Raindrops splash down on leaves, spread pathogens among plants

So we know that diseases affect plants like they affect humans but we actually don’t understand how diseases in plants get transmitted from one plant to the other. There was evidence that rainfalls are correlated with outbreaks in the field meaning we see outbreaks spreading faster or appearing in fields after rainfalls and we don’t understand why. So in the past what people have been doing is trying to correlate rainfalls with outbreaks and using a type of very large-scale mathematical models to describe spreading on fields but nobody really looked closely at what happens at the leaf level. In the past what has been thought is that a raindrop hits a plant and you have some kind of splash dynamic. So “splash” can be broadly defined as just water falling and you have, basically, droplets being emitted from that surface area. But actually what we saw was that when a raindrop hits a leaf the outcome of the creation these droplets that can emitted is very much dependent on the type of leaf you’re looking at. Obviously one can think of the surface properties as being important for the dynamics, but also the compliance and the mass of the leaf; so the inertia of the leaf actually changes completely the outcome in terms of the droplet sizes you will be emitting, their distribution in size but also their range, how far they can go. And that determines then how the next plant can be infected. So it’s important to understand the ratios of these different mechanisms because the size distribution and the speed at which these drops are generated is going to lead to a different pattern of contamination of the neighboring leaf. If you have a relatively large drop being emitted and falling on a neighboring leaf it is depositing locally on that surface area much more pathogens than what a small, tiny drop would do. The other difference is if you have a generation mechanism that produces much smaller droplets they’re more likely to evaporate quickly and then be evicted to another field through winds. But also when they deposit could contain less pathogens so it would need to do a bit more work in order to infect the next leaf. So the competition between the size, how much pathogens you have in there and how far these things can go and how quickly they can evaporate is all what is going to govern the spreading from one plant to the next. So the first outcome of this is actually being able to predict in some sense, how if you have certain crops that have certain mechanical properties you will expect to have a certain speed and pattern of outbreak in the field if a pathogen is introduced and if you know the rainfall process. So this could be tremendous help to better optimize, for example, the growth of various crops. So polyculture which is a very, very old concept, and in nature you see that all the time, crops are being alternated all the time. But this is giving us insight in how to do this in a smart way to optimize output in agriculture but also minimize the outbreaks.

6 comments

  1. There was another high speed video of rain drops recently that showed the raindrops can trap tiny air bubbles that rapidly rise through the water droplet and aerosolise substances from the surface they land on. This could potentially aerosolise pathogens that then land on nearby plants. Something to possibly look into.

  2. Could be an interesting avenue to study the evolution of altruism in plants, if such a thing exists.

    Does a plant spread water to it's surrounding plants?  Compare to the plant when sick, does it's wilting leaf spread more water or less water to other plants?  If a plant (when sick) spreads it's water to other plants, it may be 'poisoning the well', a strategy of 'Well if i can't survive, then neither shall any of my closest competitors, mwahahaha!'

    But if a plant, especially when sick and wilted, keeps it's water from splashing out;  It would be altruistic.  It would be protecting it's surrounding plants from infection.  'Live on my brothers, remember meeeeeeee (blarg im ded)'.

  3. Video very interesting for the development of research on pathogens dispersion and also didactic for classes of graduate programs in phytopathology.

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