They're a bacteria and I was told that sunlight does them in. But try and
get the powder to stick to the bottom of the leaves... if you can manage to
get it there.


I hear that if you get it to stick to the bottom of the leaves, it'll
last longer because the sun can't get to it. But for squash, you
really want it on the stems.

http://cecalaveras.ucdavis.edu/bt.htm

Yes, that has a few insights about persistence.