Contrary to popular belief, agricultural crops might be singing, "Rain, rain go away," instead of "Let it rain, let it pour." While the importance of rain for providing water and nutrients to plant life is well-understood, it also contributes to the dispersal of microscopic pathogen particles. Pathogens, such as bacteria, viruses or fungi, cause harmful plant disease and often lead to the destruction of agricultural fields. With many possible dispersal methods, it can often be difficult to assess the damage of a pathogen's impact before it's too late.

At the 70th annual meeting of the American Physical Society's Division of Fluid Dynamics, being held Nov. 19-21, at the Colorado Convention Center in Denver, Colorado, researchers from Virginia Tech will present their work on rain drop dispersal mechanisms of rust fungus on wheat plants.

"Our research focused on how the rain drop dispersed the micro-sized pathogenic particles over long distance[s], said Sunghwan (Sunny) Jung, associate professor in Virginia Tech's Department of Biomedical Engineering and Mechanics. "Previously people studied this topic for a while and claimed that the splashing caused the pathogen dispersal, but in this study, we find out that there are two other mechanisms to disperse pathogens."

Two mechanisms of pathogen dispersal have been discovered. The first is a vortex ring mechanism, where one ring of pathogenic particles propagates and radially disperses after a drop impacts a plant surface. In the other, the pathogens are dispersed by an elastic collision mechanism. Jung describes this in terms of billiard balls: "You have billiard balls forming a triangle shape so if you hit the one ball in the front the other balls will be kicked out."

The identification of these mechanisms was not without its challenges. Pathogens are microscopic, around 10 micrometers, and extremely fast moving with speeds of 1 meter per second, so deducing mechanisms based on their liberation patterns was no simple task. The research team employed strong laser lights and high-speed cameras to capture and track the pathogens.

"We shined a strong laser on a pathogen site, and then released a drop to observe how pathogens get off from the surface," Jung said.

At the root of this problem is its oppositional nature: Rain is a natural process supplying thousands of gallons of fresh water to crops every year, but at the same time dispersing pathogens and damaging agricultural yields. This research provides insights into possible agricultural solutions for this issue. "We are trying to characterize how far these pathogens are flying from one plant to the others, then we can suggest what is the optimal distance or array of the crops in the field," Jung said.

When the researchers looked specifically at the yellow rust (Puccinia triticina) on wheat plants, the pathogenic damage is clear.

"Since wheat provides roughly one quarter of the world's food supply, spreading pathogens can cause a big problem in terms of the grain yield every year," Jung said. "Minimizing pathogenic damage by taking into account mechanisms of dispersal and optimal crop array can not only protect the wheat crop from continued yield loss, but protect the entire agricultural systems from continued pathogenic spread."