The researchers conducted quantitative studies leading to diagrams showing the probability of presence of pathogens in relation to the distance from their original host plant. These diagrams of probability can serve to estimate the risk of propagation in the case where the spacing between plants is less than the maximum distance for propagation.

These results were then validated on real plants (tomato, strawberry, lemon, and coffee leafs). The experiments confirmed the estimated sizes and distances obtained with the artificial leafs in the laboratory. The study therefore determines a theoretical optimal distance at which plants should be planted.

In this way, it is possible to quantify the risk of outbreak. Indeed, the bigger the leaves become, the higher their potential for pathogen dispersal. Thus, there might be stages in the development of plant that are more critical in transmission than others. This can suggest moments that are more critical than others in terms of pesticides spraying as well.

“These 80-90cm spacings”, explains Tristan Gilet, “will not be a source of worry to mixed-farming enthusiasts. Polyculture is an ancient practice - abandoned since the advent of industrial agriculture - which consists in mixing/alternating several species of plants in the same field. It was observed that polyculture is more robust in terms of resistance to disease, but nobody has been able to explain why and the reasons were attributed to immunology. Our study shows the intrinsic mechanical properties of plants could themselves help contain the spread of disease.“

Almost 15% of crops are lost

Do all raindrops participate to dispersion ? The smaller the drops the less power they have to fragment and eject the leaf pathogens. The researchers therefore studied the biggest and fastest raindrops. There is a physical limit to the formation of drops in the air. When raindrops get too large, the strong relative wind that they experience causes them to transform into a kind of parachute: the air engulfs in the raindrop and later fragments it into smaller droplets. There is therefore a maximal size for raindrops, just before this fragmentation phenomenon; it is the most critical size for the propagation of pathogens.

The different lines of research conducted in the microfluidics lab stem from the desire to understand natural phenomena…but are also valuable for industry and economy. This certainly applies to this research on the way in which diseases are spread among crops, conducted in direct collaboration with Professor Bourouiba at MIT(1).

Fungi, bacteria and viruses are responsible for the loss of 15% of global agricultural production. However, our current approaches to fight them are limited to and genetic modifications. Understanding how diseases spread provides a third addition to our mitigation tools.

Lydia Bourouiba is the Esther and Harold E. Edgerton Career Development Assistant Professor at the Massachusetts Institute of Technology and Associate Faculty at the Institute for Medical Engineering and Science. She is a physical applied mathematician, who worked in fluid dynamics and epidemiology. Her research group at MIT focuses on problems at the interface of fluid dynamics and disease transmission with the aim of elucidating the mechanisms shaping pathogen transmission dynamics where drops and bubbles, multiphase and complex flows are at the core. She directs The fluid dynamics of disease transmission laboratory:  lbourouiba.mit.edu