The latter research work was initiated when Tristan Gilet was working as a post-doctoral researcher at the Massachusetts Institute of Technology (MIT). There, he met Lydia Bourouiba, a researcher and now the Esther and Harold E. Edgerton Assistant Professor  at MIT, founder and director of the MIT Fluid Dynamics of Disease Transmission Laboratory (1). The two researchers then began to take interest in the way pathogens propagate in crops. The first observation: in the scientific literature, there is little or no description of what actually happens at the scale of a drop! The only element that farmers and agronomists are aware of is the fact that the rain is the main vector for the propagation of disease, but the mechanism by which this vector operates remains unknown.

 “Our first contribution as fluid experts was to film what happens when it rains on a plant”, explains Tristan Gilet. “We could reasonably have expected the pathogens present on the leaf to be washed away following the first impact of a raindrop. This was seen not to be the case because pathogens are generally contained in a mucilage, a viscous substance that requires dissolution first ”!

The researchers noticed that this extra requirement for dissolution initiates a mechanism that considerably increases the dispersion ability of the pathogens. The sequence is as follows: at rain onset , drops impact on the leaves, including areas that are populated with pathogens. As the water remains on the plant for a short time in a drop shape, the viscous mucilage dissolves and becomes contained within these drops. As rains continues, other raindrops strike the contaminated drops, sometimes right on top of each other, but much more often one on the side of the other. The impacting drop then washes away the contaminated drop. This latter can then fly away and land onto a neighboring leaf on the same plant, or on another plant nearby, thus spreading the infectious agent. This discovery has been the subject of a first publication (2). “It is a very efficient fragmentation scenario compared to many others”, concludes Professor Gilet. “The horizontal asymmetry of this mechanism by which the impacting drops strike the contaminated drops on the side results in a dispersion of the contaminants over very long distances”.

Another discovery by the ULg and MIT researchers in this first study is the identification of  the most frequent  fluid fragmentation scenarios that are also the most efficient at dispersal. Two of them stand out in particular. The first is the direct “splash”: the raindrop impacts besides the contaminated residue,, washing it away and consequently spreading pathogens from one plant to another. The second effect is indirect: the drop does not necessarily come into contact with another liquid residue, but it causes the leaf to move. If the leaf bends sufficiently, the resulting motion will catapult the contaminated drop away.

Modeling the impact of drops  

In a second article which has just been published (3), the same two researchers have switched from real fields and plants to a combination of real engineered leaves. These latter represent a simplified physical model with controlled mechanical parameters (e.g. the size and flexibility of the leaves) that allow for a more quantitative physical understanding of the phenomenon. Conclusions show that if the leaf is large, the impact of one drop will induce very little motion of the leaf; if the leaf is small, however, the impact will significantly bend the leaf. Leaf size and flexibility are therefore two properties of primary importance for these drop impacts. “Up to now, phytopathologists and agro-engineers have been mainly interested in the influence of rain intensity (the average volume of rain per units of time and ground surface area), without being able to identify a clear link between this average parameter and the speed of pathogen dispersal”, explains Tristan Gilet. What we have shown is that pathogen propagation mainly depend on a coupling between the individual size of raindrops and the mechanical properties of the leaf on which it lands. We observed that from one leaf to another, the maximum propagation distance can vary by a factor of up to four . This factor is very important from an economic point of view because it makes the difference between a 25-30 cm spacing (which could apply to many crops) and an 80-90cm spacing (which many consider to be "very costly”).

(1) The Fluid Dynamics of Disease Transmission Laboratory:  lbourouiba.mit.edu
(2) Rain-induced Ejection of Pathogens from Leaves : Revisiting the Hypothesis of Splash-on-Film using High-speed Visualization. Tristan Gilet and Lydia Bourouiba, Integrative and Comparative Biology 54(6), 974-84, 2014. (Read the article)
(3) Fluid fragmentation shapes rain-induced foliar disease transmission. T. Gilet and L. Bourouiba, Journal of the Royal Society Interface, 2015 (Read the article)