I think we should stop obsessing on 'crop-dusters.' There are many ways to spread this 'stuff.'
Agreed... I didn't mean to 'obsess' on crop-dusters. In the first place, that makes it sound as though there are specific planes called "crop dusters" that we could theoretically impound to eliminate the threat. In fact every general aviation aircraft in the United States could be outfitted with suitable equipment in a matter of hours. Then there are all the ways that do not involve aircraft at all.
If these guys have a plan to do this that is as well thought out as their plan to bring down the World Trade Center towers, we are in for some Serious Horror. I think the term "World War III" understates the significance of such an event, or the reaction it would bring. I know of no method beyond prayer to prevent this from happening.