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Next time you pass a wheat field on a dewy morning, you might want to say “gesundheit.”

That’s because some sick plants can “sneeze” shooting out tiny water droplets laden with pathogens, scientists report June 19 in the Journal of the Royal Society Interface. In wheat plants infected with the fungus Puccinia triticina, coalescing dew droplets flew away from the leaves they were on and carried fungal spores with them, experiments showed. The pathogen, which causes a destructive disease known as leaf rust, might then be able to infect other wheat plants (SN: 9/25/10, p. 22).

The flinging effect, which can happen on healthy plants too, is the result of a quirk of fluid dynamics:  When two water drops unite, surface tension is released and converted into kinetic energy that can hurl the fluid away.

It’s a “surface tension catapult,” says mechanical engineer Jonathan Boreyko of Virginia Tech in Blacksburg. The effect occurs only on extremely water-repellent, or superhydrophobic, surfaces, like the leaves of certain plants, including wheat (SN: 3/1/03, p. 132).

The drops can jump a few millimeters high enough to escape the layer of still air that surrounds each leaf, so that a gentle breeze could carry the water and spores to other plants, Boreyko and colleagues report. The catapulting effect was known to occur on other superhydrophobic surfaces, but this is the first time it’s been suggested that it helps transmit disease.

Understanding how leaf rust spreads could be important for controlling it. If “sneezing” turns out to be an important source of transmission, plants could be sprayed with a coating to make them no longer superhydrophobic, for example, Boreyko says.

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