A summer rain falling on a barley field is the very picture of vitality. The shower softens the dry ground; crops grow tall; grain fattens and is harvested to make bread, or beer, or feed for livestock. But zoom in close and you may witness a strikingly different scene: The same raindrops that bestow life can also spread disease and death.
Just as the common cold can travel through a handshake or a cough, plant epidemics move in various ways—by breezes or bugs, soil or seeds. Many bacteria and some types of fungal spores also jump from leaf to leaf in splatters of rainwater, a plant’s version of a sneeze. These so-called rain-dispersed pathogens surround themselves in a slimy secretion called mucilage, which protects them but also prevents them from being picked up by wind. When a raindrop strikes an infected leaf, it scoops up the microorganisms, flinging them away in a ricocheting spray.
A devastating fungus called barley scald (Rhynchosporium commune) sweeps through fields in this way. So does septoria leaf blotch (Zymoseptoria tritici), which threatens wheat, and potato blackleg (Pectobacterium atrosepticum), a bacterium that stunts and kills plants. Now, however, modern imaging technologies are enabling researchers to precisely capture foliar sneezes, revealing intricate and unexpected patterns that could inform how farmers space plants or pair different kinds of crops to shield against these diseases.
The connection between rain and contagion has long intrigued scientists. The 19th-century French microbiologist Pierre Miquel first showed, in a series of classic experiments in Paris’s famed Parc Montsouris, that rains cleared germs from the air, distributing them across the grounds. Some three decades later, a botanist named R.C. Faulwetter compared the pattern of infection in fields in South Carolina with local weather records and concluded that bacteria were kicked into the air by rainwater and then blown downwind. Yet even as Faulwetter and other researchers took their curiosity into the lab, investigating the dynamics of rain splash on blotting paper, glass plates, and other rigid surfaces, funding and technological limitations kept the action on actual leaves a black box.
Enter high-speed videography. In early 2014, Lydia Bourouiba, an expert in fluid dynamics at the Massachusetts Institute of Technology, co-authored a study that used high-speed imaging and controlled experiments to demonstrate that coughs and sneezes can launch pathogens hundreds of times farther than previously thought. Working with Tristan Gilet, an engineer at the University of Liège, she then applied the same technique to study how rainwater bounces off leaves.
The researchers experimented with plants of about 30 species, including bamboo, blueberry, English ivy, and tomato. They also built artificial leaves—plastic cantilevers with a hinged base, like little diving boards—to more precisely test how different variables, such as size and flexibility, change the pattern of splatter. Onto these real and fake leaves, they dripped water from heights above 4 meters. A high-speed camera captured the collisions at rates of 1,000 frames per second or greater.
The first thing these images revealed was that pathologists’ previous assumptions about the mechanics of rain splatter were wrong. Experiments on hard surfaces had suggested that the first drops of a rain shower dissolve the sticky mucilage, leaving pathogens suspended in a thin film of water that coats the surface of a leaf. Then, when a new drop hits, it forms a crown-shaped wall of liquid that rises upward and outward until its rim breaks up under surface tension, ejecting chains of diminutive disease-carrying droplets in a burst known as a coronal splash.
But Bourouiba and Gilet’s videos recorded no such film. Instead, they showed that water tends to pool in beads on a leaf’s surface, like drops on your skin after you step out of the shower. When a new raindrop lands on this beaded surface, the resulting spray ejects droplets in a very different pattern. And the shape and reach of the splash changes depending on the flexibility—as well as the size and mass—of the leaf.
The stiffest leaves, such as those of a banana or prayer plant, tend to fling water horizontally in an arc that the researchers call crescent moon detachment. It works like this: A raindrop lands on a leaf and flattens into a tiny puddle. As the leaf gives under the impact of the drop, the puddle slides toward its tip, slipping underneath the nearest stationary bead and stretching it into a sheet, which separates into threads and then droplets. Finally, transferred momentum from the raindrop casts these droplets away.
The more supple the leaf, however, the more the arcing spray dips toward the ground, and the less effective the crescent moon mechanism is for spewing infection afar. From the most flexible leaves, such as a tomato or strawberry leaf, pathogens escape by another route called inertial detachment. In this scenario, an infected water bead perches on a leaf like a kid standing still on a trampoline. A raindrop hurtling down is like a friend taking a flying leap onto the bounce mat. When the drop lands, the leaf takes a deep bow, generating a large centrifugal acceleration that sends the contaminated bead toward the tip of the leaf. As the leaf rebounds to its original position, the bead, like the kid, is hurled into the air.
This catapulting action “can propel larger drops farther away, when compared to the crescent moon,” says Bourouiba, who published her findings earlier this year. And the larger the drop, the larger the load of pathogens carried from one plant to the next. So while the stiffest leaves may scatter pathogens up to four times more widely than less rigid leaves, as Bourouiba and Gilet’s results suggest, the springiest leaves may launch the deadliest projectiles.
Such knowledge could help better control outbreaks, says Bruce Fitt, a plant pathologist at the University of Hertfordshire in the United Kingdom. Bourouiba agrees. For example, she suggests, farmers might space plants just far enough apart so that rain splatter can’t bounce between them—a distance calculated from factors such as the size and flexibility of their leaves. To save space, farmers could seed alternating rows with two different types of crops whose unique splash patterns prevent their pathogen footprints from overlapping.
The minute choreography of water and leaves “is really at the root” of disease spread, Bourouiba argues. It may be hard to glimpse compared to human-scale forces like wind. But its impact is anything but small.
Elizabeth Preston is a science writer and editor in Massachusetts. Her blog, Inkfish, is published by Discover. Follow her on Twitter @Inkfish.
Bourouiba, L., Dehandschoewercker, E., & Bush, J.W.M. Violent respiratory events: on coughing and sneezing. Journal of Fluid Mechanics 745, 537-563 (2014).
Gilet, T. & Bourouiba, L. Fluid fragmentation shapes rain-induced foliar disease transmission. Journal of the Royal Society Interface 12 (2015). Retrieved from DOI: 10.1098/rsif.2014.1092.
Gilet, T. & Bourouiba, L. Rain-induced ejection of pathogens from leaves: Revisiting the mechanism of splash-on-film using high-speed visualization. Integrative and Comparative Biology 54, 974–984 (2014).