In reading this passage on page 196 on Belladonna from The farmer's Encyclopædia, and Dictionary of Rural Affairs By Cuthbert William Johnson on Google books:
Johnson says that according to Buchanan, Macbeth poisoned the Danes with Belladona. I have been trying to find where Buchanan actually says the poison was Belladona, but the closest I've found is this reference in the General Index of Buchanan's "History of Scotland" on Google books, page 667-668.
Which can be found here:
It seems to indicate that the reference is to page 330 or 332 but those pages do not seem to be speaking of Macbeth at all.
What I'm looking for is a direct reference from George Buchanan's "The History of Scotland" where he says that Macbeth specifically used Belladonna to poison the Danes.
Edit: I have also found reference to Macbeth using Belladonna here:
...I see I can't post more that two links... It was a link to a Google book of the following:
From page 193 of The Naturalist: Illustrative of the Animal, Vegetable, and Mineral Kingdoms..., Volumes 4-5 by Neville Wood, Esq. with the source attribution given again to Buchanan.