Life-giving rain, which all plants need to grow and survive, can also be a threat as it spread disease from one plant to another, researchers have found.

Farmers have long noticed that some crop diseases, for instance wheat rust, will spread further and faster after a rain storm, and now U.S and Belgian researchers say they've identified a surprising culprit -- the humble raindrop.

Using high-speed video, they've identified mechanisms whereby a drop of rain falling on a leaf can spread a plant pathogen a considerable distance, the researchers at MIT and Belgium's University of Liege report in the Journal of the Royal Society Interface.

The reason this can happen, they say, is that certain pathogens -- including the fungal parasite responsible for wheat rust -- do not coat an infected leaf in thin films but instead grow into discrete droplets on the leaf's surface.

In hundreds of filmed trials using real and simulated rain, utilizing high-speed videography at 1,000 frames per second, the researchers found that raindrops hitting the pathogen droplets can launch the infecting agents off the leaf and high into the air.

Two separate "launch modes" were seen, they report.

One is where "a raindrop flattens upon impact, sliding underneath the [pathogen] droplet and launching it up in an arc," the researchers say in an MIT release.

In a second mechanism, the raindrop does not need to touch the pathogen droplet directly "but instead pushes the leaf down, causing the droplet to slide downward and then catapult out" as the leaf springs back upward.

Understanding the link between the mechanical properties of a plant -- particularly the flexibility of its leaves -- and the spread of various diseases could help farmers create more pathogen-resistant fields, says MIT researcher and lead study author Lydia Bourouiba.

One method would be to employ what's known as polyculture, planting multiple crop varieties in the same space rather than a large stand of a single crop.

"We can start thinking of how to smartly reinvent polyculture, where you have alternating species of plants with complimentary mechanical properties at various stages of their growth," she says.

"Polyculture is an old concept if you look at native cultures, but this is one way to scientifically show that by alternating plants in one field, you can mechanically and naturally reduce the range of transmission of a pathogen during rainfall."

One crop with less-flexible leaves could serve as a shield, she says. It would get contaminated, but because of its chosen mechanical properties would be less likely to project the pathogen to the next plant.

"So you could start reducing the efficacy of spread in one species, while still using agricultural space effectively," she explains.

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