‘Tiny tornadoes’ around leaves spread deadly plant pathogens

When raindrops hit a leaf of a wheat plant infected with rust – a pathogenic spore that has decimated crops globally – the leaf flutters, creating tiny swirling vortices of air that disperse the spores, where they could end up infecting healthy plants.

An analysis of this effect using high-speed cameras, described in the Jan. 31 issue of Science Advances, could be a first step toward designing a strategy to help reduce pathogens – not just spores but also bacteria, oomycetes and viruses – from spreading from leaves.

Each year about 20% of wheat is lost due to a variety of diseases, many of them airborne. For example, airborne wheat rust spores can potentially get picked up by intercontinental winds and travel across continents and even oceans, making disease control difficult. This is the first study to analyze spore dispersion at its source, where rain droplets shake flexible leaves to initially disperse the pathogens.

Applying theoretical analysis to the high-speed camera footage, the researchers were able to predict the trajectory of spores and how they are carried by a swirling motion created by the vibrating leaves.

“It’s kind of a tiny tornado in the air,” said Sunghwan Jung, the paper’s corresponding author and professor and director of graduate studies in the Department of Biological and Environmental Engineering in the College of Agriculture and Life Sciences. Zixuan Wu, a doctoral student in Jung’s lab, is the paper’s first author.

For their analysis, Jung and colleagues borrowed techniques typically used to study geophysical flows, which are large-scale oceanic and atmospheric air currents. Scientists who specialize in fluid mechanics want to predict the patterns of larger vortices in oceans for events such as oil spills to understand where the pollution may go. Researchers may want to track smoke or air pollution in the air. The study’s authors took those analytical techniques and downsized them by a few orders of magnitude to understand and predict the swirls they found in the air around a bouncing wheat leaf.

“We describe the magnitudes of these kinds of swirling motion, and then when they will form and how spores move around, so everything is predictable,” Jung said.

The research is an example of taking techniques that are often sequestered within a specialized field and applying them outside their usual applications.

Because of restrictions to working with actual live spores, the researchers used miniature hollow glass particles to mimic spores. They used their methods to better understand how many spores might come off a leaf, where they might go and how they move away from an infected plant. Ultimately, the study may inform future research that finds a way to prevent spores from infecting healthy plants at their source.

“We couldn’t figure out the solution yet,” Jung said. “But if we can control these kinds of vortex structures around the leaf somehow, then we can reduce the spread of spores to new plants.”

Mark Sorrells, professor in the School of Integrative Plant Science Plant Breeding and Genetics Section (CALS), is a co-author. Saikat Basu at South Dakota State University, Seungho Kim at Pusan National University in South Korea and Francisco Beron-Vera at the University of Miami are also co-authors.

The study was funded by the National Science Foundation.

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Becka Bowyer