By John Kerr 09/03/2017

New research reveals how raindrops on soil create bioaerosols – tiny droplets of bacteria-laden water – which can help spread harmful microbes, including kiwifruit pathogen Psa.

Although soil bacteria are usually pretty slow at getting around, wet weather has been suggested to give them a hand travelling large distances. But exactly how rain gets bacteria from the soil into the air has been something of a mystery – until now.

New research, published today in the journal Nature Communications, painstakingly details the exact mechanism that allows bacteria to get airborne with the help of rain.

Using high-resolution imaging, Cullen Buie and colleagues from MIT’s Department of Mechanical Engineering tracked the fine mist released by a fizz of bubbles created when a drop of water hits soil. The researchers found that the tiny droplets in this mist carried up to several thousand bacteria from the soil and is some cases the bacteria remained alive for more than an hour afterward.


“Imagine you had a plant infected with a pathogen in a certain area, and that pathogen spread to the local soil,” Buie says. “We’ve now found that rain could further disperse it. Manmade droplets from sprinkler systems could also lead to this type of dispersal. So this [study] has implications for how you might contain a pathogen.”

The team calculated that precipitation around the world may be responsible for 1 to 25 percent of the total amount of bacteria emitted from land.

Read more about the research on

Spreading a kiwifruit killer

Effects of kiwifruit vine disease, Psa2.jpg
Bay of Plenty Regional Council

The authors note that their research is important for studying the spread of all manner of bacteria that could harm humans, animals and plants. However, one of the sample bacteria they used in their research –Pseudomonas syringae – has direct relevance to New Zealand. A variant of this bacteria, Pseudomonas syringae  pv actinidiae (Psa) is all too well known among NZ kiwifruit growers as the cause of kiwifruit vine disease.

A 2010 outbreak of the disease in the Bay of Plenty has been calculated to cost the NZ kiwifruit industry up to $885 million over 15 years.

Psa is known to spread more easily with the help of wet and wild weather and this new research offers a deeper understanding of exactly how Psa might be fizzed up into the air by raindrops and whisked away in bioaerosols. It will also offer further avenues for research on how to best predict and limit the spread of the costly disease. Plant & Food Research has already been working on using weather data to model the spread of the the disease in Bay of Plenty orchards.