Fluid fragmentation shapes rain-induced foliar disease transmission

Abstract

Plant diseases represent a growing threat to the global food supply. The factors contributing to pathogen transmission from plant to plant remain poorly understood. Statistical correlations between rainfalls and plant disease outbreaks were reported; however, the detailed mechanisms linking the two were relegated to a black box. In this combined experimental and theoretical study, we focus on the impact dynamics of raindrops on infected leaves, one drop at a time. We find that the deposition range of most of the pathogen-bearing droplets is constrained by a hydrodynamical condition and we quantify the effect of leaf size and compliance on such constraint. Moreover, we identify and characterize two dominant fluid fragmentation scenarios as responsible for the dispersal of most pathogen-bearing droplets emitted from infected leaves: (i) the crescent-moon ejection is driven by the direct interaction between the impacting raindrop and the contaminated sessile drop and (ii) the inertial detachment is driven by the motion imparted to the leaf by the raindrop, leading to catapult-like droplet ejections. We find that at first, decreasing leaf size or increasing compliance reduces the range of pathogen-bearing droplets and the subsequent epidemic onset efficiency. However, this conclusion only applies for the crescent moon ejection. Above a certain compliance threshold a more effective mechanism of contaminated fluid ejection, the inertial detachment, emerges. This compliance threshold is determined by the ratio between the leaf velocity and the characteristic velocity of fluid fragmentation. The inertial detachment mechanism enhances the range of deposition of the larger contaminated droplets and suggests a change in epidemic onset pattern and a more efficient potential of infection of neighbouring plants. Dimensionless parameters and scaling laws are provided to rationalize our observations. Our results link for the first time the mechanical properties of foliage with the onset dynamics of foliar epidemics through the lens of fluid fragmentation. We discuss how the reported findings can inform the design of mitigation strategies acting at the early stage of a foliar disease outbreak.

Jet dynamics post drop impact on a deep pool

“…Nevertheless, one aspect has been barely characterized so far: the jet dynamics and subsequent aerosols produced by a drop impact [13]. This question is crucial in different situations such as disease transmission [14] and liquid atomization [15] or its contribution to sea spray when a drop impacts the ocean [11,16].…”

Edge effect: Liquid sheet and droplets formed by drop impact close to an edge

“…Increasing attention has also been paid to impacts on small solid targets of size comparable to that of the drop diameter [38][39][40][41][42] or binary drop collision [43,44]. In the crescent-moon impact scenario, an intrinsic horizontal asymmetry leads to a nonaxisymmetric liquid sheet [19]. The latter induces asymmetry in the speed and direction of ejected droplets.…”

“…Thus, generalizing results to various plants and precipitations remains a challenge [7,17]. Recent studies have provided insight into raindrop-leaf interactions [18,19], identifying the important role of wetting and of sessile drops on leaves. They identified several dispersal mechanisms as a function of leaf mechanical properties that are both common and efficient at dispersing pathogens.…”

“…The latter induces asymmetry in the speed and direction of ejected droplets. Inclination, compliance, and finite size of the leaves all amplify this asymmetry [19]. Drop impacts that yield asymmetric behaviors were also studied with (i) horizontal gradients of texture and wetting properties [45][46][47][48][49][50], (ii) varying inclination or tangential speed of the substrate [51][52][53], and (iii) nonaxisymmetric target shapes [54,55].…”

La physique de la pluie conditionne la propagation des maladies en agriculture

“…De nombreux paramètres physiques ont un effet significatif qui doit être finement caractérisé. En particulier, nous avons ici montré que la géométrie et les propriétés mécaniques (flexibilité) des feuilles conditionnent fortement la dispersion des pathogènes [9]. La distance parcourue peut varier de 30 cm à près de 80 cm !…”