![]() This is, for example, the case during violent expirations such as sneezes. ![]() In recent studies of the fluid dynamics of human disease transmission, it was found that the pathogen footprint of an infected host is shaped by the size distribution of the pathogen-bearing droplets it ejects. The mechanisms determining such contamination, in particular, the range and pattern of dispersal of the pathogens remain unknown. ![]() In a laboratory setting, simulated rain led to the contamination of the surroundings of an infected plant. For example, following rainfalls, new lesions at the bottom of wheat leaves were observed to appear and precede outbreaks of Septoria tritici, Septoria nodorum, yellow rust and tan spot. When trapped in sticky mucilage, spores- and bacteria-inducing foliar diseases are dispersed by rainfalls. Indeed, moist conditions allow for mucilage dissolution and formation of pathogen-loaded fluid on leaves. Despite the great variety of plant morphologies, rainfall was identified as a common precursor of foliar disease outbreaks. In the USA, plant pathogens regularly cost more than $220 billion annually. In the near future, one-third of all wheat crops could be lost to re-emerging strains of rust. Plant diseases aggravate the conditions of the billion malnourished individuals worldwide and cause up to 60% of annual wheat loss. We discuss how the reported findings can inform the design of mitigation strategies acting at the early stage of a foliar disease outbreak. 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. Dimensionless parameters and scaling laws are provided to rationalize our observations. 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. This compliance threshold is determined by the ratio between the leaf velocity and the characteristic velocity of fluid fragmentation. Above a certain compliance threshold a more effective mechanism of contaminated fluid ejection, the inertial detachment, emerges. However, this conclusion only applies for the crescent moon ejection. We find that at first, decreasing leaf size or increasing compliance reduces the range of pathogen-bearing droplets and the subsequent epidemic onset efficiency. 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 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. In this combined experimental and theoretical study, we focus on the impact dynamics of raindrops on infected leaves, one drop at a time. ![]() Statistical correlations between rainfalls and plant disease outbreaks were reported however, the detailed mechanisms linking the two were relegated to a black box. The factors contributing to pathogen transmission from plant to plant remain poorly understood. Plant diseases represent a growing threat to the global food supply. ![]()
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