Farmers testify that certain crop diseases like wheat rust seem to spread much farther and faster after a rainstorm. Researchers had various ideas on why this might be the case. But thanks to high-speed video, a team from M.I.T. and the University of Liege in Belgium has just found an unlikely culprit: raindrops.

Close examination of leaves showed that certain pathogens such a wheat rust, which is a fungal parasite, do not coat leaves as thin films, which was commonly thought. They grow as droplets on a leaf’s surface. Because of that, when raindrops strike the pathogens they can knock the pathogens into the air, sending them pretty far and wide. High-speed video (see above) of hundreds of trials with real and simulated raindrops and pathogen droplets showed the collisions in action, and even revealed two primary launch mechanisms. To quote an M.I.T. press release, one case is when “a raindrop flattens upon impact, sliding underneath the [pathogen] droplet and launching it up in an arc.” The second case is when “a raindrop never actually touches a [pathogen] droplet, but instead pushes the leaf down, causing the droplet to slide downward and then catapult out" as the leaf bounces back up.

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. My own conclusion for right now: the video is pretty cool.

Video and still image courtesy of M.I.T. News Office