
