A question of context ...

In reality, this is not necessarily a problem. Everything depends upon the crop in question and, above all, the growth stage of the plant being targeted. The phenomenon of droplet fragmentation may, indeed, be favourable to the effectiveness of the product if the crop is made up of plants which are already well developed. In this case, the entire ‘canopy’ is covered by micro-droplets, which strengthens the potential effectiveness of the product. It is only if the plants are still small, i.e. seedlings which are barely emerging from the soil, that fragmentation is a real issue, for example, when it comes to getting rid of weeds.

The complexity of the phenomenon can be better gauged if we take into account the fact that the hydrophobic nature of the surface of certain plants - which is the case of many (self-propagating) weeds such as black grass - prevents the drops from adhering correctly. Instead of covering the surface of the plant, the drops bounce off it, to the detriment of the effectiveness of the product. The density of the hairs on the foliar surface may also contribute to maintaining the drops intact. To encourage them to cover the plant, manufacturers of add tension-active agents are to the spray mixture. Other types of additives may also be used, for example, to increase the viscosity of the drops which thus lose part of their energy through a ‘shock absorption’ effect. These additives must be added in a sufficiently fine way so as not to be counter-productive as the result, for example, of the speed of impact of the drops which itself depends on the nozzle and pressure of spray used by the farmer ... 

Going beyond the black box

In short, before the active ingredient is absorbed by the plant and can develop its lethal effects, maximum drop retention on the surface of the leaves must be ensured. Without any retention, the phytosanitary product cannot be effective! This is where the work of Mathieu Massinon, author of a thesis at Gembloux Agro-Bio Tech, comes into play. Entitled ‘Approche multi-échelles de la rétention des produits phytosanitaires sur plantes superhydrophobes’, and drafted under the supervision of Prof. Frédéric Lebeau (Precision Agriculture Unit), he successfully defended his thesis in February. ‘Whether they are inherent in the physical structure of the drops or modulated by the additives used, the parameters involved in the retention of phytosanitary products on the surface of plants are many and complex’, explains the young researcher (Read on this topic From one leaf to another). ‘Agronomists who assess the effectiveness of phyto products through field trials cannot understand everything. They are confronted with a sort of black box inside of which the effect of antagonistic phenomena cannot be excluded, if only because of the variability of the climatic conditions. In my thesis, I postulated that it is possible to go beyond observations which, although indispensable, remain too empirical. How can phenomena which are as complex as coverage and fragmentation be optimally harnessed? If we can make some progress towards this objective, we may not only reduce the quantity of products used in the fields, but also reduce the dissemination of active ingredients as a pure loss in the environment. Moreover, using the finest possible dose depending on various parameters comes down to limiting the phenomenon of  resistance to certain active ingredients which, are now used, in some regions, on a really concerning scale’.