Thinking through the Effect of Dew on Spray Efficacy

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The question has been asked of me from time to time about the possible effect of water deposited on the plants by dew on spray efficacy. Spraying after all is using water as a carrier, and having even more water on the plant only adds to the amount being deposited on the field. Does this dilute the spray or cause a bunch of the mix to run off the leaf, and cause a lessened efficacy?

To start to get my arms around this problem, last year after a particularly heavy dew, I went out and gathered a sample of 43 leaflets. I am an early riser and this sort of thing is not a problem. These 43 leaflets had a total of 14.7 ml of water on them. Considering that an average strawberry plant has 90 leaflets (30 leaves), this comes out to 31 ml per plant x 21,750 plants per acre = 674,250 ml dew water per acre or 178 gallons. Not insignificant.

Now figuring out what to do about all of this water already on the plant is not as straightforward as one might think. This is 178 gallons of water that could dilute the spray or create run off, and so people are thinking maybe we should be concerned about it?

The video from Sprayers 101 provided to me by colleague Franz Niederholzer might be helpful in getting our minds in the right frame to understand of what to think about the effect of dew water on spray efficacy:

http://sprayers101.com/dew-runoff/

A few bullet points from the video:

Courser, bigger spray droplets create more run-off of dew saturated leaves than do fine droplets.
However, the run off, which in the experiment shared on the video caused about 30% loss of product, still didn't end up with lessened efficacy. This could be because of calculated play in the product or the fact that the waxy cuticle of the leaf is well hydrated from the dew and more able to take up product than when dry.
Don't forget that the flat, horizontally oriented leaf surface of the strawberry can aggregate water rather than let it run off as with say a grass, so run off might not actually be as severe as we are thinking. The first picture below is proof positive of the ability of strawberry leaves to hold a lot of water!
We should not forget the role of the surfactant, which will cause mixing of the applied water with the water already on the leaf. This could also induce more run off.

The final thought on the video is probably the most important one. Spraying a wet plant is probably not as bad as we think it is, due to the opposing effects of more run off counteracted by great plant absorption because of cuticle hydration. It might be a wash, in other words.

Very heavy dew on strawberry leaves on a recent morning. Note the water accumulated in the cupped leaf.