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Life-Giving Rain Also Spreads Deadly Plant Disease

High-speed cameras show how leaf flexibility influences raindrop dispersal into the air — along with pathogens picked up from infected plants

Dyed droplets are propelled off leaves by simulated rainfall ( Tristan Gilet and Lydia Bourouiba/MIT)
smithsonian.com

Recently, MIT researchers found that raindrops release a delicate spray of aerosols when they hit surfaces — a discovery that they suspected contributes to the smell of rain. But it may also have a less whimsical function—providing minute droplets-based rides for bacteria and other microbes.

Another MIT researcher partnered with a scientist in Belgium and confirmed that, indeed, splashes of rain can propel contaminated fluid into the air and onto other plants. Their work helps explain why farmers notice that plant disease outbreaks often follow a storm.

A press release from MIT explains:

The team ran experiments with dozens of types of common foliage, including ivy, bamboo, peppermint, and banana leaves. They conducted hundreds of experiments for each type of foliage, using 30 examples of real plant foliage and 12 artificially engineered materials. In initial trials, the researchers simulated rainfall by running water through a container pricked with tiny holes. The container was suspended several meters in the air, high enough for drops to reach terminal velocity — the speed of an actual raindrop ­upon impact.

The researchers captured the sequence of events as raindrops hit each leaf, using high-speed videography at 1,000 frames per second. 

The images showed that water hitting the leaves left scattered droplets on the surface. Those might harbor pathogens — and when the next raindrops hit the leaves, the droplets are scattered into the air. Dyed water revealed the that these dispersing droplets can be catapulted if another drop hits the leaf nearby or launched in a crescent shape if hit by a raindrop directly. (The new drop first flattens over the dyed drop and then slides underneath before launching it.) The flexibility of the leaves played a key role in how far dispersing droplets flew. The researchers published their findings in the Journal of the Royal Society Interface.

Mark Fischetti for Scientific American explains why this is important:

The work might help plant breeders alter the mechanical traits of crop leaves. Or it might encourage farmers to plant fields with rows of alternating crops, which could prevent airborne pathogens launched from one row from reaching the same kind of plant two rows away.

Fungal diseases are especially prone to breaking out after rain and can be devastating. Researchers and farmers are warning that wheat rust, one such fungal disease, could destroy world crops if the right (or wrong, from our perspective) weather conditions occur. Work on developing rust-resistant wheat varieties aims to prevent this looming disaster. Perhaps this discovery may help — any possible way to head off the fungus’s advance is welcome. 

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